Context. On 2022 January 20, the Energetic Particle Detector (EPD) on board Solar Orbiter measured a solar energetic particle (SEP) event showing unusual first arriving particles from the anti-Sun direction. Near-Earth spacecraft separated by 17 degrees in longitude to the west of Solar Orbiter measured classic anti-sunward-directed fluxes. STEREO-A and MAVEN, separated by 18 degrees to the east and by 143 degrees to the west of Solar Orbiter, respectively, also observed the event, suggesting that particles spread over at least 160 degrees in the heliosphere. Aims. The aim of the present study is to investigate how SEPs are accelerated and transported towards Solar Orbiter and near-Earth spacecraft, as well as to examine the influence of a magnetic cloud (MC) present in the heliosphere at the time of the event onset on the propagation of energetic particles. Methods. We analysed remote-sensing data, including flare, coronal mass ejection (CME), and radio emission to identify the parent solar source of the event. We investigated energetic particles, solar wind plasma, and magnetic field data from multiple spacecraft. Results. Solar Orbiter was embedded in a MC erupting on 16 January from the same active region as that related to the SEP event on 20 January. The SEP event is related to a M5.5 flare and a fast CME-driven shock of similar to 1433 km s(-1), which accelerated and injected particles within and outside the MC. Taken together, the hard SEP spectra, the presence of a Type II radio burst, and the co-temporal Type III radio burst being observed from 80 MHz that appears to emanate from the Type II burst, suggest that the shock is likely the main accelerator of the particles. Conclusions. Our detailed analysis of the SEP event strongly suggests that the energetic particles are mainly accelerated by a CME-driven shock and are injected into and outside of a previous MC present in the heliosphere at the time of the particle onset. The sunward-propagating SEPs measured by Solar Orbiter are produced by the injection of particles along the longer (western) leg of the MC still connected to the Sun at the time of the release of the particles. The determined electron propagation path length inside the MC is around 30% longer than the estimated length of the loop leg of the MC itself (based on the graduated cylindrical shell model), which is consistent with the low number of field line rotations.

Solar Orbiter (SO) observations provide an unprecedented opportunity to study the evolution of solar energetic particle (SEP) events from different locations within the heliosphere. In this work, we have compiled a catalogue of SEP events based on observations of both electrons and protons from the High Energy Telescope (HET) of the Energetic Particle Detector (EPD) that occurred in 2020 to 2023 during the ascending phase of Solar Cycle 25. A scan of simultaneous So/HET intensity-time observations for ~10 MeV protons and near relativistic (~1 MeV) electrons has been performed. We have identified all enhancements observed above the background levels of these particular channels and surveyed available solar wind data by the SO/ Solar Wind Analyzer (SWA) and the SO/Magnetometer (MAG) during the identified events. Moreover, we employed Velocity Dispersion Analysis (VDA) for protons and electrons and Time-shifting Analysis (TSA) for electrons, alone, with the aim to infer the SEP release times at the Sun. Our resulting catalogue includes 75 SEP events. For each of these events (and for each species), we provide the onset and peak time, the peak flux value and fluence. We also identify the solar associations/sources for the SEP events, by comparing the inferred release times of the SEPs to the related light curves from the SO/Spectrometer/Telescope for Imaging X-rays (STIX), the standard flare list obtained from the GOES X-ray Sensor and their associated coronal mass ejections (CMEs). We find that a significant portion of all SEP events in our sample (48%; 36/75) reached 50 MeV for protons and thus are Space Weather relevant. Finally, a statistical analysis of our observations is presented. We have investigated correlations between peak particle fluxes (for protons and electrons) and event fluences, as well as peak particle fluxes (event fluences), flare magnitude and CME speed. We also calculate the connection angle to the apparent source and identify a subsample of the events that are better connected to the solar event. In addition, the e/p ratio is calculated and a division of the sample based on Fe-rich and 3He-rich events is discussed. Acknowledgement: Research leading to these results has received funding from the Horizon Europe programme project No 101135044 (SPEARHEAD).

