Solar activity is an essential factor for the study of many aspects of the geophysical and astronomical sciences. A very simple measure of solar activity is counting sunspots using telescopes. This task can be done even with small telescopes since the Sun is apparently a very large and luminous star. For this reason, it is possible to rescue the ancient observations of sunspots made in the past centuries to obtain an image of the evolution of solar activity during the last four centuries.

The first attempt to reconstruct solar activity from these records was made by Rudolf Wolf, who defined the Sunspot Number index in the 19th century. The Zurich Observatory (and later the Brussels Observatory) was in charge of continuing Wolf's work to the present day. In 1998, Hoyt and Schatten presented a new reconstruction of solar activity that was very different from Wolf's reconstruction (Vaquero and Vázquez, 2009). Many of these differences were solved by Clette et al. (2014).

Currently, research to improve the Sunspot Number is focused on (i) improving the database by reviewing old observations, and (ii) improving the methodologies to convert raw data into the Sunspot Number index. In this work, we try to present the latest advances in this task (Muñoz-Jaramillo and Vaquero, 2019; Arlt and Vaquero, 2020).

References

Arlt, R., Vaquero, J.M. (2020) Living Reviews in Solar Physics 17, 1.

Clette, F. et al. (2014) Space Science Reviews 186, 35.

Muñoz-Jaramillo, A., Vaquero, J.M. (2019) Nature Astronomy 3, 205.

Vaquero, J.M. and Vázquez, M. (2009) The Sun recorded through history (Springer).

How to cite: Vaquero, J. M.: A long-term geophysical and astronomical dataset: sunspot counting from 1610 to 2021, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15499, https://doi.org/10.5194/egusphere-egu21-15499, 2021.

It has been a long standing problem as to how the solar corona can maintain its million K temperature, while the photosphere, which is the lowest layer of the solar atmosphere, is only at a temperature of 5800 K. A very promising theory to explain this is the “nanoflare” hypothesis, which suggests that numerous flares of energies ~10²⁴ ergs are always happening in the solar corona, and maintain its million K temperature. However, detecting these nanoflares directly is challenging with the current instrumentation as they are hypothesised to occur at very small spatial, temporal and energy scales. These nanoflares are expected to produce nonthermal electrons, which are expected to emit in the radio band. These nonthermal emissions are often brighter than their thermal counterparts and might be detectable with current radio instruments. Due to their importance multiple searches for these nonthermal emissions have been done, but thus far they have been  limited to active regions. The quiet corona is also hot, and often comprises the bulk of the coronal region, so it is equally important to understand the physical processes which maintain this medium at MK temperatures. We describe the results from our effort to use the data from the Murchison Widefield Array (MWA) to search for impulsive radio emissions in the quiet solar corona. By pushing the detection threshold of nonthermal emission by about two orders of magnitude lower than previous studies, we have uncovered ubiquitous very impulsive nonthermal emissions from the quiet sun. We refer to these emissions as Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs). Using independent observations spanning very different solar conditions we show that WINQSEs are present throughout the quiet corona at all times. Their occurrence rate lies in the range of many hundreds to about a thousand per minute, implying that on average order 10 or so WINQSEs are present in every 0.5 s MWA image. Preliminary estimates suggest that WINQSEs have a bandwidth of ~2 MHz. Buoyed by  their possible connection to the hypothesised “nanoflares”, we are pursuing several projects to characterise and understand them. These include developing machine learning algorithms to identify WINQSEs in radio images and characterise their morphologies; exploring the ability of the present generation EUV and X-ray instruments to estimate the energy corresponding to the brightest of WINQSEs; and attempting very high time resolution imaging to explore their temporal structure. In this talk, I will present the results from the past and ongoing projects about WINQSEs and argue that these might be a key step towards detecting “nanoflares” and the resolution of the coronal heating problem.

How to cite: Mondal, S., Oberoi, D., Biswas, A., Bawaji, S., Alam, U., Behera, A., Kansabanik, D., Swainston, N., Bhat, R., and Morgan, J.:  First radio evidence for ubiquitous magnetic reconnections and impulsive heating in the quiet solar corona , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14168, https://doi.org/10.5194/egusphere-egu21-14168, 2021.

Solar flares release enormous magnetic energy into the corona, producing the heating of ambient plasma and acceleration of particles. The flaring process is complex and often shows multiple spatially separated temporal individual episodes of energy releases, which can be hard to resolve based on the instrument capability. We analysed the multi-wavelength imaging and spectroscopy observations of multiple electron acceleration episodes during a GOES B1.7-class two-ribbon flare observed simultaneously with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, the Reuven Ramatay High Energy Solar Spectroscopic Imager (RHESSI) in X-rays, and the Solar Dynamics Observatory in extreme ultraviolet (EUV).
We observed a total of six radio bursts. First three bursts were co-temporal, but not co-spatial nonthermal X-ray source and represent multiple electron acceleration episodes. We model the radio spectra by optically thick gyrosynchrotron emission and estimate the magnetic field strength and nonthermal electron spectral parameters in each acceleration episode. We note that the nonthermal parameters derived from X-rays differ considerably from the nonthermal parameters inferred from the radio and originates in the lower corona. Although co-temporal, our multi-wavelength analysis shows that different electron populations produce multiple acceleration episodes in radio and X-rays wavelengths. 

How to cite: Sharma, R., Battaglia, M., Luo, Y., Chen, B., and Yu, S.: Radio and X-ray Observations of Short-lived Episodes of Electron Acceleration in a Solar Microflare   , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14293, https://doi.org/10.5194/egusphere-egu21-14293, 2021.

The properties of the magnetic fields of the solar photosphere are investigated, in particular, the distribution of fields of different polarity over the solar surface. As primary data, synoptic maps of the photospheric magnetic field of the Kitt Peak National Solar Observatory for 1978-2016 were used. Using the vector summation method, the non-axisymmetric component of the magnetic field is determined. It was found that the nonaxisymmetric component of weak magnetic fields B < 5 G changes in antiphase with the flux of these fields. Magnetic fields of B < 5 G constitute a significant part of the total magnetic field of the Sun, since they occupy more than 60% of the area of the photosphere. The fine structure of the distribution of weak fields can  be observed by setting the upper limit to the strength of the  fields  included in the time–latitude diagram. This allows to eliminate the contribution of the strong fields of sunspots.

On the time-latitude diagram for weak magnetic fields (B < 5 G), bands of differing colors correspond to the streams of the magnetic fields moving in the direction to the Sun’s poles.. These streams or surges show the alternation of the dominant polarity - positive or negative - which is clearly seen in all four cycles. The slopes of the bands indicate the velocity of the fields movement towards the poles. The surges can be divided into two groups. The surges of the first group belong to the so-called Rush-to-the-Poles. These are bands with the width of about three years, which begin at approximately 40° of latitude and have the same polarity as the trailing sunspots. They reach high latitudes and cause the polarity reversal of the polar field. However, in addition to these surges, for most of the solar  cycle (the descending phase, the minimum and the ascending phase), there are narrower surges of both polarities (with the width less than one year), which extend from the equator almost to the poles. These surges are most clearly visible in the southern hemisphere when the southern pole is positive. Consideration of the latitude-time diagrams separately for positive and negative polarities showed that the alternating dominance of one of the polarities is associated with the antiphase development  of the positive and negative fields of the surges. The widths of surges and the periodicity of their appearance vary significantly for the two hemispheres and from one solar cycle to the other. The mean period of the polarity alternation is about 1.5 years.

How to cite: Baranov, D., Vernova, E., and Tyasto, M.: Surges of the weak magnetic field in the photosphere of the Sun, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4954, https://doi.org/10.5194/egusphere-egu21-4954, 2021.

Increasing observations show that coronal jets may result in bubble-shaped coronal mass ejections (CMEs), but the genesis of jet-driven CMEs and their nature are not fully understood. Here, we report a direct stereoscopic observation on the magnetic coupling from a coronal blowout jet to a stellar-sized CME.  Observations in the EUV passbands of SDO/AIA show that this whole event starts with a small-scale active-region filament whose eruption occurs at a coronal geyser site due to flux emergence and cancellation. By interacting with an overlying null-point configuration, this erupting filament first breaks one of its legs and triggers an unwinding blowout jet. The release of magnetic twist in its jet spire is estimated at around 1.5−2.0 turns. This prominent twist transport in jet spire rapidly creates a newborn large-scale flux rope from the jet base to a remote site. As a result, the newborn large-scale flux rope erupts into the outer coronae causing an Earth-directed bubble-shaped CME. In particular, two sets of distinct flare post-flare loops form in its source region in sequence, indicating this eruptive event couples with twice flare reconnection. This observation highlights a real pathway for jet-CME magnetic coupling and provides a new hint for the buildup of large-scale CME flux ropes. 

How to cite: Chen, H., Yang, J., Hong, J., Li, H., and Duan, Y.: Direct Observation of A Large-scale CME Flux Rope Event Arising from an Unwinding Coronal Jet , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3616, https://doi.org/10.5194/egusphere-egu21-3616, 2021.

Helioseismic response to solar flares ("sunquakes") occurs due to localized force or/and momentum impacts observed during the flare impulsive phase in the lower atmosphere. Such impacts may be caused by precipitation of high-energy particles, downward shocks, or magnetic Lorentz force. Understanding the mechanism of sunquakes is a key problem of the flare energy release and transport. Our statistical analysis of M-X class flares observed by the Solar Dynamics Observatory during Solar Cycle 24 has shown that contrary to expectations, many relatively weak M-class flares produced strong sunquakes, while for some powerful X-class flares, helioseismic waves were not observed or were weak. The analysis also revealed that there were active regions characterized by the most efficient generation of sunquakes during the solar cycle. We found that the sunquake power correlates with maximal values of the X-ray flux derivative better than with the X-ray class. The sunquake data challenge the current theories of solar flares.

How to cite: Kosovichev, A. and Sharykin, I.: Characteristics of Sunquake Events Observed in Solar Cycle 24, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1461, https://doi.org/10.5194/egusphere-egu21-1461, 2021.

The corona of the Sun, and probably also of other stars, is built up by loops defined through the magnetic field. They vividly appear in solar observations in the extreme UV and X-rays. High-resolution observations show individual strands with diameters down to a few 100 km, and so far it remains open what defines these strands, in particular their width, and where the energy to heat them is generated.

The aim of our study is to understand how the magnetic field couples the different layers of the solar atmosphere, how the energy generated by magnetoconvection is transported into the upper atmosphere and dissipated, and how this process determines the scales of observed bright strands in the loop.

To this end, we conduct 3D resistive MHD simulations with the MURaM code. We include the effects of heat conduction, radiative transfer and optically thin radiative losses.
We study an isolated coronal loop that is rooted with both footpoints in a shallow convection zone layer. To properly resolve the internal structure of the loop while limiting the size of the computational box, the coronal loop is modelled as a straightened magnetic flux tube. By including part of the convection zone, we drive the evolution of the corona self-consistently by magnetoconvection.

We find that the energy injected into the loop is generated by internal coherent motions within strong magnetic elements. 
The resulting Poynting flux is channelled into the loop in vortex tubes forming a magnetic connection between the photosphere and corona, where it is dissipated and heats the upper atmosphere.

The coronal emission as it would be observed in solar extreme UV or X-ray observations, e.g. with AIA or XRT, shows transient bright strands.
The widths of these strands are consistent with observations. From our model we find that the width of the strands is governed by the size of the individual photospheric magnetic field concentrations where the field lines through these strands are rooted. Essentially, each coronal strand is mainly rooted in a single magnetic patch in the photosphere, and the energy to heat the strand is generated by internal motions within this magnetic concentration.

