Context. The radial gradients of cosmic rays are key parameters in studies of the transport of particles in space. Solar Orbiter, launched on 2020 February 10, approaches the Sun approximately every half-a-year, with its closest perihelion distance of 0.29 au at the end of 2022 during the nominal mission phase. The two double-ended high-energy telescopes (HET) on board Solar Orbiter measure energetic particles in the energy range between a few MeV/nuc and a few hundreds of MeV/nuc. These particles are dominated by anomalous cosmic rays (ACRs) and galactic cosmic rays (GCRs) during solar quiet times. Aims. By obtaining the radial gradient of ACR helium in the inner heliosphere, we can advance our understanding of how the transport of cosmic rays is affected by the particle drift effect and the large-scale magnetic field. Methods. We analyzed helium observations at Solar Orbiter/HET between 11.1 and 49 MeV/nuc. Since our study is focused on quiet time measurements, we removed the periods of solar energetic particle (SEP) events. The intensities were averaged over the Carrington rotation period. The helium observations from the Electron Proton and Helium Instrument (EPHIN) on board SOHO were utilized as the baseline to correct the long-term variation caused by the solar modulations. Results. We present the first observation of ACR helium at Solar Orbiter/HET between February 2020 and July 2022 in the inner heliosphere before the sun became fully active. We derived a radial gradient of ACR helium of between 0.3 and 1 au. The averaged radial gradient between 11.1 and 49 MeV/nuc is about 22 +/- 4%/au and the averaged value between 11.1 and 41.2 MeV/nuc was raised to 32 +/- 8%/au after removing the GCR contribution, which was estimated using a GCR model. In addition, the temporal variation of radial gradients indicates that gradients tend to increase with the enhancement of the solar modulation and the increased tilt angle of the heliospheric current sheet.

Sudden Solar Energetic Particle (SEP) events can have a major impact on technology and humans in space. Therefore forecasts and early warning systems working to support those missions are desirable. One example is REleASE, which utilizes the close correlation of near relativistic electrons and the slower but more hazardous protons.The original (2009) REleASE system used electron measurements from SOHO/EPHIN. During the HESPERIA project it was expended to include ACE/EPAM. Both systems issue short term warnings before there is a significant flux increase of >20 MeV protons at L1.We now successfully adapted the method to work with the High Energy Telescope (HET) and the Solar Electron Proton Telescope (SEPT) on board of STEREO-A. The resulting forecasts are publicly available in real time and can be accessed on a dedicated website. Furthermore, we gained valuable insights from adapting the method to the SEPT that uses the magnet/foil technique to separate electrons from ions, which can pose several difficulties.With now two REleASE systems operational we have the possibility to directly compare forecasts from different points in the heliosphere.The SOHO/EPHIN and STEREO/SEPT project is supported under Grant 50 OC 2302532 by the German Bundesministerium für Wirtschaft through the Deutsches Zentrum für Luft- und Raumfahrt (DLR). This study has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101004159535 (SERPENTINE). The REleASE system is supported by NASA via the grant TXS0150642.

Corotating interaction regions (CIRs) are long-lasting solar wind structures that persist over multiple solar rotations. These structures accelerate particles throughout the heliosphere and can impart significant energy onto planetary ionospheres and magnetospheres. Understanding how CIRs and their associated energetic particles evolve radially with heliocentric distance is of great interest and can give insight into acceleration mechanisms that occur within these structures. CIR solar wind and particle properties have been examined at numerous heliocentric distances but have been largely unexplored at Mars. We examine the properties of a CIR observed over two Carrington rotations by the Solar Terrestrial Relations Observatory (STEREO)-A at 1 au and the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft at 1.5 au. This CIR was observed during near-radial alignment of the spacecraft, allowing for the exclusion of significant longitudinal variations in the CIR's properties. We find that during both rotations, the scaled solar wind density and interplanetary magnetic field are slightly higher at MAVEN than measurements at STEREO-A, and the CIR compression occurs over similar periods at both spacecraft during each event. Suprathermal particle enhancements are observed at both spacecraft during both passes of the CIR. Variations in the particle spectra between observations indicate suprathermal particle acceleration at both spacecraft, with higher-energy populations observed at 1.5 au compared to 1 au. Spectral anisotropies observed at MAVEN also indicate a combination of local and nonlocal particle acceleration, further demonstrating that the acceleration mechanisms associated with CIRs are complex and evolve with heliocentric distance.