A series of solar energetic electron (SEE) events was observed from 2022 November 9 to November 15 by Solar Orbiter, STEREO-A, and near-Earth spacecraft. At least 32 SEE intensity enhancements at energies >10 keV were clearly distinguishable in Solar Orbiter particle data, with 13 of them occurring on November 11. Several of these events were accompanied by less than or similar to 10 MeV proton and less than or similar to 2 MeV nucleon(-1) heavy-ion intensity enhancements. By combining remote-sensing and in situ data from the three viewpoints (Solar Orbiter and STEREO-A were similar to 20 degrees and similar to 15 degrees east of Earth, respectively), we determine that the origin of this rapid succession of events was a series of brightenings and jetlike eruptions detected in extreme ultraviolet (EUV) observations from the vicinity of two active regions. We find a close association between these EUV phenomena, the occurrence of hard X-ray flares, type III radio bursts, and the release of SEEs. For the most intense events, usually associated with extended EUV jets, the distance between the site of these solar eruptions and the estimated magnetic connectivity regions of each spacecraft with the Sun did not prevent the arrival of electrons at the three locations. The capability of jets to drive coronal fronts does not necessarily imply the observation of an SEE event. Two peculiar SEE events on November 9 and 14, observed only at electron energies less than or similar to 50 keV but rich in less than or similar to 1 MeV nucleon(-1) heavy ions, originated from slow-rising confined EUV emissions, for which the process resulting in energetic particle release to interplanetary space is unclear.

Solar Orbiter (SO) observations provide an unprecedented opportunity to study the evolution of solar energetic particle (SEP) events from different locations within the heliosphere. In this work, we have compiled a catalogue of SEP events based on observations of both electrons and protons from the High Energy Telescope (HET) of the Energetic Particle Detector (EPD) that occurred in 2020 to 2023 during the ascending phase of Solar Cycle 25. A scan of simultaneous So/HET intensity-time observations for ~10 MeV protons and near relativistic (~1 MeV) electrons has been performed. We have identified all enhancements observed above the background levels of these particular channels and surveyed available solar wind data by the SO/ Solar Wind Analyzer (SWA) and the SO/Magnetometer (MAG) during the identified events. Moreover, we employed Velocity Dispersion Analysis (VDA) for protons and electrons and Time-shifting Analysis (TSA) for electrons, alone, with the aim to infer the SEP release times at the Sun. Our resulting catalogue includes 75 SEP events. For each of these events (and for each species), we provide the onset and peak time, the peak flux value and fluence. We also identify the solar associations/sources for the SEP events, by comparing the inferred release times of the SEPs to the related light curves from the SO/Spectrometer/Telescope for Imaging X-rays (STIX), the standard flare list obtained from the GOES X-ray Sensor and their associated coronal mass ejections (CMEs). We find that a significant portion of all SEP events in our sample (48%; 36/75) reached 50 MeV for protons and thus are Space Weather relevant. Finally, a statistical analysis of our observations is presented. We have investigated correlations between peak particle fluxes (for protons and electrons) and event fluences, as well as peak particle fluxes (event fluences), flare magnitude and CME speed. We also calculate the connection angle to the apparent source and identify a subsample of the events that are better connected to the solar event. In addition, the e/p ratio is calculated and a division of the sample based on Fe-rich and 3He-rich events is discussed.  Acknowledgement: Research leading to these results has received funding from the Horizon Europe programme project No 101135044 (SPEARHEAD).

Aims. We perform a statistical study of the relations between the properties of solar energetic electron (SEE) events measured by the MESSENGER mission from 2010 to 2015 and the parameters of the respective parent solar activity phenomena to identify the potential correlations between them. During the time of analysis MESSENGER heliocentric distance varied between 0.31 and 0.47 au. Results. There is an asymmetry to the east in the range of connection angles (CAs) for which the SEE events present the highest peak intensities, where the CA is the longitudinal separation between the footpoint of the magnetic field connecting to the spacecraft and the flare location. Based on this asymmetry, we define the subsample of well-connected events as when -65$^{\circ}\leq$ CA $\leq+33^{\circ}$. Conclusions. Based on the comparison of the correlation coefficients presented in this study using near 0.4 au data, (1) both flare and shock-related processes may contribute to the acceleration of near relativistic electrons in large SEE events, in agreement with previous studies based on near 1 au data; and (2) the maximum speed of the CME-driven shock is a better parameter to investigate particle acceleration related mechanisms than the average CME speed, as suggested by the stronger correlation with the SEE peak intensities.