With this model we can build a coherent picture of how energy and matter are transported into the upper solar atmosphere and how these processes structure the interior of coronal loops.

How to cite: Breu, C., Peter, H., Cameron, R., Solanki, S., Chitta, P., and Przybylski, D.: Coronal loops in a box: 3D models of their internal structure, dynamics and heating, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13091, https://doi.org/10.5194/egusphere-egu21-13091, 2021.

Coronal loops are building blocks of solar active regions (ARs). However, their formation is not well understood. Here we present direct observational evidence for the formation of coronal loops through magnetic reconnection as new magnetic fluxes emerge to the solar atmosphere. Observations in the EUV passbands of SDO/AIA clearly show the newly formed loops following magnetic reconnection within a vertical current sheet. Formation of the loops is also seen in the Hα images taken by NVST. The SDO/HMI observations show that a positive-polarity flux concentration moves toward a negative-polarity one with a speed of ~0.5 km s^-1 before the apparent formation of coronal loops. During the formation of coronal loops, we found signatures of flux cancellation and subsequent enhancement of the transverse field between the two polarities. We have reconstructed the three-dimensional magnetic field structure through a magnetohydrostatic model, which shows field lines consistent with the loops in AIA images. Numerous bright blobs with a width of ~1.5 Mm appear intermittently in the current sheet and move upward with apparent velocities of ~80 km s^-1. We have also identified plasma blobs moving to the footpoints of the newly formed large loops, with apparent velocities ranging from 30 to 50 km s^-1. A differential emission measure analysis shows that the temperature, emission measure and density of the bright blobs are 2.5-3.5 MK, 1.1-2.3×10²⁸ cm^-5 and 8.9-12.9×10⁹ cm^-3, respectively. Power spectral analysis of these blobs indicates that the magnetic reconnection is inconsistent with the turbulent reconnection scenario.

How to cite: Hou, Z., Tian, H., Chen, H., Zhu, X., He, J., Bai, X., Huang, Z., and Xia, L.: Formation of solar coronal loops through magnetic reconnection in an emerging active region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1013, https://doi.org/10.5194/egusphere-egu21-1013, 2021.

We studied 43 coronal dimming events associated with Earth-directed coronal mass ejections (CMEs) that were observed in quasi-quadrature by the SDO and STEREO satellites. We derived the properties of the dimmings as observed above the limb by STEREO EUVI, and compared them with the mass and speed of the associated CMEs. The unique satellite constellation allowed us to compare our findings with the results from Dissauer et al. (2018, 2019), who studied these events observed against the solar disk by SDO AIA. Such statistics is done for the first time and confirms the close relation between characteristic dimming and CME parameters for the off-limb viewpoint. We find that the dimming areas are typically larger for off-limb observations (mean value of 1.24±1.23×10¹¹ km² against 3.51±0.71×10¹⁰ km² for on-disk), while the decrease in the total extreme ultraviolet intensity is similar (c=0.60±0.14). The off-limb dimming areas and brightnesses are strongly correlated with the CME mass (c=0.82±0.06 and 0.75±0.08), whereas the dimming area and brightness change rate correlate with the CME speed (c∼0.6). Our findings suggest that coronal dimmings have the potential to provide early estimates of the Earth-directed CMEs parameters, relevant for space weather forecasts, for satellite locations at both L1 and L5.

How to cite: Chikunova, G., Dissauer, K., Podladchikova, T., and Veronig, A.: Coronal dimmings associated with coronal mass ejections on the solar limb, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3134, https://doi.org/10.5194/egusphere-egu21-3134, 2021.

Major flares and coronal mass ejections (CMEs) tend to originate from the compact polarity inversion lines (PILs) in the solar active regions (ARs). Recently, a scenario named as “collisional shearing” is proposed by Chintzoglou et al. (2019) to explain the phenomenon, which suggests that the collision between different emerging bipoles is able to form the compact PIL, driving the shearing and flux cancellation that are responsible to the subsequent large activities. In this work, through tracking the evolution of 19 emerging ARs from their birth until they produce the first major flares or CMEs, we investigated the source PILs of the activities, i.e., the active PILs, to explore the generality of “collisional shearing”. We find that none of the active PILs is the self PIL (sPIL) of a single bipole. We further find that 11 eruptions originate from the collisional PILs (cPILs) formed due to the collision between different bipoles, 6 from the conjoined systems of sPIL and cPIL, and 2 from the conjoined systems of sPIL and ePIL (external  PIL between the AR and the nearby preexisting polarities). Collision accompanied by shearing and flux cancellation is found developing at all PILs prior to the eruptions, with 84% (16/19) cases having collisional length longer than 18 Mm. Moreover, we find that the magnitude of the flares is positively correlated with the collisional length of the active PILs, indicating that the intenser activities tend to originate from the PILs with severer collision. The results suggest that the “collisional shearing”, i.e., bipole-bipole interaction during the flux emergence is a common process in driving the major activities in emerging ARs.

How to cite: Liu, L., Wang, Y., Zhou, Z., and Cui, J.: The Source Locations of Major Flares and CMEs in the Emerging Active Regions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-31, https://doi.org/10.5194/egusphere-egu21-31, 2021.

We present a novel method to derive the shock density compression ratio of coronal shock waves that are occasionally observed as halo coronal mass ejections (CMEs). Our method uses the three-dimensional (3-D) geometry and enables us to access the reliable shock density compression ratio. We show the 3-D properties of coronal shock waves seen from multiple vantage point observations, i.e., geometry, kinematics, and compression ratio (Mach number). The significant findings are as follows: (1) Halo CMEs are the manifestation of spherically shaped fast-mode waves/shocks, rather than a matter of the projection of expanding flux ropes. The footprints of halo CMEs on the coronal base are the so-called EIT/EUV waves. (2) These spherical fronts arise from a driven shock (bow- or piston-type) close to the CME nose, and it is gradually becoming a freely propagating (decaying) fast-mode shock wave at the flank. (3) The shock density compressions peak around the CME nose and decrease at larger position angles (flank). (4) Finally, the supercritical region extends over a large area of the shock and lasts longer than past reports.  These results offer a simple unified picture of the different manifestations for CME-associated (shock) waves, such as EUV waves and SEP events observed in various regimes and heliocentric distances. We conclude that CME shocks can accelerate energetic particles in the corona over extended spatial and temporal scales and are likely responsible for the wide longitudinal distribution of these particles in the inner heliosphere.

How to cite: Kwon, R. Y.: The three-dimensional density compression ratio of shock fronts observed as halo coronal mass ejections , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10578, https://doi.org/10.5194/egusphere-egu21-10578, 2021.

Coronal Mass Ejections (CMEs) and their interplanetary counterparts (ICMEs) are the major sources for strong space weather disturbances. We present a study of statistical properties of fast CMEs (v≥1000 km/s) that occurred during solar cycles 23 and 24. We apply the Max Spectrum and the declustering threshold time methods. The Max Spectrum can detect the predominant clusters, and the declustering threshold time method provides details on the typical clustering properties and timescales. Our analysis shows that during the different phases of solar cycles 23 and 24, fast CMEs preferentially occur as isolated events and in clusters with, on average, two members. However, clusters with more members appear, particularly during the maximum phases of the solar cycles. During different solar cycle phases, the typical declustering timescales of fast CMEs are τ_c =28-32 hrs, irrespective of the very different occurrence frequencies of CMEs during a solar minimum and maximum. These findings suggest that  τ_c   for extreme events may reflect the characteristic energy build-up time for large flare and CME-prolific active regions. Statistically associating the clustering properties of fast CMEs with the disturbance storm time index at Earth suggests that fast CMEs occurring in clusters tend to produce larger geomagnetic storms than isolated fast CMEs. Our results highlight the importance of CME-CME interaction and their impact on Space Weather.

How to cite: Rodriguez Gomez, J. M., Podlachikova, T., Veronig, A., Ruzmaikin, A., Feynman, J., and Petrukovich, A.: Clustering of Fast Coronal Mass Ejections during Solar Cycles 23 and 24 and Implications for CME–CME Interactions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3990, https://doi.org/10.5194/egusphere-egu21-3990, 2021.

The Sun produces highly dynamic and eruptive events that can drive shocks through the corona. These shocks can accelerate electrons, which result in plasma emission in the form of a type II radio burst. Despite a large number of type II radio bursts observations, the precise origin of coronal shocks is still subject to investigation. Here we present a well-observed solar eruptive event that occurred on 16 October 2015, focusing on a jet observed in the extreme ultraviolet by the SDO Atmospheric Imaging Assembly, a streamer observed in white-light by the Large Angle and  Spectrometric Coronagraph, and a metric type II radio burst observed by the LOw-Frequency Array (LOFAR) radio telescope. For the first time, LOFAR has interferometrically imaged the fundamental and harmonic sources of a type II radio burst and revealed that the sources did not appear to be co-spatial, as would be expected from the plasma emission mechanism. We correct for the separation between the fundamental and harmonic using a model which accounts for the scattering of radio waves by electron density fluctuations in a turbulent plasma. This allows us to show the type II radio sources were located ∼0.5 R_sun above the jet and propagated at a speed of ∼1000 km s⁻¹, which was significantly faster than the jet speed of ∼200 km s⁻¹. This suggests that the type II burst was generated by a piston shock driven by the jet in the low corona.

How to cite: Maguire, C., Carley, E., Zucca, P., Vilmer, N., and Gallagher, P.: LOFAR observations of a jet-driven piston shock in the low solar corona, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7602, https://doi.org/10.5194/egusphere-egu21-7602, 2021.

Coronal Mass Ejections (CMEs) are large-scale explosive eruptions of magnetised plasma from the Sun into the Heliosphere. Measuring the physical parameters of CMEs is crucial for understanding their physics and for assessing their geo-effectiveness. Radio observations offer the most direct means for estimating these plasma parameters when gyrosynchrotron (GS) emission is detected from the CME plasma. However, since the first detection by Bastian et al.2001, only a handful of studies have successfully detected GS emission from CME plasma. This is usually attributed to the challenges involved in obtaining the high dynamic range imaging required for observing this faint gyrosynchrotron emission in the vicinity of active solar emissions.

The newly developed imaging pipeline (Mondal et al., 2019) designed for the data from Murchison Widefield Array (MWA) marks a significant improvement in metrewave solar radio imaging. Our work suggests that we should now be able to routinely detect GS emission from CME plasma. We present an example where we have successfully detected radio emission from CME plasma and modelled it as GS emission, leading to reliable estimates of CME magnetic field as well as the distribution of energetic electrons (Mondal et al. 2020). In a different example we are able to detect the radio emission from the CME plasma out to as far as 8.3 solar radii. We find that the observed spectra are not always consistent with simple GS models. This highlights that more complicated physics might be at play and points to the need for building more detailed models for interpreting these emissions. We hope that with the availability of polarimetric imaging capability, which we are in the process of developing, this technique will provide a robust way to routinely measure CME magnetic fields along with its other physical parameters. We note that these are the weakest detections of GS emissions from CME plasma reported yet.

How to cite: Kansabanik, D., Mondal, S., Oberoi, D., and Vourlidas, A.: High fidelity spectroscopic imaging at low radio frequencies to estimate plasma parameters of solar coronal mass ejections at higher coronal heights, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11089, https://doi.org/10.5194/egusphere-egu21-11089, 2021.

