Robert F. Wimmer-Schweingruber

Aims. We investigated two successive solar energetic electron (SEE) events observed by Solar Orbiter, associated with the same active region and separated by ∼25 minutes, on December 24, 2022. Methods. We performed the forward fitting to in situ temporal profiles to derive the solar injection profiles of SEEs, and utilized the extended pan-spectrum (EPS) fitting method to self-consistently determine the shape of SEE energy spectra. Then we examined the temporal relationship between SEE solar injections and candidate solar flares and/or coronal jets. Results. After considering the effects of interplanetary propagation and instrumental response, we found that the SEE #1 (#2) event can be formed by three (two) solar injections that dominate, respectively, at ≲22 keV, 22–290 keV, and 290–746 keV (at ≲26 keV and 26–185 keV). In both events, the low-energy SEE injection starts earlier, peaks earlier and lasts longer as the electron energy decreases, producing a very hard power-law energy spectrum. The high-energy injection is roughly independent of energy in its timing, with a short duration of ∼3–6 minutes and a soft energy spectrum, which coincides with a B-C class flare, HXR burst, and coronal jet. The #1 event shows another, middle-energy injection with the start-time (duration) getting earlier (longer) but the peak-time growing earlier and then later as the electron energy decreases, and with a hard energy spectrum. Conclusions. In both SEE events, the low-energy injection likely comes from a progressive acceleration process occurring high in the corona, while the high-energy injection can arise from an instantaneous acceleration of the lower-energy electrons, accompanied by an HXR burst and/or coronal jet. In the #1 event, the middle-energy injection can result from an additional process, leading to a much more efficient acceleration at ≳25 keV. Thus, successive SEE events may form through very dynamic and complex processes.

During the first Earth gravity assist maneuver of JUICE on 20 August 2024 the spacecraft passed through the Earth plasmasphere for about 2 h. Before closest approach at a distance of 2.1 R _E the Jovian Electron and Ion spectrometer JEI of the PEP instrument suite was switched on for 40 min in an ion mode test configuration. The high plasma density of about 3000/cm ³ (observed by the RPWI plasma wave instrument) led to a negative charging of the spacecraft which allowed a rare observation of the cold and dense plasmaspheric ion populations. Since the ions are only corotating with the Earth at a velocity of about 1 km s ⁻¹ at this distance the observed ion speed is dominated by the spacecraft velocity of about 8 km s ⁻¹ . For this reason ions with different mass appear at different energies in the energy spectrum observed by the JEI sensor. In addition the spacecraft potential leads to specific filtering of ion masses in the observed angular distribution. By calculating the sensor response function for these specific observing conditions it is possible to quantify densities of the different ion species. This response calculation depends critically on the response of the JEI channel electron multipliers to the ion velocity and mass. Since the sensor was operated with a low post-acceleration further laboratory calibrations may be needed for this specific setup. Nevertheless we can already conclude from the observations that ions with mass > 4 amu q ⁻¹ contribute at least 25 % to the observed total ion density. A flux peak observed in the energy spectrum at 15 eV can only be explained by the presence of heavy molecular ions with mass ∼ 30 amu q ⁻¹ . Molecular ions have only been rarely detected in the outer Earth plasmasphere. The observations indicate that the JEI sensor can also be used to achieve ion composition measurements in the exospheres of the Jovian moons.

Context. The BepiColombo Environmental Radiation Monitor (BERM) on board the BepiColombo mission on its way to Mercury is designed to measure the radiation environment of the spacecraft during the mission. The Energetic Particle Detector (EPD), part of the scientific payload of the Solar Orbiter mission, provides measurements of a broad spectrum of energetic particles in the inner heliosphere. Some solar energetic particle (SEP) events have been observed by both missions, and this offers valuable opportunities for comparative and intercalibration studies. Aims. We intercalibrate BERM and EPD in order to enhance our understanding of the BERM detection capabilities of SEP events. This intercalibration effort also serves to reinforce the validity and scientific reliability of the results produced with BERM data in future analyses. Methods. To support the intercalibration, we analysed the interplanetary context of each SEP event by combining solar wind plasma and magnetic field observations. We examined the proton anisotropies measured by Solar Orbiter and selected an isotropic interval during the decay phase of each SEP event to compute the proton energy spectrum. The resulting spectrum from Solar Orbiter was fitted and subsequently compared to the measurements from BERM, which allowed us to derive intercalibration factors between the two instruments. Results. The proton spectral fluxes measured by BERM and EPD were found to be similar, resulting in calibration factors of 0.90 ± 0.08 and 1.05 ± 0.12 for proton channels of 2.15 MeV and 6.85 MeV, respectively. Conclusions. The BERM instrument on board BepiColombo is a valuable tool for measuring SEP events in the heliosphere, providing an excellent opportunity to study and characterise the energetic particle environment in the solar wind between 0.3 and 1.0 au.