Context. We present a list of 61 solar energetic electron (SEE) events measured by the MESSENGER mission and the radial dependences of some parameters associated with these SEE events. The analysis covers the period from 2010 to 2015, when the heliocentric distance of MESSENGER varied between 0.31 and 0.47 au. We also show the radial dependences for a shorter list of 12 SEE events measured in February and March 2022 by spacecraft near 1 au and by Solar Orbiter at about its first close perihelion at 0.32 au. Aims. We study the radial dependences of the electron peak intensity and the energy spectrum of the electron intensity at the time of the SEE event peak intensity, taking advantage of multi-spacecraft measurements. Methods. We compiled the list of SEE events measured by MESSENGER and Solar Orbiter using hourly averages to find the prompt component of the near-relativistic (∼70–110 keV) electron peak intensities and to calculate the peak-intensity energy spectra. We also obtained the peak intensities and energy spectra for the same events as measured by the STEREO-A, -B, ACE, or Wind spacecraft when one of these spacecraft was in close nominal magnetic connection with MESSENGER or Solar Orbiter to derive the radial dependences of these SEE parameters. Results. (1) Because the background intensity level of the particle instrument on board MESSENGER is high, the SEE events measured by this mission are necessarily large and intense; most of them are accompanied by a shock driven by a coronal mass ejection and are widely spread in heliolongitude. The SEE events display relativistic (∼1 MeV) electron intensity enhancements. For this SEE sample, we found that (2) the SEE peak intensity shows a radial dependence that can be expressed as R α , where the median value of the α index is α Med = −3.3±1.4 for a subsample of 28 events for which the nominal magnetic footpoints of the near 0.3 au and 1 au spacecraft were close in heliographic longitude. (3) The mean spectral index δ of a subset of 42 events for which the energy spectrum could be analysed is ⟨ δ ⟩= − 1.9 ± 0.3, which is harder than the value found in previous studies using data from spacecraft near 1 au. SEE events observed by Solar Orbiter also display harder energy spectra than previous studies using data obtained near 1 au. Conclusions. There is a wide variability in the radial dependence of the electron peak intensities, but on average and within uncertainties, the ∝ R −3 dependence found in previous observational and modelling studies is confirmed. The electron spectral index found in the energy range around ∼200 keV ( δ 200) of the backward-scattered population near 0.3 au measured by MESSENGER is harder by a median factor of ∼20% and ∼10% when comparing to the near 1 au anti-sunward propagating beam and the backward-scattered population, respectively.

Context. We present a list of 61 solar energetic electron (SEE) events measured by the MESSENGER mission and the radial dependences of some parameters associated with these SEE events. The analysis covers the period from 2010 to 2015, when the heliocentric distance of MESSENGER varied between 0.31 and 0.47 au. We also show the radial dependences for a shorter list of 12 SEE events measured in February and March 2022 by spacecraft near 1 au and by Solar Orbiter at about its first close perihelion at 0.32 au. Aims. We study the radial dependences of the electron peak intensity and the energy spectrum of the electron intensity at the time of the SEE event peak intensity, taking advantage of multi-spacecraft measurements. Methods. We compiled the list of SEE events measured by MESSENGER and Solar Orbiter using hourly averages to find the prompt component of the near-relativistic (similar to 70-110 keV) electron peak intensities and to calculate the peak-intensity energy spectra. We also obtained the peak intensities and energy spectra for the same events as measured by the STEREO-A, -B, ACE, or Wind spacecraft when one of these spacecraft was in close nominal magnetic connection with MESSENGER or Solar Orbiter to derive the radial dependences of these SEE parameters. Results. (1) Because the background intensity level of the particle instrument on board MESSENGER is high, the SEE events measured by this mission are necessarily large and intense; most of them are accompanied by a shock driven by a coronal mass ejection and are widely spread in heliolongitude. The SEE events display relativistic (similar to 1 MeV) electron intensity enhancements. For this SEE sample, we found that (2) the SEE peak intensity shows a radial dependence that can be expressed as R-alpha, where the median value of the alpha index is alpha(Med) = 3.3 +/- 1.4 for a subsample of 28 events for which the nominal magnetic footpoints of the near 0.3 au and 1 au spacecraft were close in heliographic longitude. (3) The mean spectral index delta of a subset of 42 events for which the energy spectrum could be analysed is = -1.9 +/- 0.3, which is harder than the value found in previous studies using data from spacecraft near 1 au. SEE events observed by Solar Orbiter also display harder energy spectra than previous studies using data obtained near 1 au. Conclusions. There is a wide variability in the radial dependence of the electron peak intensities, but on average and within uncertainties, the proportional to R-3 dependence found in previous observational and modelling studies is confirmed. The electron spectral index found in the energy range around similar to 200 keV (delta 200) of the backward-scattered population near 0.3 au measured by MESSENGER is harder by a median factor of similar to 20% and similar to 10% when comparing to the near 1 au anti-sunward propagating beam and the backward-scattered population, respectively.