The X-Ray Telescope (XRT) onboard the Hinode satellite has a specially designed Wolter type grazing-incidence (GI) optics with a paraboloid-hyperboloid mirror assembly to measure the solar coronal plasma of temperatures up to 10 MK with a resolution of about one arc sec. One of the main purposes of this scientific mission is to investigate the detailed mechanism of energy transfer processes from the photosphere to the upper coronal region leading to its heating and the solar wind acceleration. An astronomical telescope is in general designed such that the best-focused image of an object is achieved at or very close to the optical axis, and inevitably the optical performance deteriorates away from the on-axis position. The Sun is, however, a large astronomical object and thus targets near the limb of full-disk images are placed at the outskirt of the field of view. The design of a solar telescope should thus consider the uniformity of imaging quality over a wide FOV, and it is particularly so for X-ray telescopes whose targets can be in the corona high above the limb.

We will explain in this presentation the importance of detailed calibration of the off-axis optical characteristics for Hinode/XRT. It have been revealed that the scattered light caused by the GI mirror surface has a power-law distribution and shows an energy dependence. We will also introduce the basic scheme of how the level of scattering wing is determined and connected to the core from the analysis of highly saturated in-flight data. Vignetting is another important optical characteristics for describing the telescope's performance, which reflects the ability to collect incoming light at different locations and photon energies. We have evaluated the vignetting effect in Hinode/XRT by analyzing the ground experimental data and found that the degree of vignetting varies linearly from the optical center and its pattern shows an energy dependence. Many interesting results on the calibration of Hinode/XRT optical characteristics will be introduced and discussed thoroughly. 

How to cite: Shin, J., Kano, R., Sakurai, T., Kim, Y.-H., and Moon, Y.-J.: Detailed Calibration of the Off-Axis Optical Characteristics for the X-Ray Telescope onboard Hinode, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13004, https://doi.org/10.5194/egusphere-egu21-13004, 2021.

During Solar Minimum, the Sun is perceived to be quite inactive with barely any spots emerging on the solar surface. Consequently, we observe a drop in the number of highly energetic events such as solar flares and coronal mass ejections (CMEs), which are often associated with active regions on the photosphere. However, our magnetofrictional simulations during the minimum period suggest that the solar corona could still be significantly dynamic while evolving in response to the large-scale shearing velocities on the solar surface. The non-potential evolution of the corona leads to the accumulation of magnetic free energy and helicity, which is periodically lost through eruptive events. Our study shows that these events can be categorised into two distinct classes. One set of events are caused due to full-scale eruption of low-lying coronal flux ropes and could be associated with occasional filament erupting CMEs observed during Solar Minimum. The other set of events are not driven by destabilisation of low-lying structures but rather by eruption from overlying sheared arcades. These could be linked with streamer blowouts or stealth CMEs. The two classes differ considerably in the amount of magnetic flux and helicity shed through the outer coronal boundary. We additionally investigate how other measurables such as current, open magnetic flux, free energy, coronal holes area, and the horizontal component of the magnetic field on the outer model boundary vary during the two classes of event. This study demonstrates and emphasises the importance and necessity of understanding the dynamics of the coronal magnetic field during Solar Minimum.

How to cite: Bhowmik, P. and Yeates, A.: Two classes of eruptive events during Solar Minimum, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6760, https://doi.org/10.5194/egusphere-egu21-6760, 2021.

Society’s dependence on technology has increased during the past years. Therefore, understanding the hazardous events including space weather events that lead to technological problems is now critical. As solar wind is the driver of space weather, identifying extreme solar wind is important. In this work extreme value theory is used to characterize the solar wind parameters most relevant to space weather: interplanetary magnetic field strength and proton speed. This is done using an extreme value distribution for all data above a certain threshold for each parameter. Analysis demonstrates that these thresholds are around 900 km/s for the proton speed and around 95 nT for the interplanetary magnetic field. Based on 20 years of solar wind data, we made an estimation for the interplanetary magnetic field and solar wind proton speed with return periods corresponding to 4 and 6 solar cycles with a 99% confidence interval.

How to cite: Larrodera, C., Nikitina, L., and Cid, C.: Extreme event theory applied to the solar wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2076, https://doi.org/10.5194/egusphere-egu21-2076, 2021.

The solar corona is a highly-structured plasma which can reach temperatures of more than 2 MK. At low frequencies (decimetric and metric wavelengths), scattering and refraction of electromagnetic waves are thought to considerably increase the imaged radio source sizes (up to a few arcminutes). However, exactly how source size relates to scattering due to turbulence is still subject to investigation. The theoretical predictions relating source broadening to propagation effects have not been fully confirmed by observations, due to the rarity of high spatial resolution observations of the solar corona at low frequencies. Here, the LOw Frequency ARray (LOFAR) was used to observe the solar corona at 120–180 MHz using baselines of up to 3.5 km (corresponding to a resolution of 1–2’) during the partial solar eclipse of 2015 March 20. A lunar de-occultation technique was used to achieve higher spatial resolution (0.6’) than that attainable via standard interferometric imaging (2.4’). This provides a means of studying the contribution of scattering to apparent source size broadening. This study shows that the de-occultation technique can reveal a more structured quiet corona that is not resolved from standard imaging, implying scattering may be overestimated in this region when using standard imaging techniques. However, an active region source was measured to be 4’ using both de-occultation and standard imaging. This may be explained by increased scattering of radio waves by turbulent density fluctuations in active regions, which is more severe than in the quiet Sun.

How to cite: Ryan, A. M., Gallagher, P. T., Carley, E. P., Brentjens, M. A., Murphy, P. C., Vocks, C., Morosan, D. E., Reid, H., Magdalenic, J., Breitling, F., Zucca, P., Fallows, R., Mann, G., Kerdraon, A., and Halfwerk, R.: LOFAR Imaging of the Solar Corona during the 2015 March 20 Solar Eclipse, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11094, https://doi.org/10.5194/egusphere-egu21-11094, 2021.

Two large-scale interaction regions between the fast solar wind emanating from coronal holes and the slow solar wind coming from streamer belt are usually distinguished. When the fast stream pushes up against the slow solar wind ahead of it, a compressed interaction region that co-rotates with the Sun (CIR) is created. It was already shown that the relative abundance of alpha particles, which usually serve as one of solar wind source identifiers can change within this region. By symmetry, when the fast stream outruns the slow stream, a corotating rarefaction region (CRR) is formed. CRRs are characterized by a monotonic decrease of the solar wind speed, and they are associated with the regions of small longitudinal extent on the Sun. In our study, we use near-Earth measurements complemented by observations at different heliocentric distances, and focus on the behavior of alpha particles in the CRRs because we found that the large variations of the relative helium abundance (AHe) can also be observed there. Unlike in the CIRs, these variations are usually not connected with the solar wind speed and alpha-proton relative drift changes. We thus apply a superposed-epoch analysis of identified CRRs with a motivation to determine the global profile of alpha particle parameters through these regions. Next, we concentrate on the cases with largest AHe variations and investigate whether they can be associated with the changes of the solar wind source region or whether there is a relation between the AHe variations and the non-thermal features in the proton velocity distribution functions like the temperature anisotropy and/or presence of the proton beam.

How to cite: Durovcova, T., Šafránková, J., and Němeček, Z.: Study of alpha particle properties across rarefaction regions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2864, https://doi.org/10.5194/egusphere-egu21-2864, 2021.

³He-rich solar energetic particle (SEP) events are characterized by a peculiar elemental composition with rare species like ³He or ultra-heavy ions tremendously enhanced over the solar system abundances.
We report on ³He rich SEP periods measured by the Suprathermal Ion Telescope (SIT) onboard STEREO-A beginning in 2007 until 2020, covering the whole solar cycle 24.
The mass resolution capabilities of SIT do not allow to easily distinguish between ³He and ⁴He especially in cases of a low ³He to ⁴He ratio.
We therefore developed a semi-automatic detection algorithm to find time periods during which a ³He enhancement can be statistically determined.
Using this method we found 112 ³He rich periods.
These periods were further examined in regards of their ³He/⁴He and Fe/O ratio. 
Previously about ten ³He-rich SEP periods measured by SIT on STEREO-A have been reported.
An association with in-situ electron measurements by STEREO-SEPT and STEREO-STE showed that ~60% of the 112 periods are accompanied with electron events.
The here presented catalogue of ³He rich periods is intended to serve as a reference for the community.

How to cite: Köberle, M., Bucik, R., Dresing, N., Heber, B., Klassen, A., and Wang, L.: 3He rich periods measured by the Suprathermal Ion Telescope (SIT) on STEREO-A during solar cycle 24, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12772, https://doi.org/10.5194/egusphere-egu21-12772, 2021.

We study the 27-day variations of galactic cosmic rays (GCRs) based on neutron monitor (NM), ACE/CRIS, STEREO and SOHO/EPHIN measurements, in solar minima 23/24 and 24/25 characterized by the opposite polarities of solar magnetic cycle. Now there is an opportunity to re-analyze the polarity dependence of the amplitudes of the recurrent GCR variations in 2007-2009 for negative A < 0 solar magnetic polarity and to compare it with the clear periodic variations related to solar rotation in 2017-2019 for positive A > 0. We use the Fourier analysis method to study the periodicity in the GCR fluxes. Since the GCR recurrence is a consequence of solar rotation, we analyze not only GCR fluxes, but also solar and heliospheric parameters examining the relationships between the 27-day GCR variations and heliospheric, as well as, solar wind parameters. We find that the polarity dependence of the amplitudes of the 27-day variations of the GCR intensity and anisotropy for NMs data is kept for the last two solar minima: 23/24 (2007-2009) and 24/25 (2017-2019) with greater amplitudes in positive A > 0 solar magnetic polarity. ACE/CRIS, SOHO/EPHIN and STEREO measurements are not governed by this principle of greater amplitudes in positive A > 0 polarity. GCR recurrence caused by the solar rotation for low energy (< 1GeV) cosmic rays is more sensitive to the enhanced diffusion effects, resulting in the same level of the 27-day amplitudes for positive and negative polarities. While high energy (> 1GeV) cosmic rays registered by NMs, are more sensitive to the large-scale drift effect leading to the 22-year Hale cycle in the 27-day GCR variation, with the larger amplitudes in the A > 0 polarity than in the A < 0.

How to cite: Modzelewska, R. and Gil, A.: 27-day variations of the galactic cosmic rays intensity and anisotropy in solar minima 23/24 and 24/25 by ACE/CRIS, STEREO, SOHO/EPHIN and neutron monitors, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13185, https://doi.org/10.5194/egusphere-egu21-13185, 2021.

The Interstellar Probe is a space mission to discover physical interactions shaping globally the boundary of our Sun`s heliosphere and its dynamics and for the first time directly sample the properties of the local interstellar medium (LISM). Interstellar Probe will go through the boundary of the heliosphere to the LISM enabling for the first time to explore the boundary with a dedicated instrumentation, to take the image of the global heliosphere by looking back and explore in-situ the unknown LISM. The pragmatic concept study of such mission with a lifetime 50 years that can be implemented by 2030 was funded by NASA and has been led by the Johns Hopkins University Applied Physics Laboratory (APL). The study brought together a diverse community of more than 400 scientists and engineers spanning a wide range of science disciplines across the world.

Compelling science questions for the Interstellar Probe mission have been with us for many decades. Recent discoveries from a number of space missions exploring the heliosphere raised new questions strengthening the science case. The very shape of the heliosphere, a manifestation of complex global interactions between the solar wind and the LISM, remains the biggest mystery. Interpretations of imaging the heliosphere in energetic neutral atoms (ENAs) in different energy ranges on IBEX and Cassini/INCA from inside show contradictory pictures. Global physics-based models also do not agree on the global shape. Interstellar Probe on outbound trajectory will image the heliosphere from outside for the first time and will provide a unique determination of the global shape.