Aims. Solar Orbiter observations provide an unprecedented opportunity to study solar energetic particle (SEP) events at heliocentric distances ≲1 au. In this work, we compile a catalogue of SEP events, utilising Solar Orbiter measurements. Methods. The catalogue is based on measurements from the High Energy Telescope (HET) of the Energetic Particle Detector (EPD) instrument suite on Solar Orbiter during the ascending phase of solar cycle 25 (2020–2025). We performed a parallel survey of near-relativistic electrons (∼1 MeV) and protons (∼8 MeV), identifying particle enhancements above background levels and registering SEP events. For each SEP event and particle species, we calculated the SEP characteristics (e.g. onset and peak times, peak intensity values, the fluence, and e/p intensity ratios). Our analysis techniques included a velocity dispersion analysis (VDA) and time-shifting analysis (TSA) to infer the SEP release times at the Sun. We compared the SEP release times with solar flare observations from the Spectrometer Telescope for Imaging X-rays (STIX) and X-Ray Sensor (XRS) on Geostationary Operational Environmental Satellites (GOES) and identified the parent solar sources. We also identified the associated coronal mass ejections (CMEs). The SEP events were cross-checked with magnetic field and solar wind data from the Solar Wind Analyser (SWA) and Magnetometer (MAG) and the relevant plasma quantities were tabulated. In addition, the connection angle (CA) for each event was calculated and reported. Results. Our catalogue includes 177 SEP events. The present study focuses on both electron and proton events, with 50 of these events exhibiting high-energy protons (∼50 MeV). A strong correlation is found between peak intensity and fluence for both electrons ( cc = 0.86 ± 0.03) and protons ( cc = 0.95 ± 0.02). Using the CA, events were classified as well-connected (±20°) and poorly connected. The SEP peak fluxes and fluences show weak correlations with flare soft X-ray peak flux ( cc ∼0.17-0.21), while stronger correlations are observed with a CME projected speed for well-connected events ( cc ∼ 0.61 for electrons and ∼0.47 for protons). Across all events with a measurable electron-to-proton (e/p) ratio, the ratio spans more than three orders of magnitude and forms a continuous distribution, with no clear distinction between impulsive and gradual populations. Combined with compositional information, higher e/p ratios are generally associated with ³ He-rich and Fe-rich events and lower values with shock-driven events, although a significant overlap exists. Finally, scaling relations reveal an energy dependence: low-energy protons (∼8-25 MeV) are influenced by both magnetic connectivity and source composition, whereas high-energy protons (∼50 MeV) are primarily governed by source composition ( ³ He and Fe signatures).