Context.On 2020 November 29, the first widespread solar energetic particle (SEP) event of solar cycle 25 was observed at four widely separated locations in the inner (≲1 AU) heliosphere. Relativistic electrons as well as protons with energies > 50 MeV were observed by Solar Orbiter (SolO), Parker Solar Probe, the Solar Terrestrial Relations Observatory (STEREO)-A and multiple near-Earth spacecraft. The SEP event was associated with an M4.4 class X-ray flare and accompanied by a coronal mass ejection and an extreme ultraviolet (EUV) wave as well as a type II radio burst and multiple type III radio bursts.Aims.We present multi-spacecraft particle observations and place them in context with source observations from remote sensing instruments and discuss how such observations may further our understanding of particle acceleration and transport in this widespread event.Methods.Velocity dispersion analysis (VDA) and time shift analysis (TSA) were used to infer the particle release times at the Sun. Solar wind plasma and magnetic field measurements were examined to identify structures that influence the properties of the energetic particles such as their intensity. Pitch angle distributions and first-order anisotropies were analyzed in order to characterize the particle propagation in the interplanetary medium.Results.We find that during the 2020 November 29 SEP event, particles spread over more than 230° in longitude close to 1 AU. The particle onset delays observed at the different spacecraft are larger as the flare–footpoint angle increases and are consistent with those from previous STEREO observations. Comparing the timing when the EUV wave intersects the estimated magnetic footpoints of each spacecraft with particle release times from TSA and VDA, we conclude that a simple scenario where the particle release is only determined by the EUV wave propagation is unlikely for this event. Observations of anisotropic particle distributions at SolO, Wind, and STEREO-A do not rule out that particles are injected over a wide longitudinal range close to the Sun. However, the low values of the first-order anisotropy observed by near-Earth spacecraft suggest that diffusive propagation processes are likely involved.

We report observations of a relatively long period of3He-rich solar energetic particles (SEPs) measured by Solar Orbiter. The period consists of several well-resolved ion injections. The high-resolution STEREO-A imaging observations reveal that the injections coincide with extreme ultraviolet jets and brightenings near the east limb, not far from the nominal magnetic connection of Solar Orbiter. The jets originated in two adjacent, large, and complex active regions, as observed by the Solar Dynamics Observatory when the regions rotated into the Earth’s view. It appears that the sustained ion injections were related to the complex configuration of the sunspot group and the long period of3He-rich SEPs to the longitudinal extent covered by the group during the analyzed time period.

Context. Solar Orbiter, launched in February 2020, started its cruise phase in June 2020, in coincidence with its first perihelion at 0.51 au from the Sun. The in situ instruments onboard, including the Energetic Particle Detector (EPD), operate continuously during the cruise phase enabling the observation of solar energetic particles. Aims. In situ measurements of the first near-relativistic solar electron events observed in July 2020 by EPD are analyzed and the solar origins and the conditions for the interplanetary transport of these particles investigated. Methods. Electron observations from keV energies to the near-relativistic range were combined with the detection of type III radio bursts and extreme ultraviolet (EUV) observations from multiple spacecraft in order to identify the solar origin of the electron events. Electron anisotropies and timing as well as the plasma and magnetic field environment were evaluated to characterize the interplanetary transport conditions. Results. All electron events were clearly associated with type III radio bursts. EUV jets were also found in association with all of them except one. A diversity of time profiles and pitch-angle distributions was observed. Different source locations and different magnetic connectivity and transport conditions were likely involved. The July 11 event was also detected by Wind, separated 107 degrees in longitude from Solar Orbiter. For the July 22 event, the Suprathermal Electron and Proton sensor of EPD allowed for us to not only resolve multiple electron injections at low energies, but it also provided an exceptionally high pitch-angle resolution of a very anisotropic beam. This, together with radio observations of local Langmuir waves suggest a very good magnetic connection during the July 22 event. This scenario is challenged by a high-frequency occultation of the type III radio burst and a nominally non-direct connection to the source; therefore, magnetic connectivity requires further investigation.

