With the launch of Solar Orbiter (SolO) on Feb. 10th 2020, a new era of multi-spacecraft solar energetic particles (SEP) observations has started. The unique orbit of the mission allows the observation of SEP events close to the Sun (<0.28 au), which can occasionally be compared to corresponding observations made by other spacecraft at 1au. Such multi-spacecraft observations of the same event at different radial distances provide an excellent opportunity to study the radial evolution of SEP events.In this study, we identify SEP events for which SolO and either Wind or STEREO-A had a small longitudinal separation (<15°) between their magnetic foot-points at the Sun. For all SEP events that satisfy our selection criteria we determine the onset times and rise times as well as peak fluxes and peak values of the first-order anisotropy for electrons in the energy range from ∼50−85 keV. We compare the event parameters observed at the different spacecraft regarding their radial changes. In our sample we find strong event-to-event variations in the radial dependency of all derived event parameters. For the majority of events, the peak flux and the maximum value of the first-order anisotropy decrease with increasing radial distance to the Sun, while the rise time increases with radial distance in the majority of events. The derived onset delays observed between two spacecraft were found to be too long to be explained by ideal Parker spirals in multiple events.We present an overview of the most interesting observations and discuss the wide variability in the radial dependency of the event parameters analysed in this study.

Context. After their acceleration and release at the Sun, solar energetic particles (SEPs) are injected into the interplanetary medium and are bound to the interplanetary magnetic field (IMF) by the Lorentz force. The expansion of the IMF close to the Sun focuses the particle pitch-angle distribution, and scattering counteracts this focusing. Solar Orbiter observed an unusual solar particle event on 9 April 2022 when it was at 0.43 astronomical units (au) from the Sun. Aims. We show that the inferred IMF along which the SEPs traveled was about three times longer than the nominal length of the Parker spiral and provide an explanation for this apparently long path. Methods. We used velocity dispersion analysis (VDA) information to infer the spiral length along which the electrons and ions traveled and infer their solar release times and arrival direction. Results. The path length inferred from VDA is approximately three times longer than the nominal Parker spiral. Nevertheless, the pitch-angle distribution of the particles of this event is highly anisotropic, and the electrons and ions appear to be streaming along the same IMF structures. The angular width of the streaming population is estimated to be approximately 30 degrees. The highly anisotropic ion beam was observed for more than 12 h. This may be due to the low level of fluctuations in the IMF, which in turn is very probably due to this event being inside an interplanetary coronal mass ejection The slow and small rotation in the IMF suggests a flux-rope structure. Small flux dropouts are associated with very small changes in pitch angle, which may be explained by different flux tubes connecting to different locations in the flare region. Conclusions. The unusually long path length along which the electrons and ions have propagated virtually scatter-free together with the short-term flux dropouts offer excellent opportunities to study the transport of SEPs within interplanetary structures. The 9 April 2022 solar particle event offers an especially rich number of unique observations that can be used to limit SEP transport models.

The Sun drives a supersonic wind which inflates a giant plasma bubble in our very local interstellar neighborhood, the heliosphere. Its boundaries and the turbulent magnetic field shield the solar system from much of the interstellar medium as well as the low-energy portion of galactic cosmic rays (GCRs) which are accelerated primarily by super-nova-driven shocks in our galaxy. The heliosphere is bathed in an extremely variable background of energetic ions and electrons which originate from a number of sources. Solar energetic particles (SEPs) are accelerated in the vicinity of the Sun, whereas shocks driven by solar disturbances are observed to accelerate energetic storm particles (ESPs). Moreover, a dilute population with a distinct composition forms the anomalous cosmic rays (ACRs) which are of a mixed interstellar-heliospheric origin. Particles are also accelerated at planetary bow shocks. In February 2020, the European Space Agency (ESA) launched Solar Orbiter, a science mission to answer the question how the Sun creates and controls the heliosphere. Its orbit brings it within 0.3 astronomical units (au) from the Sun and will also reach moderately high solar latitudes to allow to understand why solar activity changes with time. The spacecraft carries instruments which observe the Sun and its surrounding remotely, others that measure the local environment, and some can track solar disturbances as they travel away from the Sun. The Energetic Particle Detector (EPD) on Solar Orbiter measures suprathermal and energetic particles in the energy range from a few keV up to (near-) relativistic energies (tens of MeV for electrons and about $> 100$ MeV/nuc for ions). Together with the other sophisticated instruments on Solar Orbiter it is designed to unravel how solar eruptions produce energetic particle radiation that fills the heliosphere. Since launch, EPD has made several advances about GCRs, SEPs, ACRs, ESPs, and the particles around the Venusian magnetosphere.