The LISM is a completely new area for exploration and discovery. We have a crude understanding of the LISM inferred from in-situ measurements inside the heliosphere of interstellar helium, pick-up-ions, ENAs, remote observations of solar backscattered Lyman-alpha emission and absorption line spectroscopy in the lines of sight of stars. We have no in-situ measurements of most LISM properties, e.g. ionization, plasma and neutral gas, magnetic field, composition, dust, and scales of possible inhomogeneities. Voyagers with limited capabilities have explored 30 AU beyond the heliosphere which appeared to be a region of significant heliospheric influence. The LISM properties are among the key unknowns to understand the Sun`s galactic neighborhood and how it shapes our heliosphere. Interstellar Probe will be the first NASA mission to discover the very nature of the LISM and shed light on whether the Sun enters a new region in the LISM in the near future.

In this presentation we give an overview of heliophysics science for the Interstellar Probe mission focusing on the critical science questions of the three objectives for the mission. We will discuss in more details a need for direct measurements in the LISM uniquely enabled by the Interstellar Probe.

How to cite: Provornikova, E., Brandt, P. C., McNutt, Jr., R. L., DeMajistre, R., Roelof, E. C., Mostafavi, P., Turner, D., Hill, M. E., Linsky, J. L., Redfield, S., Galli, A., Lisse, C., Mandt, K., Rymer, A., and Runyon, K.: Unique heliophysics science opportunities along the Interstellar Probe journey up to 1000 AU from the Sun, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10504, https://doi.org/10.5194/egusphere-egu21-10504, 2021.

An Interstellar Probe mission to the Very Local Interstellar Medium (VLISM) would bring new scientific discoveries of the mechanisms upholding our vast heliosphere and directly sample the unexplored Local Interstellar Clouds that our Sun is moving through in relatively short galactic timescales. As such, it would represent Humanity's first explicit step in to the galaxy and become perhaps NASA's boldest step in space exploration. Such a mission has been discussed and studied since 1960, but the stumbling block has often been propulsion. Now this hurdle has been overcome by the availability of new and larger launch vehicles. An international team of scientists and experts are now progressing towards the final year of a NASA-funded study led by The Johns Hopkins University Applied Physics Laboratory (APL) to develop pragmatic example mission concepts for an Interstellar Probe with a nominal design lifetime of 50 years. Together with the Space Launch System (SLS) Office at the NASA Marshall Space Flight Center, the team has analyzed dozens of launch configurations and demonstrate that asymptotic speeds in excess of 7.5 Astronomical Units (AU) per year can be achieved using existing or near-term propulsion stages with a powered or passive Jupiter Gravity Assist (JGA). These speeds are more than twice that of the fastest escaping man-made spacecraft to date, which is Voyager 1 currently at 3.59 AU/year. An Interstellar Probe would therefore reach the Termination Shock (TS) in less than 12 years and cross the Heliopause into the VLISM after about 16 years from launch.

In this presentation we provide an overview and update of the study, the science mission concept, discuss the compelling discoveries that await, and the associated example science payload, measurements and operations ensuring a historic data return that would push the boundaries of space exploration by going where no one has gone before.

How to cite: Brandt, P., McNutt, R., Provornikova, E., Lisse, C., Mandt, K., Runyon, K., Rymer, A., Mostafavi, P., DeMajistre, R., Roelof, E., Turner, D., Hill, M., Kinnison, J., Rogers, G., Smith, C., Fountain, G., Copeland, D., Kollmann, P., Ashtari, R., and Stough, R. and the The Interstellar Probe Study Team: Interstellar Probe: A Mission to the Heliospheric Boundary and Interstellar Medium for the Next Decade, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3308, https://doi.org/10.5194/egusphere-egu21-3308, 2021.

Galactic cosmic rays, and sporadic high energy solar energetic particles, are energetic enough to pierce the Earth’s protective magnetosphere and interact with the atmosphere. Here, a secondary particle cascade leads to enhanced radiation levels which is of importance, for instance, to aviation dosimetry and related studies. At ground level, these secondary particles can be observed (indirectly) by means of neutron monitors, and this has been done for more than 70 years, providing a valuable long-term cosmic ray record. In this talk, we introduce the different primary particle populations, discuss their acceleration and modulation, and connect this with long-term neutron monitor measurements.

How to cite: Strauss, D. T.: Cosmic rays at ground level; a brief introduction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6212, https://doi.org/10.5194/egusphere-egu21-6212, 2021.

SEPs are correlated with the 11-year solar cycle due to their production by flares and interaction with the inner heliosphere, while GCRs are anti-correlated with it due to the modulation of the heliospheric magnetic field. The solar magnetic field along the cycle varies in amplitude but also in geometry, causing diffusion of the particles along and across the field lines; the solar wind distribution also evolves, and its turbulence affects particle trajectories.

We combine 3D MHD compressible numerical simulations to compute the configuration of the magnetic field and the associated polytropic solar wind up to 1 AU, with analytical prescriptions of the corresponding parallel and perpendicular diffusion coefficients for SEPs and GCRs. First, we analyze separately the impact of the magnetic field amplitude and geometry for a 100 MeV proton. By varying the amplitude, we change the amplitude of the diffusion by the same factor, and the radial gradients by changing the spread of the current sheet. By varying the geometry, we change the latitudinal gradients of diffusion by changing the position of the current sheets. We also vary the energy, and show that the statistical distribution of parallel diffusion is different for SEPs and GCRs. Then, we use realistic solar configurations, showing that diffusion is highly non-axisymmetric due to the configuration of the current sheets, and that the distribution varies a lot with the distance to the Sun, especially at minimum of activity. With this model, we are thus able to study the direct influence of the Sun on Earth spatial environment in terms of energetic particles. 

How to cite: Perri, B., Brun, A. S., Strugarek, A., and Réville, V.: Energetic particles and the solar cycle: Impact of solar magnetic field amplitude and geometry on SEPs and GCRs diffusion coefficients, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6394, https://doi.org/10.5194/egusphere-egu21-6394, 2021.

The random walk of magnetic field lines is an important ingredient in understanding how the connectivity of the magnetic field affects the spatial transport and diffusion of charged particles. As solar energetic particles (SEPs) propagate away from near-solar sources, they interact with the fluctuating magnetic field, which modifies their distributions. We develop a formalism in which the differential equation describing the field line random walk contains both effects due to localized magnetic displacements and a non-stochastic contribution from the large-scale expansion. We use this formalism together with a global magnetohydrodynamic simulation of the inner-heliospheric solar wind, which includes a turbulence transport model, to estimate the diffusive spreading of magnetic field lines that originate in different regions of the solar atmosphere. We first use this model to quantify field line spreading at 1 au, starting from a localized solar source region, and find rms angular spreads of about 20 – 60 degrees. In the second instance, we use the model to estimate the size of the source regions from which field lines observed at 1 au may have originated, thus quantifying the uncertainty in calculations of magnetic connectivity; the angular uncertainty is estimated to be about 20 degrees. Finally, we estimate the filamentation distance, i.e., the heliocentric distance up to which field lines originating in magnetic islands can remain strongly trapped in filamentary structures. We emphasize the key role of slab-like fluctuations in the transition from filamentary to more diffusive transport at greater heliocentric distances. This research has been supported in part by grant RTA6280002 from Thailand Science Research and Innovation and the Parker Solar Probe mission under the ISOIS project (contract NNN06AA01C) and a subcontract to University of Delaware from Princeton University (SUB0000165).  MLG acknowledges support from the Parker Solar Probe FIELDS MAG team.  Additional support is acknowledged from the  NASA LWS program  (NNX17AB79G) and the HSR program (80NSSC18K1210 & 80NSSC18K1648).

How to cite: Ruffolo, D., Chhiber, R., Matthaeus, W. H., Usmanov, A. V., Tooprakai, P., Chuychai, P., and Goldstein, M. L.: Random Walk and Trapping of Interplanetary Magnetic Field Lines: Global Simulation, Magnetic Connectivity, and Implications for Solar Energetic Particles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3719, https://doi.org/10.5194/egusphere-egu21-3719, 2021.

The influence of the heliospheric current sheet (HCS) on the propagation of high energy solar protons is explored using 3D test particle modelling. The test particle model, which includes drift effects, is used to simulate specific past ground level enhancement (GLE) events which cover a range of HCS configurations. For example, the effects of a source location close to and far from the HCS for events both poorly and well-connected to Earth are examined. Similarly, the effect of the Earth’s location relative to the HCS is explored. The modelling is performed for high energy (300-1200 MeV) protons to represent the energetic conditions under which GLEs occur. The derived intensity profiles at 1AU are compared to observations from HEPAD onboard GOES, as well as STEREO (at locations away from Earth) and neutron monitor data. 

How to cite: Waterfall, C. and Dalla, S.: Role of the heliospheric current sheet in high energy proton transport through modelling of historic GLE events , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6223, https://doi.org/10.5194/egusphere-egu21-6223, 2021.

In-situ observations of solar energetic particle events are determined by a combination of acceleration, injection, and transport processes which are often hard to disentangle. However, the energy spectrum of impulsive electron events is believed to carry the imprint of the flare acceleration process which can be studied by analyzing the hard X-ray (HXR) spectrum of the flare.

Using STEREO/SEPT electron data of the whole STEREO mission we have identified 64 solar energetic electron event candidates where the HXR solar counterpart of the event was observed by RHESSI. After cleaning of the data set and an independent verification by the timing of associated interplanetary type III radio bursts, we find 17 events which lend themselves for a comparison of the spectral indices observed in situ and at the Sun.

Special attention is paid to the choice of the in-situ electron spectral index used for comparison as most of the events show spectral transitions (breaks) in the measurement range of SEPT. We find that both the lower and higher spectral indices correlate similarly well with the HXR spectra yielding correlation coefficients of 0.8 but indicating opposite relations with the flare spectrum in terms of the thin- or thick target model. The correlations show no dependence on the electron onset delay, nor on the longitudinal separation between flare and spacecraft magnetic footpoint at the Sun. However, the correlations increase, if only events with significant anisotropy are used indicating that transport effects play a role in shaping the spectra observed in-situ. We will discuss the different transport effects that need to be taken into account and which may even lead to a vanishing imprint of the flare acceleration.

How to cite: Dresing, N., Warmuth, A., Effenberger, F., Klein, L., Glesener, L., Musset, S., and Bruedern, M.: Connecting solar flare hard X-ray spectra to in-situ electron spectra using RHESSI and STEREO/SEPT observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9543, https://doi.org/10.5194/egusphere-egu21-9543, 2021.

How to cite: Mironova, I.: The effect of Forbush decreases on the polar-night HOx concentration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7498, https://doi.org/10.5194/egusphere-egu21-7498, 2021.

The solar system is constantly changing, and it is important for us to understand how our climate and weather changes in response to the solar activity during both long-time scales (e.g. the 11-year solar cycle) and short time scales (e.g. days to weeks during For-bush Decreases (FDs)). Solar variability causes a corresponding modulation of the incident number of cosmic rays in Earth's atmosphere. Previous work by [Veretenenko and Pudovkin, 1997], [Svensmark and Friis-Christensen, 1997], [Palle Bago and Butler, 2000], [Svensmark et al., 2016], [Harrison and Ambaum, 2010], and other researchers have discussed this cause-effect relationship from an experimental and theoretical approach. Since the 1970s, global observations of the Earth's system by satellites are offering an invaluable source of information about cloud parameters.