Context. Following a multi-year minimum of solar activity, a solar energetic particle event on 2020 Nov. 29 was observed by multiple spacecraft covering a wide range of solar longitudes including ACE, the Solar Terrestrial Relations Observatory-A, and the recently launched Parker Solar Probe and Solar Orbiter. Aims. Multi-point observations of a solar particle event, combined with remote-sensing imaging of flaring, shocks, and coronal mass ejections allows for a global picture of the event to be synthesized, and made available to the modeling community to test, constrain, and refine models of particle acceleration and transport according to such parameters as shock geometries and particle mass-to-charge ratios. Methods. Detailed measurements of heavy ion intensities, time dependence, fluences, and spectral slopes provided the required test data for this study. Results. The heavy ion abundances, timing, and spectral forms for this event fall well within the range found in prior surveys at 1 au. The spectra were well fitted by broken power law shapes; the Fe/O ratio was somewhat lower than the average of other events. In addition, 3 He/ 4 He was very low, with only the upper limits established here.

We report observations of five impulsive solar energetic particle (SEP) events observed inside 1 au during the first perihelion pass of the Solar Orbiter mission, which was launched in February 2020. These small events were all reasonably associated with active regions observed from Earth but which had rotated out of view by the time of the Solar Orbiter observations. Even though most of the events were small, their spectral forms, 3 He content, and association with type III bursts convincingly identifies them as 3 He-rich impulsive SEP events with properties similar to those previously observed at 1 au. Three of the events showed fast ion rise times, and two of them had long-lasting anisotropies consistent with the Compton-Getting effect.

Context. The Solar Orbiter spacecraft cruised in the inner heliosphere during Feb. 2020 – Jan. 2021, moving between ∼0.5–1.0 au radial distance. The Energetic Particle Detector suite operated continuously during this period. Aims. The Suprathermal Ion Spectrograph and High Energy Telescope observations made during intervals in between transient intensity increases were used to determine the low energy ion spectra and composition during quiet times. Methods. Energetic particle spectra and major ion components, including 3He, were measured over the range ∼0.1–100 MeV nucleon−1. The radial dependence of 4.4 MeV nucleon−1 4He and O was measured. A short interval of extremely low intensities (“super-quiet”) was also studied. Results. Spectra measured during the quiet period showed transitions, including galactic cosmic rays (> 50 MeV nucleon−1), anomalous cosmic rays (a few to ∼50 MeV nucleon−1), and a steeply rising “turn-up” spectrum below a few MeV nucleon−1 whose composition resembled impulsive, 3He-rich solar energetic particle events. The radial dependence had large uncertainties but was consistent with a small gradient. During the super-quiet interval, the higher energy components remained similar to the quiet period, while the approximately flat low energy 4He spectrum extended downward, reaching ∼300 keV nucleon−1 before transitioning to a steeply rising spectrum.

The first orbit of Solar Orbiter provided comprehensive measurements of six corotating interaction regions (CIRs) within 1 au. Five of these CIRs were also observed by ACE at 1 au, allowing for comparisons of the suprathermal ion intensities and spectra at different radial distances. Only subtle modulations of the 4 He spectral slopes are observed between Solar Orbiter and ACE. Additionally, the radial gradients of 226−320 keV/nuc 4 He ion intensities between Solar Orbiter and ACE are similar to that of 1.53 MeV H reported by Van Hollebeke et al. (1978, J. Geophys. Res., 83, A10). These observations provide a new addition to the study of the radial dependence of CIR-associated suprathermal ions in the inner heliosphere.