Aims. We study the radial evolution of near-relativistic solar energetic electron (SEE) events observed by at least two spacecraft at different heliocentric distances and with small separation angles between their magnetic footpoints at the Sun. Methods. We identified SEE events for which Solar Orbiter and either Wind or STEREO-A had a small longitudinal separation (< 15°) between their nominal magnetic footpoints. For the approximation of the footpoint separation, we followed a ballistic back-mapping approach using in situ solar wind speed measurements. For all the SEE events that satisfied our selection criteria, we determined the onset times, rise times, peak fluxes, and peak values of the first-order anisotropy for electrons in the energy range from ∼50 − 85 keV. We compared the event parameters observed at different spacecraft and derived exponential indices αp for each parameter p, assuming an Rα-dependence on the heliocentric distance R. Results. In our sample of SEE events, we find strong event-to-event variations in the radial dependence of all derived parameters. For the majority of events, the peak flux decreases with increasing radial distance. For the first-order anisotropy and the rise time no clear radial dependence was found. The derived onset delays observed between two spacecraft were found to be too long to be explained by ideal Parker spirals in multiple events. Conclusions. The rudimentary methods presented in this study lead to event parameters with large uncertainties. The absence of a clear radial dependence on the first-order anisotropy and the rise time as well as the ambiguous onset timing of the SEE events found in this study could be the result of general limitations in the methods we used. Further studies, including analyses of the directional fluxes and transport simulations that take the individual instrument responses into account, would allow a better interpretation of the radial evolution of SEE events.

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.

The Energetic Particle Detector (EPD) onboard Solar Orbiter is a suite of multiple sensors (Suprathermal Electrons Protons, STEP; Suprathermal Ion Spectrograph, SIS; Electron Proton Telescope, EPT; High Energy Telescope, HET), which measures particle intensities over a wide range of energies (from suprathermal to relativistic energies) and for different species (electron, protons, and heavy ions) in different directions. The EPD data center (http://espada.uah.es/epd) offers a primer venue to inspect the Solar Energetic Particle (SEP) activity, both to promptly check the most recent solar activity using quicklook plots based on low-latency data sets, and to perform deeper studies with data validated for scientific use. Among others, a series of plots and relevant information, such as the spacecraft maneuvers or sensor updates, are provided to the community. This facility gives access to all the data from the EPD sensors (which can be also found in the Solar Orbiter Archive), including Level 2 (calibrated) as well as more elaborated Level 3 data in the near future, which have further processing. An application programming interface (API) is also offered for accessing EPD data. Besides, during the first year and a half of observations, Solar Orbiter has completed three orbits, and EPD has measured several increases in particle fluxes, due to heliospheric and solar-origin events. Some of the events have been analysed and the flux enhancements have been tagged for future studies. This work aims to let the community know the availability of the instrument data products, and to explain how to properly use the provided data products and plots, as well as to summarise all the available studies published until now.

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.

Solar Orbiter was launched in February 2020 carrying the most complete set of in-situ and remote sensing instruments, for the study of the Sun and the heliosphere. The Energetic Particle Detector (EPD) on board of Solar Orbiter was switched on on 28 February 2020 and, since then, it has provided us with measurements of the energetic particles traveling through the inner heliosphere. The EPD suite is composed of a set of different sensors measuring electrons, protons and ions in a wide range of energies. The Electron-Proton Telescope (EPT) was designed to measure electrons and ions with energies of 35-4000keV and 45-7000keV respectively. By utilizing the so-called magnet/foil-technique, EPT is capable of measuring energetic particles with a high temporal and energy resolution while obtaining directional information from its four different fields of view. Although EPT is well suited for the study of solar energetic particle events, instrumental effects such as the contamination of EPT data products by GCR particles need to be understood for a correct interpretation of the data. We will present our current understanding of the background and calibration of EPT based on the data gathered during the first year of Solar Orbiter’s mission.

The first solar electron events detected by Solar Orbiter were observed by the Energetic Particle Detector (EPD) suite during July 11-23, 2020, when the spacecraft was at heliocentric distances between 0.61 and 0.69 au. We combined EPD electron observations from 4 keV to the relativistic range (few MeV), radio dynamic spectra and extreme ultraviolet (EUV) observations from multiple spacecraft in order to identify the solar origin of these electron events. Electron anisotropies and timing as well as the plasma and magnetic field environment were evaluated to characterize the interplanetary transport conditions. We found that all the 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 (ranging from almost isotropic to beam-like) was observed. These observations indicate that different source locations and different magnetic connectivity and transport conditions were likely involved. The broad spectral range covered by EPD with excellent energy resolution and the high time cadence ensure that future observations close to the Sun will contribute to the understanding of the acceleration, release, and transport processes of energetic particles. EPD observations will play a key role in the identification of the sources of impulsive events and the links between the near-relativistic electrons and the ion populations enriched in 3He and heavy ions

Solar Orbiter’s Energetic Particle Detector (EPD) was commissioned in early 2020 and has since been returning data from the inner heliosphere. Despite the low activity in the current deep and extended solar minimum, EPD has observed a number of solar particle events and numerous other enhancements of energetic particles. As one of the four complementary EPD sensors, the Electron-Proton Telescope (EPT) covers the gap between the high and low particle-energy measurements of HET and STEP. With four double-ended telescopes, EPT is capable of measuring electrons and ions in an energy range of 35-400keV and 45-7000keV respectively, while providing anisotropy information from four different viewing directions. We will present a first overview of EPT measurements, exhibiting some of the EPT data products which are made available by the European Space Agency (ESA). In order to provide the community a deep insight into the data, we will go through different aspects of the measurements, including the current status of the intercalibration with the other EPD instruments.

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.