In this study, we used the newly calibrated PATMOS-x (Pathfinder Atmospheres Extended) data set during the period from 1978 to the present. A method for capturing the connection between cosmic rays and meteorological measurements has been conducted by superposition analysis of FD events for time series (36 days) and the Monte Carlo bootstrap test to evaluate significance level of the integrated signal for 9 days after the minimum in FD. We have reviewed results, primarily about cloud emissivity (Achieved Significance Level (ASL >99%), surface brightness temperature (ASL >99%), and cloud fraction (ASL >99%). Some of the results support the proposed relationship between solar activity and temperature. This result indicates that the amount of incident cosmic rays decreases due to FDs, global average temperature increases [Friis-Christensen and Lassen, 1991], [Harrison and Ambaum, 2010]. In addition, PATMOS-x parameters of cloud probability, cloud mask, and cloud fraction, which all means cloud coverage on the Earth shows statistically significant signals following FDs. In some previous research, IR-detected cloud fraction from International Satellite Cloud Climate Project (ISCCP) and combined liquid and ice cloud fraction, effective emissivity from the Moderate Resolution Imaging Spectroradiometer (MODIS) also show connection with FDs, see [Svensmark et al., 2009], [Svensmark et al., 2016], [Marsh and Svensmark, 2000a], Todd and Kniveton [2004]. The relationship between the observed changes in cloud amount and the resulting solar forcing is discussed. On the other hand, “Cloud water content" from Special Sensor Microwave Imager (SSM/I), “Liquid water path", and “Optical thickness" from MODIS also showed as significant signals by FDs, see [Svensmark et al., 2009], [Svensmark et al., 2016], however a similar parameter about “optical thickness" and “integrated total cloud water over whole column g/m2" from PATMOS-x dataset does not have high significant signals by a bootstrap test with ASL of 77.03 and 92.51% respectively. Moreover, significant results are reported for several new cloud parameters from the PATMOS-x dataset (e.g. cloud type, brightness temperature, measurements by different wavelength 0.65, 0.86, 3.75, 11.0, and 12.0 μm and others) and Fu-Liou model is used for estimation of changed radiations in the atmosphere. An interaction between CCN and radiation has not been investigated well yet. It is necessary to still more to learn about these results for further understanding of Earth’s atmosphere.

How to cite: Matsumoto, H., Svensmark, H., and Enghoff, M.: The Effect of Forbush Decreases on Atmospheric Aerosols and Clouds from The PATMOS-x Satellite from 1978 to 2018, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-858, https://doi.org/10.5194/egusphere-egu21-858, 2021.

The northern polar vortex experiences considerable inter-annual variability, which is also reflected to tropospheric weather. Recent research has established a link between polar vortex variations and energetic electron precipitation (EEP) from the near-Earth space into the polar atmosphere, which is mediated by EEP-induced chemical changes causing ozone loss in the mesosphere and stratosphere. However, the most dramatic changes in the polar vortex are due to strong enhancements of planetary wave activity, which typically result in a sudden stratospheric warming (SSW), a momentary breakdown of the polar vortex. Here we use the SSWs as an indicator of high planetary wave activity and consider their influence of SSWs on the atmospheric response to EEP in 1957-2017 using combined ERA-40 and ERA-Interim re-analysis data and geomagnetic activity as a proxy of EEP. We find that the EEP-related enhancement of the polar vortex and other associated dynamical responses are seen only during winters when a SSW occurs, and that the EEP-related changes take place slightly before the SSW onset. We show that the atmospheric conditions preceding SSWs favor enhanced wave-mean-flow interaction, which can dynamically amplify the initial polar vortex enhancement caused by ozone loss. These results highlight the importance of considering SSWs and sufficient level of planetary wave activity as a necessary condition for observing the effects of EEP on the polar vortex dynamics.

How to cite: Asikainen, T., Salminen, A., Maliniemi, V., and Mursula, K.: Influence of energetic particle precipitation on polar vortex mediated by planetary wave activity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3376, https://doi.org/10.5194/egusphere-egu21-3376, 2021.

In this presentation, we will show the first measurements performed by EPD since the end of the commissioning phase until the latest results obtained. During these months EPD has been scanning the inner heliosphere at different heliocentric distances and heliolongitues allowing - together with other spacecraft - to investigate the spatio-temporal behavior of the particle populations in the inner heliosphere during solar minimum conditions. Solar Orbiter was launched from Cape Canaveral on February 10th, 2020, thus beginning the journey to its encounter with the Sun. Solar Orbiter carries ten scientific instruments, six remote sensing and four in situ, that will allow the mission main goal: how the Sun creates and controls the heliosphere. Among the in situ instruments, the Energetic Particle Detector (EPD) measures electrons, protons and heavy ions with high temporal resolution over a wide energy range, from suprathermal energies up to several hundreds of MeV/nucleon.

How to cite: Rodriguez-Pacheco, J. and the EPD Team: First Results from Solar Orbiter’s Energetic Particle Detector, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15792, https://doi.org/10.5194/egusphere-egu21-15792, 2021.

As part of the Energetic Particle Detector (EPD) suite onboard Solar Orbiter, the High Energy Telescope has been launched on its mission to the Sun on February 9, 2020, and has been measuring energetic particles since it was first switched on about two weeks after launch. Using their double-ended telescopes, the two HET units provide measurements of ions above 7 MeV/nuc and electrons above 300 keV in four viewing directions. HET observed several Solar Energetic Particle (SEPs) events during the cruise phase, including the first one with a broad energy coverage (up to ~100MeV) on 29 Nov 2020. Being the first larger SEP event in a phase of rising solar activity, these measurements have already attracted extensive attention of the community. Apart from the SEPs, the HET can be used to observe the Galactic cosmic radiation (GCR) and its temporal variation. The GCR measurements can be also utilized for the validation of the energy response of HET. The overall spectra observed by HET are as expected, except for calibration issues in some specific energy bins that we are still investigating. Finally, the HET also observed several Forbush Decreases (FD), i.e. cosmic ray decreases caused by CMEs and their embedded magnetic field. Here, the capabilities and data products of HET, as well as first measurements of SEPs, GCR and FDs are presented. 

How to cite: Xu, Z., Freiherr von Forstner, J. L., Kühl, P., Janitzek, N., Martín, C., Kulkarni, S. R., Böttcher, S. I., Wimmer-Schweingruber, R. F., Rodríguez-Pacheco, J., Mason, G. M., and Ho, G. C. and the Solar Orbiter EPD team: The High Energy Telescope (HET) on the SolarOrbiter Mission: Overview and First Data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6776, https://doi.org/10.5194/egusphere-egu21-6776, 2021.

Solar Orbiter carries a total of 10 instrument suites making up the payload for the mission.  One of these, the Solar Wind Analyser (SWA) instrument, is comprised of 3 sensor units which are together served by a central DPU unit.  Of particular focus in this presentation are the early measurements from one of these sensors, the Electron Analyser System (EAS).  EAS is a dual-head, top-hat electrostatic analyser system that is capable of making 3D measurements of solar wind electrons at energies below ~5 keV from a vantage point at the end of a 4-metre boom extending into the shadow of the spacecraft.  The sensor was accommodated in this location to both maximise the unobstructed field of view and to minimise the effect of spacecraft related disturbances on the low-energy (less than a few tens of eV) electrons expected the core population of the solar wind.

To date the SWA instrument sensors have operated sporadically during the mission cruise phase, which began in June 2020.  This is due to a number of operational issues faced by the SWA team, which mean we have not been able to take data in a continuous manner.  However, the data that has been taken shows the clear promise of the SWA measurements, in general, once these issues can be overcome.  For example, EAS is using a novel sample steering mechanism in burst mode which, with reference to a magnetic field vector shared onboard by the MAG instrument, allows the capture of the electron pitch angle distribution at unusually high time resolution.  We discuss these observations here, and illustrate the potential science returns from the burst mode.  We also present results from the new EAS observations in the vicinity of reconnecting current sheets in the solar wind, to more generally illustrate the capability of the sensor. 

How to cite: Owen, C. and the the International SWA, MAG and RPW teams.: Early Observations from the Solar Orbiter SWA/Electron Analyser System, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12501, https://doi.org/10.5194/egusphere-egu21-12501, 2021.

The Solar Orbiter mission was launched in 2020 into an orbit that will explore the inner heliosphere. During its orbit, periods of quasi-corotation with the Sun will enable determination of the source regions on the Sun for solar wind structures.  The Solar Wind Analyser (SWA) is a suite of instruments that provide in-situ measurements of solar wind electrons, protons, alpha particles, and heavy ions.  The SWA-Heavy Ion Sensor (HIS) is optimized to measure heavy ions in the solar wind, pickup ions, and suprathermal ions in an energy range spanning from 0.5- 75keV/e.  We present measurements of heavy ion composition from SWA-HIS taken during the cruise phase of the mission to highlight the capabilities of the instrument and the observations we expect to collect over the next 10 years. We discuss how SWA-HIS will enable linkages between the Sun and the solar wind to reveal the nature of the acceleration and release of the solar wind and the sources and structure of the solar wind.  We will also provide an overview of the available data and accessibility of the public datasets. 

How to cite: Lepri, S. T., Livi, S. A., Raines, J. M., Galvin, A. B., Kistler, L. M., Dewey, R. M., Alterman, B. L., Allegrini, F., Collier, M. R., and Owen, C. J.: Updates and Early Results from the Heavy Ion Sensor on Solar Orbiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12435, https://doi.org/10.5194/egusphere-egu21-12435, 2021.

We show in situ observations of ICMEs during the first year of Solar Orbiter observations based on magnetic field data from the MAG instrument in conjunction with in situ and imaging observations from the Heliospheric System Observatory. The in situ magnetic field data from four other currently active spacecraft - Parker Solar Probe, BepiColombo, STEREO-Ahead and Wind -  are also searched for ICME signatures, and all clear ICME events that could be identified by classic signatures such as elevated and rotating magnetic fields of sufficiently long durations are included in a living online catalog. Furthermore, we provide a visualization of the in situ magnetic field data alongside spacecraft positions and propagating CME fronts, which are based on modeling of STEREO-A heliospheric imager data. This allows us to identify ICME events that could be unambiguously followed from their inception on the Sun to their impact at the aforementioned spacecraft, and highlights sought-after lineup events, in which the same ICME is observed at multiple points in space, such as the well-studied 2020 April 15-20 ICME. We discuss the ICME rate observed so far, and provide an outlook on the expected ICME rate in solar cycle 25 based on different forecasts for the cycle amplitude (see Möstl et al. 2020, https://doi.org/10.3847/1538-4357/abb9a1).

How to cite: Möstl, C., Weiss, A. J., Bailey, R. L., Reiss, M. A., Amerstorfer, T., Hinterreiter, J., Bauer, M., Amerstorfer, U. V., Davies, E. E., Horbury, T., Barnes, D., Davies, J. A., Harrison, R. A., Heyner, D., Richter, I., Auster, H.-U., Magnes, W., and Baumjohann, W.: Overview of interplanetary coronal mass ejections observed by Solar Orbiter, Parker Solar Probe, Bepi Colombo, Wind and STEREO-A, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-592, https://doi.org/10.5194/egusphere-egu21-592, 2021.

We present initial results for a triple-point analysis for the in situ magnetic field measurements of a CME observed at three independent locations. On the 19th of April 2020, Solar Orbiter observed a CME in situ at a radial distance of around 0.8 au. This CME was subsequently also detected by the Wind and Bepi Colombo satellites closer to Earth. This triple in situ measurement of a CME provides us the unique opportunity to test the consistency of the measurements with our own 3D Coronal Rope Ejection (3DCORE) model. A triple measurement allows for up to seven different data combinations to be analyzed (three single-point, three dual-point, and one single triple-point combination) which gives us information on how our analysis pipeline responds to multi-point measurements and how the results change with measurements at differing radial and longitudinal distances. The goal of this study is to test whether all three in situ measurements can still be described by a slightly bent flux rope geometry and how adding additional measurements can improve the accuracy of inferred model parameters.