Aims.We present observations of the first coronal mass ejection (CME) observed by the Solar Orbiter spacecraft on April 19, 2020 and the associated Forbush decrease (FD) measured by the High Energy Telescope (HET). This CME is a multi-spacecraft event that was also seen near Earth the following day.Methods.We highlight the capabilities of the HET for observing small short-term variations of the galactic cosmic ray count rate using its single detector counters. We applied the analytical ForbMod model to the FD measurements to reproduce the Forbush decrease at both locations. Input parameters for the model were derived from both in situ and remote-sensing observations of the CME.Results.The very slow (∼350 km s−1) stealth CME caused an FD with an amplitude of 3% in the low-energy cosmic ray measurements at HET and 2% in a comparable channel of the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on board the Lunar Reconnaissance Orbiter, as well as a 1% decrease in neutron monitor measurements. Significant differences are observed in the expansion behavior of the CME at different locations, which may be related to influence of the following high speed solar wind stream. Under certain assumptions, ForbMod is able to reproduce the observed FDs in low-energy cosmic ray measurements from HET as well as CRaTER, however, with the same input parameters, the results do not agree with the FD amplitudes at higher energies measured by neutron monitors on Earth. We study these discrepancies and provide possible explanations.Conclusions.This study highlights the notion that the novel measurements of Solar Orbiter can be coordinated with observations from other spacecraft to improve our understanding of space weather in the inner heliosphere. Multi-spacecraft observations combined with data-based modeling are also essential for understanding the propagation and evolution of CMEs, in addition to their space weather impacts.