Providing reliable forecasts of Solar Energetic Particle (SEP) events is mandatory for human spaceflight beyond low-Earth orbit, especially outside the Earth's magnetosphere. High-energy SEPs are tracked because they penetrate deeper into the terrestrial atmosphere and contribute to the radiation dose aboard spacecraft specifically over Canada and the Southern Indian Ocean, due to the tilt of the Earth on its axis. Based on the Relativistic Electron Alert System for Exploration (REleASE) forecasting scheme], the HESPERIA REleASE product was developed by the HESPERIA H2020 project (Project Coordinator: Dr. Olga Malandraki) and generating real-time predictions of the proton flux (30-50 MeV) at L1, making use of relativistic and near-relativistic electron measurements by the SOHO/EPHIN and ACE/EPAM experiments, respectively. The HESPERIA REleASE tools are operational through the Space Weather Operational Unit of the National Observatory of Athens, accessible through the dedicated website (http://www.hesperia.astro.noa.gr). HESPERIA REleASE has attracted attention from various space organizations (e.g., NASA/CCMC, SRAG), due to the real-time, highly accurate and timely performance offered. ESA selected the HESPERIA REleASE products that were integrated and provided through the ESA Space Weather (SWE) Service Network (https://swe.ssa.esa.int/noa-hesperia-federated) under the Space Radiation Expert Service Center (R-ESC). Solar cycle 25 solar radiation storms successfully predicted by HESPERIA REleASE are presented and discussed. Moreover, we present an innovative upgrade implemented, namely HESPERIA REleASE+, that is using the novel approach of combining for the first time real-time type III solar radio burst observations by the STEREO S/WAVES instrument, thus incorporating clear evidence of particle escape from the Sun, within the HESPERIA REleASE system. To this end, a robust automated algorithm has been developed for the real-time identification and classification of Type III radio burst characteristics, related to intense SEP events at Earth’s orbit. This new implementation leads to a substantial step forward in improving the accuracy and reduction of false alarms.

On November 27, 2021, Solar Orbiter has performed a gravity assist maneuver at the Earth and in December 2022 Solar Orbiter and STEREO-A were separated by less than 0.2 au. The periods around these two events provide unique opportunities to compare Solar Energetic Particle (SEP) observations from Solar Orbiter with corresponding observations from near-Earth missions and STEREO-A. The proximity of the different spacecraft offers a useful opportunity to compare the calibrations of the various particle instruments of the different spacecraft. Furthermore, the unique constellations can be used to study small-scale changes in the density distributions of the SEPs.We will present observations of near-relativistic electrons with energies from 50 keV to 400 keV as well as observations of protons with energies from 50 keV to 50 MeV. In particular, we use the period around the Earth gravity assist maneuver for a comparison of measurements from the Electron Proton Telescope (EPT) and the High Energy Telescope (HET) aboard Solar Orbiter with measurements from the Electron Proton and Alpha Monitor aboard the Advanced Composition Explorer (ACE), the 3DP instrument aboard Wind and the Electron Proton Helium Instrument (EPHIN) aboard SOHO. The period around the close encounter with STEREO-A is used for a comparison of EPT and HET observations with measurements from the High Energy Telescope (STA/HET) and the Solar Electron and Proton Telescope (SEPT) aboard the STEREO-A spacecraft. For both periods we discuss the instrument calibrations and possible physical explanations for differences in particle observations. Funding: This work was supported by the German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt, e.V., (DLR)) under grant number 50OT2002 and has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 101004159 (SERPENTINE).