How to cite: Weiss, A. J., Möstl, C., Davies, E., Owens, M. J., Amerstorfer, T., Bauer, M., Hinterreiter, J., Bailey, R. L., Reiss, M. A., Horbury, T., O'Brien, H., Evans, V., Angelini, V., Heyner, D., Richter, I., Auster, U., Magnes, W., and Baumjohann, W.: Triple-point magnetic flux rope analysis for the 2020 April 19 CME observed in situ by Solar Orbiter, Bepi Colombo, and WIND, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8878, https://doi.org/10.5194/egusphere-egu21-8878, 2021.

During 27th September 2020 NASA Parker Solar Probe (PSP) and ESA-NASA Solar Orbiter (SolO) have been located around the same Carrington longitude and their latitudinal separation was very small as well. Solar wind plasma and magnetic field data obtained throughout this time interval  allows to consider that sometimes the solar wind, observed by both spacecrafts, originates from the same coronal hole region. Inside these time intervals the SolO radial magnetic field experiences several short variations similar to the "switchbacks" regularly observed by PSP. We used the SolO SWA-PAS proton analyzer data to analyze the ion distribution function variations inside such switchback-like events to understand if such events are really "remains" of the alfvenic structures observed below 60 Rs.

How to cite: Fedorov, A., Louarn, P., Owen, C., Prech, L., Horbury, T., Barthe, A., Rouillard, A., Kasper, J., Bale, S., Bruno, R., O’Brien, H., Evans, V., Angelini, V., Larson, D., and Livi, R. and the SWA-PAS, MAG, SWEAP, and FIELDS teams: Switchback-like structures observed by Solar Orbiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4996, https://doi.org/10.5194/egusphere-egu21-4996, 2021.

The recently released spacecraft potential measured by the RPW instrument onboard Solar Orbiter has been used to estimate the solar wind electron density in the inner heliosphere. Selected intervals have been extracted to study and quantify the properties of turbulence. Empirical Mode Decomposition was used to obtain the generalized marginal Hilbert spectrum, equivalent to the structure functions analysis, additionally reducing issues typical of nonstationary time series. Results show the presence of a well defined inertial range with Kolmogorov scaling. However, the turbulence shows intermittency only in part of the samples, while other intervals have homogeneous scale-dependent fluctuations. These are observed predominantly during intervals of ion-frequency wave activity. Comparisons with compressible magnetic field intermittency (from the MAG instrument) and with an estimate of the solar wind velocity (using electric and magnetic field) are also provided to provide general context and help determine the cause for the absence of intermittency.

How to cite: Sorriso-Valvo, L., Carbone, F., Yuri Khotyaintsev, Y., Graham, D., Steinvall, K., and Telloni, D. and the The Solar Orbiter RPW and MAG Teams: Turbulence and intermittency of electron density fluctuations in the inner heliosphere: Solar Orbiter first data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9712, https://doi.org/10.5194/egusphere-egu21-9712, 2021.

Electric field observations of the Time Domain Sampler (TDS) receiver, a part of the Radio and Plasma Waves (RPW) instrument onboard Solar Orbiter, often exhibit very intense broadband emissions at frequencies below 10 kHz in the spacecraft frame. The RPW instrument has been operating almost continuously during the commissioning phase of the mission from March to May, the first perihelion in June, and through the first flyby of Venus in late December 2020. Nearly a year of observations allow us to perform a statistical study of ion-acoustic waves in the solar wind covering an interval of heliocentric distances between 0.5 AU to 1 AU. The occurrence of low-frequency waves peaks around perihelion in June at distances of 0.5 AU and decreases with increasing distances, with only a few waves detected per day in late September at ~1 AU. A more detailed analysis of triggered waveform snapshots shows the typical wave frequency at about 3 kHz and wave power about 5e-2 mV²/m². The distribution of the relative phase between two components of the projected E-field in the Spacecraft Reference Frame (SRF) shows a mostly linear wave polarization. These waves are interpreted as strongly Doppler-shifted ion-acoustic waves, generated by solar wind ion beams and often accompany large-scale solar wind structures. A detailed analysis of the Doppler-shift using solar wind data from a Proton and Alpha particle Sensor (PAS), a part of Solar Wind Analyzer (SWA), is done for several examples.

How to cite: Pisa, D., Soucek, J., Santolik, O., Maksimovic, M., Horbury, T., and Owen, C. and the SolO RPW, MAG, and SWA instrument teams: Large amplitude ion-acoustic waves observed in the solar wind by the Solar Orbiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10023, https://doi.org/10.5194/egusphere-egu21-10023, 2021.

The magnetic and velocity fluctuations of the solar wind may be strongly correlated. This characterizes the  ‘Alfvenic’ flows. Using the observations of the Proton Alfa sensor (PAS/SWA) and the magnetometer (MAG) onboard Solar Orbiter, we analyze a period of 100 hours of such alfvenic flows, at different scales. Several parameters of the turbulence are computed (V-B correlation, various spectral indexes, cross-helicity, residual energy). We explore how these parameters may vary with time and characterize different turbulent states of the flow. More specifically, using the unprecedented time resolution of PAS during burst mode, especially its capability to measure 3D distribution functions at time scale below the proton gyroperiod, we study the connection of the turbulence to the dissipation domain and analyze the fine structure of the distribution functions and their evolutions at sub-second scales. The goal is to investigate whether some characteristics of the distributions, as their more or less pronounced temperature anisotropy, may be related to the turbulence parameters and the degree of V-B correlation.

How to cite: Louarn, P., fedorov, A., Rouillard, A., Lavraud, B., Génot, V., Owen, C. J., Bruno, R., Prech, L., Livi, S., Horbury, T. S., and Maksimovic, M. and the SWA and MAG Solar Orbiter: Analysis of Alfvénic flows with Solar Orbiter: particle and magnetic observations down to kinetic scales., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10238, https://doi.org/10.5194/egusphere-egu21-10238, 2021.

Solar wind current sheets have been extensively studied at 1 AU. The recent advent of Parker Solar Probe and Solar Orbiter (SolO) has enabled us to study these structures at a range of heliocentric distances.

We present SolO observations of current sheets in the solar wind at heliocentric distances between 0.55 and 0.85 AU, some of which show signatures of ongoing magnetic reconnection. We develop a method to find the deHoffman-Teller frame which minimizes the Y-component (the component tangential to the spacecraft orbit) of the electric field. Using the electric field measurements from RPW and magnetic field measurements from MAG, we use our method to determine the deHoffman-Teller frame of solar wind current sheets. The same method can also be used on the Alfvénic turbulence and structures found in the solar wind to obtain a measure of the solar wind velocity.

Our preliminary results show a good agreement between our modified deHoffmann-Teller analysis based on the single component E-field, and the conventional deHoffman-Teller analysis based on 3D plasma velocity measurements from PAS. This opens up the possibility to use the RPW and MAG data to obtain an estimate of the solar wind velocity when particle data is unavailable.

How to cite: Steinvall, K., Khotyaintsev, Y., Cozzani, G., Vaivads, A., Owen, C., Fedorov, A., and Louarn, P. and the RPW Team, SWA Team, MAG Team: Solar Orbiter observations of solar wind current sheets and their deHoffman-Teller frames, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10630, https://doi.org/10.5194/egusphere-egu21-10630, 2021.

The recent launches of Parker Solar Probe (PSP), Solar Orbiter (SO) and BepiColombo, along with several legacy spacecraft, have provided the opportunity to study the solar wind at multiple latitudes and distances from the Sun simultaneously. We take advantage of this unique spacecraft constellation, along with low solar activity between May and July 2020, to investigate how latitude affects the solar wind and Heliospheric Current Sheet (HCS) structure. We use ballistic mapping to compare polarity and solar wind velocity between several spacecraft, showing that fine scale ripples in the HCS can be resolved down to several degrees in longitude. We show that considering solar wind velocity is also useful when investigating the HCS structure, as it can reveal times when the spacecraft is within slow, dense streamer belt wind without changing magnetic polarity. We measured the local orientation of planar magnetic structures associated with HCS crossings, finding that these were broadly consistent with the shape of the HCS but at much steeper angles due to compression from stream interaction regions. We identified several transient magnetic clouds associated with HCS crossings, and have shown that these can disrupt the local HCS orientation up to four days after their passage, but did not significantly affect the position of the HCS. This work highlights that the heliosphere should always be treated as three-dimensional, especially at solar minimum, where a few degrees in latitude can create a considerable difference in solar wind conditions.

How to cite: Laker, R., Horbury, T., Matteini, L., Woolley, T., Woodham, L., Stawarz, J., Bale, S., Davies, E., Eastwood, J., O'Brien, H., Evans, V., Angelini, V., Richter, I., Heyner, D., Owen, C., Louarn, P., and Fedorov, A.: Ripples in the Heliospheric Current Sheet: Dependence on Latitude and Transient Outflows, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12941, https://doi.org/10.5194/egusphere-egu21-12941, 2021.

We will review the very latest observations and results obtained by the Radio and Plasma Waves (RPW) Instrument on the recently launched Solar Orbiter mission. RPW is designed to measure in-situ magnetic and electric fields and waves from 'DC' to a few hundreds of kHz. RPW is also capable of measuring solar radio emissions up to 16 MHz and link them to solar flares observed by the onboard remote sensing instruments. The latest results we will present concern a wide range of phenomena including: Langmuir and Whistler Waves, dust impacts, Solar Type III bursts and observations during the recently visited Venus environment.

How to cite: Maksimovic, M. and the RPW, MAG, SWA and EPD teams: The Radio and Plasma Waves (RPW) Instrument on Solar Orbiter: latest observations and results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11899, https://doi.org/10.5194/egusphere-egu21-11899, 2021.

Impacts of dust grains on spacecraft are known to produce typical impulsive signals in the voltage waveform recorded at the terminals of electric antennas. Such signals are, as could be expected, routinely detected by the radio and plasma waves (RPW) instrument aboard Solar Orbiter, therefore providing in-situ measurements of the interplanetary dust density along the spacecraft trajectory.

We present a statistical analysis of the first year and half of dust impact data recorded by Solar Orbiter RPW between 1 AU and 0.5 AU. We discuss the results in terms of constraints that can be put on beta-meteoroids and interstellar dust fluxes, and compare them to results obtained by STEREO at 1 AU and more recently by Parker Solar Probe at 0.5 AU.

How to cite: Zaslavsky, A., Mann, I., Bale, S., Czechowski, A., Issautier, K., Lorfèvre, E., Maksimovic, M., Meyer-Vernet, N., Pisa, D., Rackovic-Babic, K., Soucek, J., and Vaverka, J.: Interplanetary dust observations with the Solar Orbiter RPW instrument: a first year of data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4482, https://doi.org/10.5194/egusphere-egu21-4482, 2021.

A large-amplitude impact-induced like electric field signal is often observed by the Radio and Plasma Wave (RPW) Instrument onboard Solar Orbiter. The signal has a sharp increase followed by an exponential decay, typically observed when spacecraft experiences a dust impact. The amplitude can reach several V/m. The impact dust size can be estimated from the electric field amplitude and is similar to the characteristic dust size near the sun expected from the zodiacal-light observations. On the other hand, the signal's decay time is the order of second, unusually long compared to the dust impact signals previously reported by the other spacecraft. We will show the characteristics of these signals and discuss the origin.