The Solar Orbiter flyby of Venus on 27 December 2020 allowed for an opportunity to measure the suprathermal to energetic ions in the Venusian system over a large range of radial distances to better understand the acceleration processes within the system and provide a characterization of galactic cosmic rays near the planet. Bursty suprathermal ion enhancements (up to ∼10 keV) were observed as far as ∼50 R V downtail. These enhancements are likely related to a combination of acceleration mechanisms in regions of strong turbulence, current sheet crossings, and boundary layer crossings, with a possible instance of ion heating due to ion cyclotron waves within the Venusian tail. Upstream of the planet, suprathermal ions are observed that might be related to pick-up acceleration of photoionized exospheric populations as far as 5 R V upstream in the solar wind as has been observed before by missions such as Pioneer Venus Orbiter and Venus Express. Near the closest approach of Solar Orbiter, the Galactic cosmic ray (GCR) count rate was observed to decrease by approximately 5 percent, which is consistent with the amount of sky obscured by the planet, suggesting a negligible abundance of GCR albedo particles at over 2 R V . Along with modulation of the GCR population very close to Venus, the Solar Orbiter observations show that the Venusian system, even far from the planet, can be an effective accelerator of ions up to ∼30 keV. This paper is part of a series of the first papers from the Solar Orbiter Venus flyby.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]The Long Period of 3He-rich Solar Energetic Particles Measured by Solar Orbiter 2020 November 17–23AuthorsRadoslavBucikiDGlennMasonRaulGomez-HerreroiDDavidLarioiDLauraBalmacedaNariakiNittaVratislavKruparNinaDresingGeorgeHoiDRobertAllenFernandoCarcaboso-MoralesiDJavierRodriguez-PachecoFredericSchulleriDAlexanderWarmuthRobertWimmer-SchweingruberiDJohanFreiherr von ForstnerGAndrewsLarsBergerIgnacioCernudaiDFranciscoEspinosa LaraiDW.LeesCesarMartin-GarciaDanielPachecoManuelPrietoiDSebastianSanchez PrietoiDCharlesSchlemmHelmutSeifertKushTyagiMilanMaksimoviciDAntonioVecchioAlexanderKollhoffPatrickKühlZigongXuiDSandraEldrumSee all authors Radoslav BucikiDCorresponding Author• Submitting AuthorSouthwest Research InstituteiDhttps://orcid.org/0000-0001-7381-6949view email addressThe email was not providedcopy email addressGlenn MasonJohns Hopkins University Applied Physics Laboratoryview email addressThe email was not providedcopy email addressRaul Gomez-HerreroiDUniversity of AlcaláiDhttps://orcid.org/0000-0002-5705-9236view email addressThe email was not providedcopy email addressDavid LarioiDNASA Goddard Space Flight CenteriDhttps://orcid.org/0000-0002-3176-8704view email addressThe email was not providedcopy email addressLaura BalmacedaGeorge Mason University Fairfaxview email addressThe email was not providedcopy email addressNariaki NittaLockheed Martin Advanced Technology Centerview email addressThe email was not providedcopy email addressVratislav KruparUniversity of Maryland Baltimore Countyview email addressThe email was not providedcopy email addressNina DresingUniversity of Turkuview email addressThe email was not providedcopy email addressGeorge HoiDJohns Hopkins University Applied Physics LaboratoryiDhttps://orcid.org/0000-0003-1093-2066view email addressThe email was not providedcopy email addressRobert AllenJohns Hopkins University Applied Physics Laboratoryview email addressThe email was not providedcopy email addressFernando Carcaboso-MoralesiDUniversity of AlcaláiDhttps://orcid.org/0000-0003-1758-6194view email addressThe email was not providedcopy email addressJavier Rodriguez-PachecoUniversity of Alcaláview email addressThe email was not providedcopy email addressFrederic SchulleriDLeibniz Institute for Astrophysics PotsdamiDhttps://orcid.org/0000-0002-2609-1604view email addressThe email was not providedcopy email addressAlexander WarmuthLeibniz Institute for Astrophysics Potsdamview email addressThe email was not providedcopy email addressRobert Wimmer-SchweingruberiDUniversity of KieliDhttps://orcid.org/0000-0002-7388-173Xview email addressThe email was not providedcopy email addressJohan Freiherr von ForstnerUniversity of Kielview email addressThe email was not providedcopy email addressG AndrewsJohns Hopkins University Applied Physics Laboratoryview email addressThe email was not providedcopy email addressLars BergerUniversity of Kielview email addressThe email was not providedcopy email addressIgnacio CernudaiDUniversity of AlcaláiDhttps://orcid.org/0000-0001-8432-5379view email addressThe email was not providedcopy email addressFrancisco Espinosa LaraiDUniversity of AlcaláiDhttps://orcid.org/0000-0001-9039-8822view email addressThe email was not providedcopy email addressW. LeesJohns Hopkins University Applied Physics Laboratoryview email addressThe email was not providedcopy email addressCesar Martin-GarciaUniversity of Kielview email addressThe email was not providedcopy email addressDaniel PachecoUniversity of Kielview email addressThe email was not providedcopy email addressManuel PrietoiDUniversity of AlcaláiDhttps://orcid.org/0000-0003-3050-3445view email addressThe email was not providedcopy email addressSebastian Sanchez PrietoiDUniversity of AlcaláiDhttps://orcid.org/0000-0002-6729-7932view email addressThe email was not providedcopy email addressCharles SchlemmJohns Hopkins University Applied Physics Laboratoryview email addressThe email was not providedcopy email addressHelmut SeifertJohns Hopkins University Applied Physics Laboratoryview email addressThe email was not providedcopy email addressKush TyagiUniversity of Colorado at Boulderview email addressThe email was not providedcopy email addressMilan MaksimoviciDLESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de ParisiDhttps://orcid.org/0000-0001-6172-5062view email addressThe email was not providedcopy email addressAntonio VecchioLESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Parisview email addressThe email was not providedcopy email addressAlexander KollhoffUniversity of Kielview email addressThe email was not providedcopy email addressPatrick KühlUniversity of Kielview email addressThe email was not providedcopy email addressZigong XuiDUniversity of KieliDhttps://orcid.org/0000-0002-9246-996Xview email addressThe email was not providedcopy email addressSandra EldrumUniversity of Kielview email addressThe email was not providedcopy email address