Context . solar energetic electron (SEE) events are a commonly observed phenomenon. However, SEE events accelerating electrons up to ultra-relativistic energies, for example, several megaelectronvolts, are rarely observed. This is due to the necessity of unique acceleration conditions. Because of this, few observations are found in the literature, and theories that explain the acceleration and transport of ultra-relativistic solar energetic electrons (URSEEs) are sparse. In February 2024, Solar Orbiter observed a series of three individual URSEE events shortly after each other. Aims . The purpose of this study is to analyse this unique triplet of URSEE events that occurred within 3 days of each other while the Sun showed high activity. Given the rarity of such URSEE events, a triplet of them occurring in a time period of 3 days raises the question of whether, and if so how, the acceleration or processes are connected. Naturally, one would assume that electrons from all three events originate from the same solar source; however, here we show evidence of at least two distinct sources. This makes this series of URSEE events even more unique and motivates this study Methods . Particle measurements obtained by the Solar Orbiter (SolO)/High-Energy Telescope (HET) and SolO/Electron Proton Telescope (EPT) are used. Here, we focus on electron energies from a few hundred keV to above ten MeV. A velocity dispersion analysis of these data sets is used in combination with radio and X-ray data to draw conclusions about the acceleration sites of the three events. Additionally, solar wind and magnetic field data are used to characterise the events. Electron measurements from Solar and Heliospheric Observatory (SOHO)/Electron Proton Helium Instrument (EPHIN), Solar TErrestrial RElations Observatory (STEREO) (High Energy Telescope (STEREO/HET), and Solar Electron Proton Telescope (STEREO/SEPT)) are utilised for a multi-spacecraft comparison. Results . The analysis of this unique triplet in 2024 results in at least two distinct sources for the events. A complex plasma environment disturbed by coronal mass ejections seems to change the connection of SolO to the Sun, and therefore makes it possible to observe URSEEs accelerated from the west limb for the first event on 9 February, and from the centre of the disc of the Sun for the second event on 10 February. This is also confirmed by the observations of SOHO/EPHIN and STEREO/HET. Since the source of the last event is located somewhere behind the west limb of the Sun, it cannot be seen from SolO or the other spacecrafts. Whether this source is connected to the source of the first event is likely, but unknown.

Context . Solar eruptive events such as flares and coronal mass ejection -driven shocks can release solar energetic particles (SEPs) into the heliosphere. The heliospheric current sheet (HCS) is a large-scale structure that separates regions of opposite magnetic polarity. Its influence on SEP propagation remains poorly understood. Aims . In this study, we attempt to provide statistical insight into the role of the HCS in SEP propagation through a statistical analysis of the solar energetic electron (SEE) events observed by Solar Orbiter. Methods . We analysed SEE events in the Comprehensive Solar Energetic Electron event Catalogue (CoSEE-Cat) and classified them into two groups: same-side events, where both the solar source and spacecraft are in the same magnetic sector, and opposite-side events. The magnetic polarities of the solar source region and the spacecraft location are determined comprehensively using approaches based on the potential field source surface (PFSS), magnetic field measurements, the pitch angle distribution of strahl electrons, and the first-order anisotropy of energetic electrons. The uncertainties in determining magnetic polarities associated with these methods are evaluated. Results . The spacecraft magnetic polarities determined by footpoint positions at the source surface and in situ observations are consistent for most events, providing a useful methodological reference for future studies. We identified 60 same-side events and 9 opposite-side events. By comparing characteristics of events in these two groups, we were able to investigate the potential influence of the HCS on SEP propagation. Conclusions . Our results show that opposite-side events tend to be more isotropic and that both the solar source and the spacecraft are closer to the HCS than in same-side events. This suggests that particle transport across the HCS is inefficient unless the source or the observer is close to the HCS. These preliminary statistical findings advance our understanding of the role of the HCS in shaping SEP transport.

The Juice flyby of Earth in August 2024 gave us the first chance to evaluate the performance of the Jovian Plasma Dynamics and Composition analyzer (JDC) in environments similar to those expected at Jupiter. JDC is one of the sensors belonging to the Particle Environment Package (PEP) on the Juice spacecraft. It measures positive and negative ions as well as electrons in the energy range 1 eV q ⁻¹ to 35 keV q ⁻¹ . One of the most challenging observations at the final destination is the one of the low-energy ion populations in the tenuous ionospheres of Jupiter's icy moons. During the Juice flyby of Earth we discovered that the energies of the positive ions observed by JDC were not easy to interpret due to a problem with the energy sweep. Using measurements made on ground, we were able to reconstruct the observed energies and construct a new sweeping scheme that solves the problem and that will greatly improve future observations. We also used a simulation to explain the effects of the spacecraft velocity and spacecraft potential on the recorded positive ion fluxes when Juice passed through the Earth's plasmasphere. The study highlights the importance of in-flight calibrations for optimizing the scientific return. Planetary flybys give access to multiple low-energy particle populations besides the mono-energetic and highly directional solar wind.