The Relativistic Electron Alert System for Exploration (REleASE) forecasting metric, developed by Posner (2007), utilizes electron data collected by the Electron Proton Helium Instrument (EPHIN) aboard the SOHO spacecraft. Our project aims to enhance the probability of detection, decrease the false alarm rate and extend the warning time by implementing various remote sensing techniques. These include automatic flare detection and localization, as well as automatic radio burst detection using the ROBUST algorithm developed at the University of Graz. Historically, a range of diagnostics for Solar Energetic Particle (SEP) events based on radio observations from Earth has been developed since the 1960s, which are to some extent utilized in contemporary prediction models. These diagnostics span from the occurrence of long-lasting broadband radio emissions (cm-m waves) to the spectra of microwave bursts (mm-cm waves). The presence of radio emission at meter wavelengths (e.g., type III bursts) is crucial, since it indicates particle injection into the high corona, but is absent in confined flares, where no particles escape from the active region and no CME is available to accelerate particles higher up.Moreover, our project explores the extent to which diagnostics across diverse frequency ranges can enhance the REleASE system. Initial results of this integration and its impact on the accuracy of SEP event forecasting will be presented.

We present results over an 11-year Solar cycle of cosmic antiprotons based on 1.1×10^{6} events in the rigidity range from 1.00 to 41.9 GV. The p[over ¯] fluxes exhibit distinct properties. The magnitude of the p[over ¯] flux temporal variation is significantly smaller than those of p, e^{-}, and e^{+}. A hysteresis between the p[over ¯] fluxes and the p fluxes is observed, whereas the p[over ¯] and e^{-} fluxes show a linear correlation. With a model-independent analysis, we found a universal relation between the shape of the rigidity spectrum and the magnitude of flux temporal variation over an 11-year Solar cycle for both positively and negatively charged particles. The simultaneous results on p[over ¯] and p, e^{-}, and e^{+} provide unique information for understanding particle transport in the Solar System as a function of mass, charge, and spectral shape.

We report the properties of precision time structures of cosmic nuclei He, Li, Be, B, C, N, and O fluxes over an 11-year solar cycle from May 2011 to November 2022 in the rigidity range from 1.92 to 60.3 GV. The nuclei fluxes show similar but not identical time variations with amplitudes decreasing with increasing rigidity. In particular, below 3.64 GV the Li, Be, and B fluxes, and below 2.15 GV the C, N, and O fluxes, are significantly less affected by solar modulation than the He flux. We observe that these differences in solar modulation are linearly correlated with the differences in the spectral indices of the cosmic nuclei fluxes. This shows, in a model-independent way, that solar modulation of galactic cosmic nuclei depends on their spectral shape. In addition, solar modulation differences due to nuclei velocity dependence on the mass-to-charge ratio (A/Z) are not observed.

The origins of energetic electrons with energies ranging from a few tens of keV to tens of MeV in the inner heliosphere are manifold. They include Galactic Cosmic Rays, Jovian electrons as well as sporadic Solar Energetic Electron (SEE) events. Their energy spectra provide insights into the acceleration at the source and transport processes in the heliosphere.The SOlar and Heliospheric Observatory (SOHO) was launched in December 1995 with the Electron Proton Helium INstrument (EPHIN) measuring electrons from 150 keV to several MeV. However, its measuring capability was reduced due to the loss of two detectors in 1997 and 2017, respectively. Thus from 2017 onwards only two electron channels, one in the range from 300 keV to one MeV and one “integral channel” that measures between 300 keV and 10 MeV.In this contribution we present a new data product for electron spectra based on the onboard histograms. This data product has the advantage of providing the total energy loss in the first two detectors with good statistics compromising energy loss determination via PHA data and counting statistics of the single channel. Using the so-called bow-tie method we were able to derive several energy channels between 300 keV and about 1 MeV. We present first results and compare them with instruments from other missions.The SOHO/EPHIN project is supported under Grant 50 OC 2102 by the German Bundesministerium für Wirtschaft through the Deutsches Zentrum für Luft- und Raumfahrt (DLR). This study has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101004159 (SERPENTINE).