Solar Orbiter strives to unveil how the Sun controls and shapes the heliosphere and fills it with energetic particle radiation. To this end, its Energetic Particle Detector (EPD) has now been in operation, providing excellent data, for just over a year. EPD measures suprathermal and energetic particles in the energy range from a few keV up to (near-) relativistic energies (few MeV for electrons and about 500 MeV/nuc for ions). We present an overview of the initial results from the first year of operations and we provide a first assessment of issues and limitations. During this first year of operations of the Solar Orbiter mission, EPD has recorded several particle events at distances between 0.5 and 1 au from the Sun. We present dynamic and time-averaged energy spectra for ions that were measured with a combination of all four EPD sensors, namely: the SupraThermal Electron and Proton sensor (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) as well as the associated energy spectra for electrons measured with STEP and EPT. We illustrate the capabilities of the EPD suite using the 10-11 December 2020 solar particle event. This event showed an enrichment of heavy ions as well as ^3He, for which we also present dynamic spectra measured with SIS. The high anisotropy of electrons at the onset of the event and its temporal evolution is also shown using data from these sensors. We discuss the ongoing in-flight calibration and a few open instrumental issues using data from the 21 July and the 10-11 December 2020 events and give guidelines and examples for the usage of the EPD data. We explain how spacecraft operations may affect EPD data and we present a list of such time periods in the appendix. A list of the most significant particle enhancements as observed by EPT during this first year is also provided.

Context.In mid-June 2020, the Solar Orbiter (SolO) mission reached its first perihelion at 0.51 au and started its cruise phase, with most of the in situ instruments operating continuously.Aims.We present the in situ particle measurements of the first proton event observed after the first perihelion obtained by the Energetic Particle Detector (EPD) suite on board SolO. The potential solar and interplanetary (IP) sources of these particles are investigated.Methods.Ion observations from ∼20 keV to ∼1 MeV are combined with available solar wind data from the Radio and Plasma Waves (RPW) instrument and magnetic field data from the magnetometer on board SolO to evaluate the energetic particle transport conditions and infer the possible acceleration mechanisms through which particles gain energy. We compare > 17–20 MeV ion count rate measurements for two solar rotations, along with the solar wind plasma data available from the Solar Wind Analyser (SWA) and RPW instruments, in order to infer the origin of the observed galactic cosmic ray (GCR) depressions.Results.The lack of an observed electron event and of velocity dispersion at various low-energy ion channels and the observed IP structure indicate a local IP source for the low-energy particles. From the analysis of the anisotropy of particle intensities, we conclude that the low-energy ions were most likely accelerated via a local second-orderFermiprocess. The observed GCR decrease on 19 June, together with the 51.8–1034.0 keV nuc−1ion enhancement, was due to a solar wind stream interaction region (SIR). The observation of a similar GCR decrease in the next solar rotation favours this interpretation and constitutes the first observation of a recurrent GCR decrease by SolO. The analysis of the recurrence times of this SIR suggests that it is the same SIR responsible for the4He events previously measured in April and May. Finally, we point out that an IP structure more complex than a common SIR cannot be discarded, mainly due to the lack of solar wind temperature measurements and the lack of a higher cadence of solar wind velocity observations.

After decades of observations of solar energetic particles from space-based observatories, relevant questions on particle injection, transport, and acceleration remain open. To address these scientific topics, accurate measurements of the particle properties in the inner heliosphere are needed. In this paper we describe the Energetic Particle Detector (EPD), an instrument suite that is part of the scientific payload aboard the Solar Orbiter mission. Solar Orbiter will approach the Sun as close as 0.28 au and will provide extra-ecliptic measurements beyond ∼30° heliographic latitude during the later stages of the mission. The EPD will measure electrons, protons, and heavy ions with high temporal resolution over a wide energy range, from suprathermal energies up to several hundreds of megaelectronvolts/nucleons. For this purpose, EPD is composed of four units: the SupraThermal Electrons and Protons (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) plus the Instrument Control Unit that serves as power and data interface with the spacecraft. The low-energy population of electrons and ions will be covered by STEP and EPT, while the high-energy range will be measured by HET. Elemental and isotopic ion composition measurements will be performed by SIS and HET, allowing full particle identification from a few kiloelectronvolts up to several hundreds of megaelectronvolts/nucleons. Angular information will be provided by the separate look directions from different sensor heads, on the ecliptic plane along the Parker spiral magnetic field both forward and backwards, and out of the ecliptic plane observing both northern and southern hemispheres. The unparalleled observations of EPD will provide key insights into long-open and crucial questions about the processes that govern energetic particles in the inner heliosphere.