The propagation of solar energetic particles (SEPs) through the heliosphere is primarily guided by the interplanetary magnetic field (IMF), which is embedded in the solar wind plasma. Large-scale IMF structures can drive transient variations in SEP intensities. Using Solar Orbiter observations, we identify a distinct class of SEP variations: SEP flux deflections (SFDs), which are commonly detected in SEP events and frequently recur multiple times within a single event. SFDs are characterized by a sudden change in SEP flux directions where the intensities drop in one direction and increase in another direction, without a significant net change in total flux magnitude. These deflections occur dispersionlessly across a broad energy range—from tens of keV to over 100 MeV—and exhibit steep intensity gradients. SFDs appear to be associated with magnetic flux tubes whose boundaries exhibit features consistent with tangential discontinuities. We further show that the solar wind inside these structures exhibits distinct plasma properties, and that the SEP streaming direction within SFDs aligns closely to the flux-tube axis. These observations are consistent with magnetic flux tubes being a prevalent structure of the solar wind, and suggest that SEPs can serve as an effective diagnostic tool for probing the topology and dynamics of solar wind structures.

Context. In collisionless shocks, energetic particles can carry enough pressure to modify the upstream plasma and the shock structure itself. This represents a regime that is often invoked in theories of cosmic-ray acceleration, but rarely observed in the heliosphere. Aims. We locate and characterise interplanetary (IP) shocks where energetic particles dynamically dominate the upstream pressure. Methods. We analysed IP shocks observed by Solar Orbiter within 1 au and computed the energetic particle pressure, P _EP , from proton measurements above 10 keV. We compared these results it with the upstream thermal P _Th and magnetic P _B pressures. Results. We identified four shocks for which P _EP ≥ P _Th + P _B . These events correspond to strong and fast shocks in the high-Mach-number tail of the Solar Orbiter shock population. In several cases, the P _EP increase coincides with a decreasing upstream bulk flow speed in the shock frame, with the resulting particle-mediated foreshocks extending up to ∼10 ⁵ ion inertial lengths, d _i . The extent of such an energetic particle-dominated region depends on the shock geometry. Conclusions. These observations provide evidence that accelerated particles can dynamically modify IP shocks. They highlight the importance of coupling among energetic particles, upstream fluctuations, and shock structure in improving our understanding of particle acceleration at collisionless shocks.

Context. The Solar Orbiter spacecraft cruised in the inner heliosphere from April 2020 to December 2025, moving between ∼0.3–1.0 au radial distance. The Energetic Particle Detector (EPD) suite operated continuously during this period. Aims. The Suprathermal Ion Spectrograph (SIS) and High Energy Telescope (HET) were used to survey the energetic particle population H-Fe from solar energetic particles (SEPs), corotating interaction regions (CIRs), ³ He rich SEPs and other sources. Methods. The particle populations were organized by source type, energy, radial location, and time in order to construct a statistical description of the ion population and characteristics of its sources and transport processes. In addition, “quiet” periods outside transient activity were included to cover the remaining time intervals. Results. The mix of particle population sources below ∼1 MeV/nucleon showed little variation with radial distance from the Sun, and changed from primarily quiet and CIR sources at solar minimum to SEP (gradual and ³ He rich) at maximum. The fluences of O summed over the entire period showed surprisingly little variation with radius once the observing time and 1/r ² correction was made, indicating that energy changing processes (either acceleration or deceleration) play a small role outside ∼0.3 au. The relative abundances of elements H-Fe did not change with radial distance, suggesting that the particle population is energized primarily within 0.4 au distance, and the spacecraft is observing a population where only transport is playing a role. The composition of the quiet periods, measured with the Fe/O ratio, was observed to increase from CIR-like to SEP-like as solar activity moved from minimum to maximum. This behavior was seen in all radial distance bins and has been previously reported from surveys at 1 au. Spectra collected during the quietest periods of the survey, April 2020–April 2021, revealed low energy steep spectra with H and ⁴ He slopes and abundances similar to CIRs but at much lower intensity levels, and heavier ions consistent with SEPs, but with low statistical accuracy. No evidence was found for other source populations.