Solar Energetic Particle (SEPs) with energies ranging from tens of keV to a few GeV, are a significant component in the description of the space environment. In this work, the HESPERIA REleASE product is emphasized, which, based on the Relativistic Electron Alert System for Exploration (REleASE) forecasting scheme, generates real-time predictions of the proton flux (30–50 meV) at L1, making use of relativistic and near-relativistic electron measurements by the SOHO/EPHIN and ACE/EPAM experiments, respectively. The HESPERIA REleASE Alert is a notification system based on the forecasts produced by the HESPERIA REleASE product and informs about the expected radiation impact in real-time using an illustration and a distribution system for registered users. We also present and discuss the Advance Warning Times derived for the compiled list of major SEP events successfully forecasted over the last 2.5 years during solar cycle 25 by HESPERIA REleASE and recent developments.

Solar energetic particle (SEP) events are increases of ions and electrons causedby solar activity namely flares and coronal mass ejections. While the mostenergetic ion population is well studied, SEP events accelerating electrons above20 MeV have only been reported from measurements by ISEE III in the 1980’sand the Kiel Electron Telescope (KET).The KET aboard Ulysses launched in 1990 and measured the electron flux inthe energy range from 4 MeV to above 6 GeV. Here we report on observationsof ultra-relativistic electrons and show spectra of electron events during solarcycle 22 and 23 until the end of 2008. The maximum electron energy exceeded100 MeV during the August 16, 2001 SEP event.This study has received funding from the European Union’s Horizon 2020research and innovation programme under grant agreement No. 101004159(SPEARHEAD).

This study investigates the periodicities of cosmic rays attributed to co-rotating interaction regions (CIRs) using AMS-02 data from late 2016 to early2017. These data enable the first-time examination of Forbush decrease amplitudes induced by CIRs, considering rigidity and charge sign dependence. The findings from the Lomb-Scargle algorithm and Superposed Epoch Analysis were compared. Results reveal that the rigidity dependence of proton decreases in the northern coronal hole aligns with existing literature, while the southern coronal hole shows no rigidity dependence. Helium modulation surpasses that of protons, in line with previous observations, while limited statistical data for positrons prevent definitive conclusions. Notably, the modulation behavior of electrons differs from that of positively charged particles.

Solar energetic particle (SEP) events are major outbursts of energetic charged particle radiation from the Sun. These events are related to solar flares and fast coronal mass ejections (CMEs). Flares are presumed to accelerate particles in magnetic reconnection processes, whereas fast (speeds > 1000 km s–1) CMEs drive shock waves through the corona that are known to be able to accelerate particles. Electron acceleration has traditionally been ascribed to reconnection in flares whereas proton acceleration is believed to be efficient in CME-driven shocks. Recent observational evidence [1], however, suggests that shocks may be important in electron acceleration as well. Almost all major eruptions are related to both flares and CMEs so the association of the accelerated particles to these eruptive phenomena is often subject to debate. Using novel spacecraft observations of strong SEP events detected in solar cycle 25, we aim at identifying the parent acceleration region of the observed electron and proton events.We have analyzed a set of 45 SEP events between Nov 2020 and May 2023 using data from multiple spacecraft including Solar Orbiter, near-Earth spacecraft (SOHO and Wind), STEREO-A and BepiColombo. We make use of peak intensities of >25-MeV protons and ~100 keV and ~1 MeV electrons and perform correlation studies of these peak intensities with each other as well as with the associated flare intensity. We separate the events into those that are well-connected (angular separation ≤ 35°) or poorly-connected (angular separation > 35°) to the flare by the interplanetary magnetic field.We find significant correlations between electron and proton peak intensities. While events detected by poorly-connected observers show a single population of events, consistent with the idea that these particles are all accelerated by the spatially-extended CME-driven shock, events observed in well-connected regions show two populations: One population has higher proton peak intensities that correlate with electron peak intensities similarly to the poorly-connected events. These are most likely shock associated. The other population has low proton intensities that are less well correlated with electron peak intensities. This population is suggested to show a dominant contribution of the flare.References:[1] Dresing, N. Kouloumvakos, A., Vainio, R., Rouillard, A., Astrophys. J. Lett., 925, L2