How to cite: Morooka, M., Khotyaintsev, Y., Eriksson, A., Edberg, N., Johansson, E., Maksimovic, M., Bale, S., Chust, T., Krasnoselskikh, V., Kretzschmar, M., Lorfèvre, E., Plettemeier, D., Souček, J., Steller, M., Štverák, Š., Trávníček, P., Vaivads, A., and Vecchio, A.: Impact induced electric field signals observed by the Solar Orbiter/RPW, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13801, https://doi.org/10.5194/egusphere-egu21-13801, 2021.

Plasma waves can play an important role in the evolution of the solar wind and the particle velocity distribution functions in particular. We analyzed the electromagnetic waves observed above a few Hz by the Radio Plasma Waves (RPW) instrument suite onboard Solar Orbiter, during its first orbit, which covered a distance from the Sun between 1 AU and 0.5 AU.  We identified the majority of the detected waves as whistler waves with frequency around  0.1 f_ce and right handed circular polarisation. We found these waves to be mostly aligned or anti aligned with the ambient magnetic field, and rarely oblique. We also present and discuss their direction of propagation and the variation of the waves' properties with heliocentric distance.

How to cite: Kretschmar, M., Chust, T., Graham, D., Krasnosekskikh, V., Colomban, L., Maksimovic, M., Horbury, T., Owen, C., and Louarn, P.: Whistler waves observed by Solar Orbiter during its first orbit, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15195, https://doi.org/10.5194/egusphere-egu21-15195, 2021.

We report Solar Orbiter observations of electromagnetic waves near the proton cyclotron frequency during the first perihelion. The waves have polarization close to circular and have wave vectors closely aligned with the background magnetic field. Such waves are potentially important for heating of the solar wind as their frequency and polarization allows effective energy exchange with solar wind protons. The Radio and Plasma Waves (RPW) instrument provides a high-cadence measurement of plasma density and electric field which we use together with the magnetic field measured by MAG to characterize these waves. In particular we compute the compressibility and the phase between the density fluctuations and the parallel component of the magnetic field, and show that these have a distinct behavior for the waves compared to the Alfvénic turbulence. We compare the observations to multi-fluid plasma dispersion and identify the waves modes corresponding to the observed waves. We discuss the importance of the waves for solar wind heating.

How to cite: Khotyaintsev, Y., Graham, D. B., Steinvall, K., Vaivads, A., Maksimovic, M., Edberg, N. J. T., Johansson, E. P. G., and Eriksson, A. I. and the RPW, MAG and SWA Teams: Using Compressibility and Electric Field to Characterize Circularly-Polarized Waves Near the Proton Cyclotron Frequency Observed by Solar Orbiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15746, https://doi.org/10.5194/egusphere-egu21-15746, 2021.

Thin current sheets are routinely observed in the solar wind. Here we report observations of thin current sheets and the associated plasma waves using the Solar Orbiter spacecraft. The Radio and Plasma Waves (RPW) instrument provides high-resolution measurements of the electric field, number density perturbations, and magnetic field fluctuations, which we use to identify and characterise the observed waves, while the magnetic field provided by the MAG instrument is used to characterise the current sheets. We discuss the role of current sheets in the generation of the observed waves and the effects of the waves on the current sheets. 

How to cite: Graham, D., Khotyaintsev, Y., Steinvall, K., Vaivads, A., Maksimovic, M., Edberg, N., Johansson, E., Eriksson, A., Kretzschmar, M., and Chust, T. and the RPW, MAG and SWA Solar Orbiter teams: Thin current sheets and the associated wave activity observed by Solar Orbiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15860, https://doi.org/10.5194/egusphere-egu21-15860, 2021.

How to cite: Bercic, L. and the et al.: Evolution of the electron velocity distribution under the presence of whistler waves in the solar wind (high-cadence Solar Orbiter observations), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16455, https://doi.org/10.5194/egusphere-egu21-16455, 2021.

How to cite: Chen, Y., Przybylski, D., Peter, H., and Tian, H.: Transient small-scale brightenings in the quiet Sun corona: a model for "campfires" observed with Solar Orbiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5061, https://doi.org/10.5194/egusphere-egu21-5061, 2021.

Linking solar activity on the surface and in the corona to the heliosphere is one of Solar Orbiter’s main goals. Its EUV spectrometer SPICE (SPectral Imaging of the Coronal Environment) will provide relative abundance measurements which will be key in this quest, as different structures on the Sun have different abundances as a consequence of the FIP (First Ionization Potential) effect. From the 16th to the 22nd of November 2020,  the Solar Orbiter remote sensing checkout window STP-122 was carried out. During this period of observations, SPICE was lucky to catch a small AR in its field of view. We carried out abundance specific observations in order to provide relative FIP bias measurements with SPICE. Furthermore, data from other types of observations carried out during that same week allow us to identify the spectral lines that could be used for abundance diagnostics. We take the SPICE instrument characteristics into account to give recommendations regarding the types of studies to carry out to obtain such abundance measurements.

How to cite: Zambrana Prado, N., Buchlin, É., and Peter, H. and the SPICE consortium team: First data for abundance diagnostics with SPICE, the EUV spectrometer on-board Solar Orbiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15555, https://doi.org/10.5194/egusphere-egu21-15555, 2021.

With the launch and commissioning of Solar Orbiter, the Spectrometer/Telescope for Imaging X-rays (STIX) is the latest hard X-ray telescope to study solar flares over a large range of flare sizes. STIX uses hard X-ray imaging spectroscopy in the range from 4 to 150 keV to diagnose the hottest temperature of solar flare plasma and the related nonthermal accelerated electrons. The unique orbit away from the Earth-Sun line in combination with the opportunity of joint observations with other Solar Orbiter instruments, STIX will provide new inputs into understanding the magnetic energy release and particle acceleration in solar flares. Commissioning observations showed that STIX is working as designed and therefore we report on the first solar microflare observations recorded on June 2020, when the spacecraft was at 0.52 AU from the Sun. STIX’s measurements are compared with Earth-orbiting observatories, such as GOES and SDO/AIA, for which we investigate and interpret the different temporal evolution. The detected early peak of the STIX profiles relative to GOES is due either by nonthermal X-ray emission of accelerated particles interacting with the dense chromosphere or the higher sensitivity of STIX toward hotter plasma.

How to cite: Battaglia, A. F., Saqri, J., Dickson, E., Xiao, H., Veronig, A., Warmuth, A., Battaglia, M., and Krucker, S. and the STIX Team: First results of the STIX hard X-ray telescope onboard Solar Orbiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4390, https://doi.org/10.5194/egusphere-egu21-4390, 2021.

The Spectrometer/Telescope for Imaging X-rays (STIX) is the instrument of the Solar Orbiter mission conceived for the observation of the hard X-ray flaring emission, with the objective of providing insights on the diagnosis of thermal and non-thermal accelerated electrons at the Sun. The STIX imaging system is composed of 30 pairs of tungsten grids, each one placed in front of a four-pixel detector, and produces as many Fourier components of the angular distribution of the flaring source, via Moiré pattern modulation. Therefore, the data recorded by STIX, named visibilities, can be interpreted as a sparse sampling of the Fourier transform of the X-ray signal and the corresponding image reconstruction problem requires the inversion of the Fourier transform from limited data, usually addressed with regularization techniques. Since the current calibration status of STIX measurements still prevents the use of visibility phases for imaging purposes, here we propose a parameter identification process based on forward fitting  just the amplitude of the experimental visibilities. Specifically, we have parameterized the flaring source by means of pre-assigned source shapes (e.g., circular and elliptical bi-variate Gaussian functions), and we relied on several approaches to non-linear optimization in order to estimating the shape parameters. In particular, we have implemented a forward-fit method based on deterministic chi-squared minimization, a stochastic optimization algorithm and a deep neural approach based on ensemble learning, also equipping them with an ad hoc statistical technique for uncertainty quantification. The performances of the three approaches are compared in the case of both microflares and M class events recorded by STIX during its commissioning phase and the validation of results is realized also exploiting the EUV information provided by the Atmospheric Imaging Assembly within the Solar Dynamics Observatory.

How to cite: piana, M., massa, P., perracchione, E., battaglia, A. F., benvenuto, F., massone, A. M., hurford, G., and krucker, S.: Early results for the STIX image reconstruction problem: imaging from visibility amplitudes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9422, https://doi.org/10.5194/egusphere-egu21-9422, 2021.

Solar flares are generally thought to be the impulsive release of magnetic energy giving rise to a wide range of solar phenomena that influence the heliosphere and in some cases even conditions of earth. Part of this liberated energy is used for particle acceleration and to heat up the solar plasma. The Spectrometer/Telescope for Imaging X-rays (STIX) instrument onboard the Solar Orbiter mission launched on February 10th 2020 promises advances in the study of solar flares of various sizes. It is capable of measuring X-ray spectra from 4 to 150 keV with 1 keV resolution binned into 32 energy bins before downlinking. With this energy range and sensitivity, STIX is capable of sampling thermal plasma with temperatures of≳10 MK, and to diagnose the nonthermal bremsstrahlung emission of flare-accelerated electrons. During the spacecraft commissioning phase in the first half of the year 2020, STIX observed 68 microflares. Of this set, 26 events could clearly be identified in at least two energy channels, all of which originated in an active region that was also visible from earth. These events provided a great opportunity to combine the STIX observations with the multi-band EUV imagery from the Atmospheric Imaging Assembly (AIA) instrument on board the earth orbiting Solar Dynamics Observatory (SDO). For the microflares that could be identified in two STIX science energy bands, it was possible to derive the temperature and emission measure (EM) of the flaring plasma assuming an isothermal source. For larger events where more detailed spectra could be derived, a more accurate analysis was performed by fitting the spectra assuming various thermal and nonthermal sources. These results are compared to the diagnostics derived from AIA images. To this aim, the Differential EmissionMeasure (DEM) was reconstructed from AIA observations to infer plasma temperatures and EM in the flaring regions. Combined with the the relative timing between the emission seen by STIX and AIA, this allows us to get deeper insight into the flare energy release and transport processes.

How to cite: Saqri, J., Veronig, A., Dickson, E., Krucker, S., Battaglia, A. F., Battaglia, M., Xiao, H., Warmuth, A., and STIX Team, T.: Plasma Diagnostics of Microflares observed by STIX and AIA, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7966, https://doi.org/10.5194/egusphere-egu21-7966, 2021.

Alfvénic fluctuations represent the dominant contributions to turbulent fluctuations in the solar wind, especially, but not limited to, the fastest streams with velocity of the order of 600-700 km/s. Alfvénic fluctuations can contribute to solar wind heating and acceleration via wave pressure and turbulent heating. Observations show that such fluctuations are characterized by a nearly constant magnetic field amplitude, a condition which remains largely to be understood and that may be an indication of how fluctuations evolve and relax in the expanding solar wind. Interestingly, measurements from Parker Solar Probe have shown the ubiquitous and persistent presence of the so-called switchbacks. These are magnetic field lines which are strongly perturbed to the point that they produce local inversions of the radial magnetic field. The corresponding signature of switchbacks in the velocity field is that of local enhancements in the radial speed (or jets) that display the typical velocity-magnetic field correlation that characterizes Alfvén waves propagating away from the Sun. While there is not yet a general consensus on what is the origin of switchbacks and their connection to coronal activity, a first necessary step to answer these important questions is to understand how they evolve and how long they can persist in the solar wind. Here we investigate the evolution of switchbacks. We address how their evolution is affected by parametric instabilities and the possible role of expansion, by comparing models with the observed radial evolution of the fluctuations’ amplitude. We finally discuss what are the implications of our results for models of switchback generation and related open questions.

