Rami Vainio

During BepiColombo second Mercury’s flyby on 23 June 2022, a ground-based campaign with the THEMIS solar telescope provided simultaneous observations of the sodium (Na) exosphere while Bepicolombo measured interplanetary and magnetospheric conditions. Between 8:53 and 9:34 UT, the exospheric Na emission abruptly increased in intensity by about 70% across the entire day side, followed by a slow recovery to the preceding level over approximately four hours. BepiColombo did not register neither relevant perturbations of the solar wind nor solar energetic particles during the Na increase. The configuration of Mercury, Earth and Solar Orbiter enabled observations of the solar regions facing Mercury from both sides. They revealed that an active region facing Mercury, but hidden from Earth, produced a flare and an extreme-ultraviolet (EUV) brightening when the Na emission increased, whereas subsequent emissions from post-flare loops produced no further increase. While photon-stimulated desorption is one of the main mechanisms releasing Na from the surface, short-therm Na variability has so far been attributed mainly to plasma precipitation. Our observations suggest that solar flares can trigger rapid Na variability at Mercury, and point to a multi-phase release from a finite reservoir of surface sodium, reminiscent of the steeper than linear dependence on solar EUV reported for the Moon on longer timescales. This result marks a significant step forward in understanding the mechanisms governing the interaction of Mercury and the Moon with the Sun and it was possible thanks to multi-messenger observations highlighting the potential of investigations during BepiColombo nominal mission.

Context. Electrons accelerated by coronal shock waves can be remotely detected as type II solar radio bursts. Radio imaging of these bursts can be used to study the presence of shocks in the solar corona especially when other clear observations of the shock are missing at other wavelengths. Aims. Our aim is to investigate the origin and source locations of two type II solar radio bursts associated with a faint and slow coronal mass ejection (CME) using imaging and spectroscopic observations from the LOw Frequency ARray (LOFAR) together with a magnetohydrodynamic (MHD) model of the solar corona. Methods. We use dynamic spectra of the observed type II bursts to track them in frequency and time. We then use the imaging observations at the corresponding frequencies and times to see where the type II sources are located relative to the Sun. Finally, we combine the imaging observations with a global MHD model of coronal densities and magnetic field to de-project the radio source locations and investigate the properties of the emitting regions. Results. Imaging of the type II bursts shows that they originate from different regions in the corona. Combining LOFAR imaging with the MHD model shows that despite their separate source locations, both type II bursts originate along a coronal streamer characterised by high density and low Alfvèn speed. Conclusions. The different source locations of the type II bursts suggest that a single coronal shock driven by a weak CME can accelerate electrons at multiple separate locations along the shock front. The results indicate that a CME interacting with a streamer can create favourable conditions for the generation of type II bursts. The high density leads to low values of Alfvèn speed, and thus, even a slow CME can drive a shock capable of accelerating electrons in these regions.

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.

Context . The RADiation-hard Electron Monitor (RADEM) aboard the JUpiter ICy moons Explorer (JUICE) mission, launched on April 14, 2023, measures high-energy protons and electrons during the cruise phase and will continue to do so during the nominal mission phase. However, ground calibration results were unable to explain the initial flight observations, which prompted an in-flight calibration campaign. Aims . Our main goal was to calibrate RADEM and develop a procedure to compute particle fluxes from the count rates obtained by the RADEM detector heads. Methods . We used galactic cosmic rays (GCRs) to calibrate RADEM’s sensors by increasing the respective thresholds and therefore modifying their response to high-energy particles. We then compared the count rates obtained in flight for each threshold to theoretical count rates calculated using the Badhwar-O’Neill 2020 (BON2020) GCR model and threshold-dependent response functions. We used these results to develop a flux-reconstruction algorithm based on the bow-tie method. Results . We derived a new set of in-flight calibration coefficients for all sensors. In several cases, the in-flight calibration slopes differ by up to an order of magnitude from pre-flight ground calibration values. Proton fluxes from solar energetic particle (SEP) events, reconstructed using the bow-tie method, show good agreement (within a factor of two) with measurements from the SOlar and Heliospheric Observatory (SOHO). Conclusions . The RADEM provides accurate measurements of proton fluxes in interplanetary space and is well suited for both single-spacecraft analyses and coordinated multi-mission studies of SEPs. While electrons have been clearly identified during the JUICE Lunar-Earth gravity assist (LEGA), reconstructing their fluxes needs a more detailed analysis.

HEliospheric pioNeer for sOlar and interplanetary threats defeNce (HENON) is a 12U CubeSat that will explore for the first time ever the Distant Retrograde Orbit in the Sun-Earth system, bringing a payload suited for Space Weather observations and science. Initially designed for the Foresail-2 nanosatellite mission, the Relativistic Electron and Proton Experiment (REPE) instrument has since evolved for deployment in a variety of future missions, including the HENON mission. REPE is a particle telescope developed to measure fluxes of high-energy electrons and protons over broad ranges of energies, relevant to the space radiation environment. The instrument is designed to measure electron energy spectrum from 0.1 to 10.4 MeV and proton energy spectrum from 2 to hundreds of MeV. We present Monte Carlo simulations of REPE performance using Geant4. We evaluate the performance in terms of sensitivity (geometric factor), energy resolution, and cross-contamination between measured species. We show that the instrument meets the scientific requirements of the mission.

Aims. The aim of the this work is to investigate electron acceleration in shocks and present an easy-to-use simulation model that can be used to check resulting energetic particle populations from shock interactions. Methods. A new open-source model is presented in the work for investigation of electron acceleration and electron beam generation and for further research within the heliophysics community. The model is a one-dimensional Monte Carlo model with physical input parameters, in which particles obey a transport equation in a large-scale field with focusing caused by magnetic field gradients and in which the effects of a small-scale turbulent field on charged particles are described by pitch-angle scattering. The shock has a finite thickness and an adjustable mean free path profile. Particles are injected monoenergetically and with energies sampled from a Maxwellian distribution to investigate attained energies and beam generation. Results. The simulation results indicate that particles can be accelerated to energies of >100 keV from a 1 keV monoenergetic injection with plasma and shock parameters corresponding to coronal shock environments. An electron beam linked to radio observations of shock waves can also be generated with sufficiently high shock obliquities and, in particular, with a Maxwellian distribution of particle injection energies. Moreover, as the model performs computationally well and corresponds to expectations based on physics, it is an excellent tool for investigating energetic electrons and radio observations corresponding to the electron beams generated in shock waves.