Predictions of the intensities of solar particle events are often fraught with uncertainties. Insufficient knowledge and understanding of the solar sources of the particles, and the conditions in the corona and in the inner heliosphere, as well as magnetic connectivity, have been limiting factors in making further progress. This presentation covers a study done by one of the Work Packages of the SERPENTINE project, and is based on the analysis of lists prepared by two other work packages of the same project, and logically consists of two parts. In the first part we performed a statistical analysis of a list of 45 multi-spacecraft events in solar cycle 25 observed by five spacecraft located in the inner Heliosphere (Solar Orbiter, Parker Solar Probe, Stereo A, Bepi Colombo), and one located at 1 AU close to the Earth (SOHO or Wind). The list, while prepared with a focus on the detection of protons above 25 MeV by two or more spacecraft, contains also information about electron observations around 100 keV and 1 MeV, respectively. Aiming to investigate the processes which are responsible for spreading energetic particles in longitude and latitude, and to estimate the importance of perpendicular diffusion in the latitudinal direction, we considered, together with other parameters, not only the longitudinal distances to the source, but also the differences in the total angle. In this part of our study we used methods such as correlation analysis and principal component analysis, and applied them to the list as a whole, as well as to different types of events. For example, we found that in the case of narrow-spread events perpendicular diffusion is sufficient to explain the spreading of particles from the solar source into the heliosphere, while in the case of wide-spread events an additional acceleration source is needed. We also evaluated the role of the speeds and sizes of the associated coronal mass ejections, as well as features of EUV waves appearing in the events, and relate different types of microwave (radio) emission to different groups of events. In the second part of this work we performed a statistical analysis of a list of 61 interplanetary shocks, observed by Solar Orbiter. By using a superposed epoch analysis, we built a statistical picture of ion time profiles around the shock front in several energy ranges. We also investigated which shock parameters are more important for particle energization by propagating interplanetary shocks, particularly in the case of an overlap in these lists.This study has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 101004159 (SERPENTINE).

Context. Solar energetic particle (SEP) events are related to solar flares and fast coronal mass ejections (CMEs). In the case of large events, which are typically associated with both a strong flare and a fast CME driving a shock front, identification of the dominant SEP acceleration mechanism is challenging. Aims. Using novel spacecraft observations of strong SEP events detected in solar cycle 25, we aim to identify the parent acceleration region of the observed electron and proton events. Methods. We analysed 45 SEP events in November 2020 - May 2023 including > 25 MeV protons using data from multiple spacecraft, including Solar Orbiter, near-Earth spacecraft (SOHO and Wind), STEREO A, BepiColombo, and Parker Solar Probe. We used peak intensities of 25-40 MeV protons and similar to 100 keV and 1 MeV electrons provided by the SERPENTINE multi-spacecraft SEP event catalogue, and studied the correlations between these peak intensities as well as with the intensity of a soft-X-ray flare associated with the SEP event. We also separated the events into those well connected and those poorly connected to the flare by the interplanetary magnetic field. Results. We find significant correlations between electron and proton peak intensities. While events detected by poorly connected observers show a single population of events, consistent with the idea that these particles are all accelerated by a spatially extended CME-driven shock, events observed in well-connected regions show two populations. One of these populations presents higher proton peak intensities that correlate with electron peak intensities, similarly to the poorly connected events. The other population shows low proton intensities that are less well correlated with electron peak intensities. Based on our findings, we propose that the latter population is a mixture of flare- and shock-accelerated events. Conclusions. Although this study focuses on relatively energetic SEP events including > 25 MeV protons often attributed to acceleration by CME-driven shocks, we find clear indications of a flare contribution to both electron and proton fluxes in those events originating in sectors magnetically well connected to the source region.

Since its start in 2021, the Solar EneRgetic ParticlE aNalysis plaTform for the INner hEliosphere (SERPENTINE) Project funded by EU H2020 program is using multi-spacecraft observations to investigate the origin of Solar Energetic Particles (SEPs) and providing new tools and datasets for the heliophysics community. SERPENTINE distributes new catalogues covering past and recent multipoint observations of SEP events, as well as their associated coronal mass ejections and interplanetary shocks. New SEP-related high-level data products from BepiColombo and Solar Orbiter missions, with added scientific value will be also provided in the near future. In this work, we summarize the structure, contents, and functionalities of the SERPENTINE Project Data Center (https://data.serpentine-h2020.eu/), a web-based interface providing open access to the various catalogues and high-level data products resulting from the project. This research has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004159 (SERPENTINE).