How to cite: Tenerani, A., Velli, M., and Matteini, L.: Theory and observations of switchbacks’ evolution in the solar wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13400, https://doi.org/10.5194/egusphere-egu21-13400, 2021.

The observations from the Parker Solar Probe during the first
perihelion revealed large numbers of local reversals in the radial
component of the magnetic field with associated velocity spikes. Since
the spacecraft was magnetically connected to a coronal hole during the
closest approach to the sun, one possible source of these spikes is
magnetic reconnection between the open field lines in the coronal hole
and an adjacent region of closed flux. Reconnection in a low beta
environment characteristic of the corona is expected to be bursty
rather than steady and is therefore capable of producing large numbers
of magnetic flux ropes with local reversals of the radial magnetic
field that can propagate outward large radial distances from the
sun. Flux ropes with a strong guide field produce signatures
consistent with the PSP observations. We have carried out simulations
of "interchange" reconnection in the corona and have explored the
local structure of flux ropes embedded within the expanding solar
wind. We have first established that traditional interchange
reconnection cannot produce the switchbacks since bent field lines
generated in the corona quickly straighten. The simulations have been
extended to the regime dominated by the production of multiple flux
ropes and we have established that flux ropes are injected into the
local solar wind. Local simulations of reconnection are also being
carried out to explore the structure of flux ropes embedded in the
solar wind for comparison with observations. Evidence is presented
that flux rope merging may be ongoing and might lead to the high
aspect ratio of the switchback structures measured in the solar wind.

How to cite: Drake, J., Agapitov, O., Swisdak, M., Badman, S., Bale, S., Horbury, T., Kasper, J., MacDowal, R., Mozer, F., Phan, T., Pulupa, M., Szabo, A., and Velli, M.: Magnetic Reconnection in the Corona as a Source of Switchbacks in the Solar Wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2865, https://doi.org/10.5194/egusphere-egu21-2865, 2021.

Magnetic switchbacks are Alfvénic structures characterized as intervals of intermittent reversals in the radial componentof magnetic field. Switchbacks comprise of magnetic spikes preceded/succeeded by quiet, pristine solar wind. Determining switch-back generation and evolution mechanisms will further our understanding of the global circulation and transportation of Sun’s openmagnetic flux. Here, we investigate switchback transition regions using measurements from fields and plasma suites aboard the Parker SolarProbe (PSP). Minimum variance analysis (MVA) is applied on switchback transition region magnetic signatures. Discontinuity analysesare performed on 273 switchback transition regions with robust MVA solutions. Our results indicate that switchbacks are rotational discontinuities (RD) in 214 (or 78%) of the cases. 21% of the switchbacktransition regions are categorized as "either" discontinuity (ED), defined as small relative changes in both magnitude and the normalcomponent of magnetic field. RD-to-ED event count is found to reduce with increasing distance from the Sun. On average, plasmabeta falls by −28% across RD-type switchback transition regions and magnetic shear angle is 60 [deg], therefore making switchbacktransition regions theoretically favorable to local magnetic reconnection. The evolution of switchbacks away from the Sun may involve increasing mass flux across RD-type switchback transition regions. The evolution mechanism(s) remain to be discovered. Our results indicate negligible magnetic curvature across switchback transition regions which may inhibit local magnetic reconnection.

How to cite: Akhavan-Tafti, M., Kasper, J., Huang, J., and Bale, S.: Discontinuity analysis and evolution of magnetic switchbacks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7879, https://doi.org/10.5194/egusphere-egu21-7879, 2021.

Parker Solar Probe (PSP) routinely observes magnetic field deflections in the solar wind at distances less than 0.3 au from the Sun. These deflections are related to structures commonly called 'switchbacks' (SBs), whose origins and characteristic properties are currently debated. Here, we use a database of visually selected SB intervals - and regions of solar wind plasma measured just before and after each SB - to examine plasma parameters, turbulent spectra from inertial to dissipation scales, and intermittency effects in these intervals. We find that many features, such as perpendicular stochastic heating rates and turbulence spectral slopes are fairly similar inside and outside of SBs. However, important kinetic properties, such as the characteristic break scale between the inertial to dissipation ranges differ inside and outside these intervals, as does the level of intermittency, which is notably enhanced inside SBs and in their close proximity, most likely due to magnetic field and velocity shears observed at the edges. We conclude that the plasma inside and outside of a SB, in most of the observed cases, belongs to the same stream, and that the evolution of these structures is most likely regulated by kinetic processes, which dominate small scale structures at the SB edges. 

How to cite: Martinović, M., Klein, K., Huang, J., Chandran, B., Kasper, J., Lichko, E., Bowen, T., Chen, C., Matteini, L., Stevens, M., Case, A., and Bale, S.: Multiscale Solar Wind Turbulence Properties inside and near Switchbacks measured by Parker Solar Probe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6686, https://doi.org/10.5194/egusphere-egu21-6686, 2021.

Switchback boundaries separate two plasmas moving with different velocities, that may have different temperatures and densities and typically manifest sharp magnetic field deflections through the boundary. They may be analyzed similarly to MHD discontinuities. The first step of their characterization consists of analysis in terms of MHD discontinuities. Such an analysis was performed by Larosa et al., (2021) who has found that 32% of them may be attributed to rotational discontinuities, 17% to tangential, about 42% to the group of discontinuities that are difficult to unambiguously define whether they are tangential or rotational, and 9% that do not belong to any of these two groups. We describe and apply hereafter for two events another approach for the characterization of the boundaries based on classification of the general type discontinuity in MHD approximation. It is based on the problem of the decay of the general type of discontinuity. It is well known [Kulikovsky and Lyubimov, 1962, Gogosov, 1959} that general type MHD discontinuity decays on 7 separate discontinuities belonging to different types of MHD waves, namely, entropic wave, two slow mode waves, two Alfvenic waves, and two fast mode waves. Entropic wave is standing in the reference frame of the discontinuity; other wave modes are supposed to run in the opposite directions from the initial discontinuity with their characteristic velocities. Making use of plasma parameters from two sides of the boundary one can evaluate the fraction of each wave mode present in the discontinuity. We apply this method to two boundary crossings. This repartition of the discontinuity allows characterizing the deviation from Alfvenicity quantitatively.

References

Larosa, A., et al., A&A, 2021, (accepted)

Kulikovsky, Lyubimov, Magnetohydrodynamics, (1962)

Gogosov, V.V., Decay of the MHD discontinuity, (1959)

How to cite: Krasnoselskikh, V., Larosa, A., Dudok de Wit, T., Agapitov, O., Froment, C., Kretzschmar, M., Jagarlamudi, V., Velli, M., Bale, S. D., Goetz, K., Harvey, P., Kasper, J., Korreck, K., Larson, D., MacDowall, R., Malaspina, D., Mozer, F., Pulupa, M., Reveillet, C., and Stevens, M.: Decomposition of the switchback boundary on MHD wave modes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15180, https://doi.org/10.5194/egusphere-egu21-15180, 2021.

The solar wind is frequently perturbed by transient structures such as magnetic folds, jets, waves and flux-ropes that propagate rapidly away from the Sun over a large range of heliocentric distances. Parker Solar Probe has revealed that rotations of the magnetic field vector occur repeatedly at small heliocentric distances, on regions that also display surprisingly large solar wind rotation rates. Sun-to-spacecraft connectivity analysis shows that a large fraction of the solar wind flows probed so far by Parker Solar Probe were formed and accelerated in the vicinity of coronal hole boundaries.
We show by means of of global MHD simulations that coronal rotation is highly structured in proximity to those boundary regions (in agreement with preceding SoHO/UVCS observations), and that enhanced poloidal and toroidal flow shear and magnetic field gradients also develop there. We identified regions of the solar corona for which solar wind speed and rotational shear are significant, that can be associated with field-aligned and/or transverse vorticity, and that can be favourable to the development of magnetic deflections. Some of these wind flow shears are driven through large radial extensions, being noticeable tens of solar radii away from the surface, and therefore have a potential impact on the propagation of such magnetic perturbations across extended heights in the solar wind. We conclude that these regions of persistent shears are undoubtedly sources of complex solar wind structures, and suggest that they can trigger instabilities capable of creating magnetic field reversals detected in-situ in the heliosphere.
Our simulations furthermore indicate that the spatial structure of the solar wind shear will become more complex as the solar cycle progresses, with strong and extended shears appearing at heliographic latitudes that will be probed by Solar Orbiter in the near future.

How to cite: Pinto, R., Poirier, N., Kouloumvakos, A., Rouillard, A., Griton, L., Fargette, N., Kieokaew, R., Lavraud, B., and Brun, A. S.: Solar wind speed and rotational shear at coronal hole boundaries, impacts on magnetic field inversions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13552, https://doi.org/10.5194/egusphere-egu21-13552, 2021.

Recent studies of the solar wind sunward of 0.25 AU using the Parker Solar Probe spacecraft reveal that that solar wind can be bimodal, alternating between near quiescent regions with low turbulent fluctuation amplitudes and Parker-like magnetic field direction and regions of highly turbulent plasma and magnetic field fluctuations associated with ‘switchbacks’ of the radial magnetic field.  

The quiescent solar wind regions are highly unstable to the formation of plasma waves near the electron cyclotron frequency (fce), possibly driven by strahl electrons, which carry the solar wind heat flux, and may provide one of the most direct particle diagnostics of the solar corona at the source of the solar wind.  These waves are most intense near ~0.7 fce and ~fce. The near-fce waves are found to become more intense and more frequent closer to the Sun, and statistical evidence indicates that their occurrence rate is related to the sunward drift of the core electron population.  

In this study, we examine high time resolution burst captures of these waves, demonstrating that each wave burst contains several distinct wave types, including electron Bernstein waves and extremely narrow band waves that are highly sensitive to the magnetic field orientation. Using properties of these waves we provide evidence to support the identification of their likely plasma wave modes and the instabilities responsible for generating these waves.  By understanding the driving instabilities responsible for these waves, we infer their ability to modify electron distribution functions in the quiescent near-Sun solar wind.  

How to cite: Malaspina, D., Wilson, L., Ergun, R., Bale, S., Bonnell, J., Dudok de Wit, T., Goetz, K., Goodrich, K., Harvey, P., MacDowall, R., Pulupa, M., Halekas, J., Case, A., Larson, D., Stevens, M., and Whittlesey, P.: Plasma Waves Near the Electron Cyclotron Frequency in the Near Sun Solar Wind: Wave Mode Identification and Driving Instabilities, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-955, https://doi.org/10.5194/egusphere-egu21-955, 2021.

How to cite: Chhiber, R., Usmanov, A., Matthaeus, W., Goldstein, M., and Bandyopadhyay, R.: Large-scale Structure and Turbulence Transport in the Young Solar Wind – Comparison of Parker Solar Probe Observations with a Global 3D Reynolds-averaged MHD Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12726, https://doi.org/10.5194/egusphere-egu21-12726, 2021.

The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 Rs, allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP's fourth solar encounter, likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with spectral index close to -5/3 rather than -3/2), a lower Alfvenicity, and a "1/f" break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ~4 degrees from the HCS, suggesting ~8 degrees as the full-width of the streamer belt wind at these distances. While the majority of the Alfvenic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind.

How to cite: Chen, C., Chandran, B., Woodham, L., Jones, S., Perez, J., Bourouaine, S., Bowen, T., Klein, K., Moncuquet, M., Kasper, J., and Bale, S.: The Near-Sun Streamer Belt Solar Wind: Turbulence and Solar Wind Acceleration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3426, https://doi.org/10.5194/egusphere-egu21-3426, 2021.