Christian Palmroos

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

Planetary magnetic fields act as protective barriers, shielding a planet’s atmosphere and surface from energetic charged particles originating from its parent star and outer space. During its fourth flyby of Mercury on 4 September 2024, the ESA/JAXA BepiColombo mission passed within just 165 km of the planet’s surface. This close approach coincided with the elevated fluxes of high-energy charged solar particles, providing an opportunity to study shielding processes of a planet in close proximity to its star. Here, utilizing observations from the Solar Intensity X-Ray and Particle Spectrometer (SIXS) onboard BepiColombo, we investigate the penetration of high energy particles from the Sun into Mercury’s magnetosphere. The variations in energetic particle fluxes depend on the particle type, direction and energy. Our analysis reveals that the disappearance of protons is related to magnetic shadowing by the planet, while electrons dropouts occur due to a wide magnetic loss cone. These findings show that sustained high-energy particle precipitation can occur during solar energetic particle events on weakly magnetized rocky planets orbiting close to their parent star. Such processes probably play a significant role in regolith alteration and exospheric variability, with broader implications for surface geochemistry and potential habitability.

Mercury hosts a small but dynamic global magnetic field capable of standing off the solar wind, resulting in a magnetosphere that is qualitatively similar in structure to Earth’s (albeit around 1% as strong). However, due to its closer proximity to the Sun, Mercury’s magnetosphere experiences much stronger solar wind pressure than that at Earth, resulting in dynamic magnetospheric processes that occur on much shorter timescales. These extreme conditions can result in processes such as particle energization, transport and precipitation onto the planetary surface, which are strongly influenced by the external solar wind conditions. Here, we present observations of energetic protons (> ~ 1 MeV) and electrons (> ~ 50 keV) from the Solar Intensity X-Ray and Particle Spectrometer (SIXS) onboard the BepiColombo spacecraft during its sixth and final Mercury swingby on 8 January 2025. Similar to the spacecraft’s fourth Mercury swingby in 2024, a solar energetic particle event occurred a few days before closest approach, resulting in elevated fluxes of energetic particles both outside and within the Hermean magnetosphere. Furthermore, the KTH22 Mercury magnetic field model was used to help interpret these energetic particle observations and to evaluate whether features in the data were consistent with possible planetary shielding of energetic protons, as well as electron loss and precipitation onto the Hermean surface.

Context. The SPEARHEAD (specification, analysis, and re-calibration of high-energy particle data) project, funded by the European Union’s Horizon Europe programme, enhances the accuracy and usability of high-energy particle measurements. It investigates particle acceleration, release, and transport during solar eruptions by refining instrument response functions and cross-calibrating datasets. This effort supports in-depth analyses of solar energetic particle (SEP) events and provides public data products. Aims. We present a comprehensive catalogue of > 100 MeV proton events identified from May 1996 to August 2024 using SOHO/ERNE penetrating-particle rates, together with associated solar phenomena derived from multi-instrument observations. Methods. The SEP events were detected through a systematic scan of ERNE/HED counter data and cross-calibrated with SOHO/EPHIN to derive peak fluxes and fluences. Each event was associated with its likely parent eruption using X-ray (XR) (GOES/XRS, RHESSI, SolO/STIX), radio (Wind/WAVES, STEREO/WAVES, ground-based observatories), and γ -ray (Fermi/LAT) observations, CMEs (SOHO/LASCO), and ground-level enhancements (GLEs) (neutron monitors). Timings and physical properties were systematically compared to investigate the relationships between SEP onset, flare evolution, CME kinematics, and radio signatures. Results. Associations were established for 96% of CMEs and 76% of X-ray flares; all events exhibited type III radio bursts, while 95% had type II, and 53% type IV. > 100 MeV γ -rays were identified in 65 events, and 22 were linked to GLEs or sub-GLEs. Statistical analyses reveal that most SEP releases closely follow flare and CME onsets, with moderate SEP-XR-CME correlations, and a strong SEP-GLE fluence link. These results indicate that high-energy SEP events are typically associated with strong solar activity signatures, with the observed intensities and timings strongly modulated by magnetic connectivity and coronal conditions. Conclusions. This catalogue provides the most extensive reference to date for high-energy SEP events over solar cycles 23–25, establishing a unified framework for future investigations of extreme particle acceleration.

The Energetic Solar Eruptions: Data and Analysis Tools (SOLER) project presents a wide array of software tools to help in the analysis of solar energetic particle (SEP) events. The current heliospheric fleet of spacecraft, which has expanded significantly in recent years, offers an unprecedented number of simultaneous vantage points and, as such, uniquely extensive data on solar eruptions and their effects throughout the heliosphere. In an effort to utilize this data to its full potential, SOLER provides software tools in the form of easy-to-use open-source Python Jupyter Notebooks. The tools are designed such that even users with limited programming experience can get the most out of them, allowing one to focus on what matters most: the science. They are available online and require no installation by the user.The tools cover a wide range of functionalities. They include automatized data loaders that handle downloading from the web and enable visualization of SEP intensity-time profiles and other in-situ measurements made by various instruments aboard the heliospheric fleet. Additional tools assist in determining the Pitch-Angle Distributions (PAD) and capabilities for background-subtraction of SEPs and first-order anisotropies, their energy spectra, including the ability to fit the spectra using a variety of mathematical models. The final set of tools is dedicated to the determination of SEP event onset times and related analysis. These tools offer a linear regression method designed to identify the times at which SEP intensities change rapidly as well as a novel combination of a modified Poisson-CUSUM scheme, statistical bootstrapping, and methodological time-averaging to estimate the most probable onset time along with the associated confidence intervals.This project has received funding from the European Union's Horizon Europe research and innovation programme under grant agreement No 101134999 (SOLER). Views and opinions expressed are those of the authors only and do not necessarily reflect those of the European Union or HaDEA. Neither the European Union nor the granting authority can be held responsible for them.