This work reports on the real‐time implementation of a local solar proton event forecasting system at the Solar Terrestrial Relations Observatory—Ahead (STEREO‐A) spacecraft, namely STEREO REleASE+. The forecasting uses the finding that relativistic electrons provide the “earliest indication” that a solar particle event has started, arriving at near‐Earth distance earlier than protons at ∼30% of the speed of light. In addition to relativistic electrons, we use a requirement of a radio burst of type III to be observed before issuing a proton alert. A similar system, HESPERIA REleASE+, has already been implemented using electrons observed by SOHO and ACE near Earth, which creates local forecasts for the Earth‐moon system. The radio observations from STEREO‐A are used in both systems, as radio bursts in part of the emission spectrum can be observed all around the Sun. Usefulness of adding a radio burst requirement to an electron‐based forecasting system lies in the potential for suppression of known sources of false alarms, that is, added robustness. While this work describes the establishment of the real‐time system, we plan on using the two local and robust forecasts to test how far away from the spacecraft the validity and usefulness of the local forecasts extend. Moreover, the STEREO REleASE+ system, currently located between Earth and Earth‐Sun L4, adds an additional safeguard for exploration of the moon, in particular from solar particle events originating behind the western limb of the Sun.

A major impact on human and robotic space exploration activities is the sudden and prompt occurrence of solar energetic ion events. In 2023 and 2024, {STEREO} is approaching the Earth from a behind position, soon passing Earth inside its orbit and thereafter moving ahead of Earth. {STEREO} thus offers several unique opportunities during this passage. In the period from June 1 to November 1, 2023, 5 {SEP} events have been measured that cause proton fluxes of above 25~MeV to rise above 0.1 /(cm$^2$\; s\; sr\; MeV). Taking into account systematic and statistical uncertainties of the particle measurements we find a good agreement between both spacecraft.The SOHO/EPHIN and STEREO/SEPTproject is supported under Grant 50~OC~2102 by the German Bundesministerium für Wirtschaft through the Deutsches Zentrum für Luft- und Raumfahrt (DLR). This study has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101004159 (SERPENTINE).

Solar Energetic Particle (SEP) events can pose a significant radiation hazard for human and robotic space exploration activities. Therefore SEP forecasting systems are needed to support operations. The REleASE system (A. Posner, 2007) utilizes the fact that near relativistic electrons (1 MeV electrons have 94% of the speed of light) travel faster than ions (30 MeV protons have 25% of the speed of light) and are always present in hazardous SEP events. Their early arrival can be used to forecast the expected proton flux. Originally REleASE uses real time data from SOHO/EPHIN near Earth. Since the instrument is aging we recently adapted the method to STEREO-A/HET and used the period from June to November 2023 when STEREO-A passed the Earth to compare the REleASE forecasts from the different instruments.This study has received funding from the National Aeronautics and Space Administration under grant agreement No. TXS0150642 (HESPERIA RELEASE).

Solar Energetic Particle Analysis Platform for the Inner Heliosphere (SERPENTINE) is a 42-months-long EU/H2020 project that started in January 2021 and focuses on the physics of Solar Energetic Particle (SEP) acceleration and transport. The project (see https://serpentine-h2020.eu) provides answers for three science questions: (Q1) what are the primary reasons for widespread SEP events; (Q2) what are the mechanisms responsible for acceleration ions from suprathermal to near-relativistic energies in coronal and interplanetary shocks; and (Q3) what is the role of shocks in the acceleration of electrons in SEP events. SERPENTINE makes use of the present capabilities provided by inner heliospheric missions such as Solar Orbiter, Parker Solar Probe and BepiColombo. In addition to the scientific objectives, the project develops and releases to the community a large number of analysis tools to facilitate the interpretation of observations. Also event catalogs and high-level datasets are produced and distributed. We will give a summary of the results of the project. Some of the science highlights include the several identified causes of widespread events related to both sources and transport (Q1), the role of local and averaged properties of shocks in ion acceleration (Q2), and the observational evidence of shocks as the primary accelerators of MeV electrons in gradual SEP events (Q3).