BepiColombo, the joint ESA/JAXA mission to Mercury, was launched in October 2018 and is scheduled to arrive at Mercury in November 2026 after an 8-year cruise. Like other planetary missions, its scientific objectives focus mostly on the nominal, orbiting phase of the mission. However, due to the long duration of the cruise phase covering distances between 1.2 and 0.3 AU, the BepiColombo mission has been able to outstandingly contribute to characterise the solar wind and transient events encountered by the spacecraft, as well as planetary environments during the flybys of Earth, Venus, and Mercury, and contribute to the characterisation of the space radiation environment in the inner Solar System and its evolution with solar activity. In this paper, we provide an overview of the cruise observations of BepiColombo, highlighting the most relevant science cases, with the aim of demonstrating the importance of planetary missions to perform cruise observations, to contribute to a broader understanding of Space Weather in the Solar System, and in turn, increase the scientific return of the mission. Graphical Abstract

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

High energy solar protons were observed by particle detectors aboard spacecraft in near-Earth orbit on May 11, 2024 and produced the 74^th ground level enhancement (GLE74) event registered by ground-based neutron monitors. This study involves a detailed reconstruction of the neutron monitor response, along with the identification of the solar eruption responsible for the emission of the primary particles, utilizing both in situ and remote-sensing. Observations spanning proton energies from a few MeV to around 1.64 GeV, collected from the Solar and Heliospheric Observatory (SOHO), the Geostationary Operational Environmental Satellite (GOES), the Solar Terrestrial Relations Observatory (STEREO-A), and neutron monitors, were integrated with records of the associated solar soft X-ray flare, coronal mass ejection, and radio bursts, to identify the solar origin of the GLE74. Additionally, a time-shift analysis was conducted to link the detected particles to their solar sources. Finally, a comparison of GLE74 to previous ones is carried out. GLE74 reached a maximum particle rigidity of at least 2.4 GV and was associated with a series of type III, type II, and type IV radio bursts. The release time of the primary solar energetic particles (SEPs) with an energy of 500 MeV was estimated to be around 01:21 UT. A significant SEP flux was observed from the anti-Sun direction with a relatively broad angular distribution, rather than a narrow, beam-like pattern, particularly during the main phase at the particle peak flux. Comparisons with previous GLEs suggest that GLE74 was a typical event in terms of solar eruption dynamics.

The closest planet to the Sun, Mercury, is subject to particularly intense fluxes of solar energetic particles (SEPs). Its relatively weak magnetic field and small magnetosphere offer some protection againts these particles, deflecting them away before they can reach the surface. The effectiveness of this shielding could be probed in detailed during BepiColombo’s fourth (4 September 2024) and sixth (8 January 2025) flybys when and SEP events happened to be ongoing and the planet was immersed in high fluxes of energetic particles. During the fourth flyby, BepiColombo reached only 165 kilometres from the Mercury’s surface. In this presentation we analysis high energy electron and proton observations provided by the Solar Intensity X-ray and Particle Spectrometer SIXS. The data reveal a deep drop out in energetic particles fluxes due planetry shadowing. In addition, these unique measurements reveal that variations in particle fluxes depend clearly on particle type, direction and energy.

Context. On 13 March 2023, when the Parker Solar Probe spacecraft (S/C) was situated on the far side of the Sun as seen from Earth, a large solar eruption took place, which created a strong solar energetic particle (SEP) event observed by multiple S/C all around the Sun. The energetic event was observed at six well-separated locations in the heliosphere, provided by the Parker Solar Probe, Solar Orbiter, BepiColombo, STEREO A, near-Earth S/C, and MAVEN at Mars. Clear signatures of an in situ shock crossing and a related energetic storm particle (ESP) event were observed at all inner-heliospheric S/C, suggesting that the interplanetary coronal mass ejection (CME)-driven shock extended all around the Sun. However, the solar event was accompanied by a series of pre-event CMEs. Aims. We aim to characterize this extreme widespread SEP event and to provide an explanation for the unusual observation of a circumsolar interplanetary shock and a corresponding circumsolar ESP event. Methods. We analyzed data from seven space missions, namely Parker Solar Probe, Solar Orbiter, BepiColombo, STEREO A, SOHO, Wind, and MAVEN, to characterize the solar eruption at the Sun, the energetic particle event, and the interplanetary context at each observer location as well as the magnetic connectivity of each observer to the Sun. We then employed magnetohydrodynamic simulations of the solar wind in which we injected various CMEs that were launched before as well as contemporaneously with the solar eruption under study. In particular, we tested two different scenarios that could have produced the observed global ESP event: (1) a single circumsolar blast-wave-like shock launched by the associated solar eruption, and (2) the combination of multiple CMEs driving shocks into different directions. Results. By comparing the simulations of the two scenarios with observations, we find that both settings are able to explain the observations. However, the blast-wave scenario performs slightly better in terms of the predicted shock arrival times at the various observers. Conclusions. Our work demonstrates that a circumsolar ESP event, driven by a single solar eruption into the inner heliosphere, is a realistic scenario.