CME Magnetic Structure and IMF Preconditioning Affecting SEP Transport

The transport of Solar Energetic Particles (SEPs) through the interplanetary space remains a chal- lenging aspect in space physics. The effects of the interplanetary magnetic field and solar wind turbulence on the SEP arrival to Earth has motivated the adoption of physics-based approaches in operational space weather forecasting. 3D physics-based models offer a framework for incor- porating the complexity of SEP transport mechanisms. The forecasting tool, SPARX (Marsh et al. (2015)), employs a relativistic full-orbit test particle approach to simulate SEP propagation to predict their arrival and help assess the potential hazard they may cause. Detection of a solar flare with a magnitude > M1.0 triggers the SPARX system to produce synthetic particle flux profiles from a database of SEP simulations to forecast observations that would be made at 1AU by a spacecraft. To enhance SPARX’s modeling capabilities, we focus here on validating the model through a systematic analysis of recent SEP events. The performance evaluation methodology implemented in Dalla et al. (2018) used a comprehensive list of X-class solar flares between 1997 and 2017 where for every flare, SPARX was operated in forecast mode. The outputs were analysed for SEP event prediction and performance metrics such as Probability of Detection (POD), False Alarm Ratio (FAR) and Critical Success Index (CSI) were calculated. This evaluation methodology will be adapted for events post 2017 to incorporate a larger dataset and thereby enabling a statistically robust validation of the tool. We chose a proton energy threshold of > 20 MeV for maintaining consistency for model evaluation. We present preliminary analysis of SPARX-generated synthetic SEP flux profiles against multi-spacecraft observations from SOLO, SOHO, and STEREO-A, assessing its current efficiency in reproducing key event characteristics such as onset times and intensity profiles. We discuss how these findings guide next-stage improvements in SPARX. This work aims to contribute to bridging the gap between 3D physics-based SEP modeling and real-time forecasting, ultimately advancing our capability to assess and mitigate SEP-driven space weather hazards. Future enhancement consists of incorporating cross-field transport of SEPs and evaluating its impact on SPARX’s forecasting accuracy.

Context. Solar energetic particle (SEP) events are critical for understanding particle acceleration and transport in the heliosphere. While most SEP events involve outward streaming particles along open magnetic field lines, bidirectional events characterized by simultaneous sunward and anti-sunward particle flows offer unique insights into magnetic field topology and the interplay of multiple acceleration sources. Aims. We investigate the origin and transport of energetic particles in two rare bidirectional anisotropic SEP events observed by Solar Orbiter, with a particular emphasis on their association with magnetic flux ropes. Methods. Energetic particles, solar wind plasma, magnetic field, and solar radio measurements were analysed. Via the velocity dispersion analysis, we determined release times and path lengths for distinct particle populations. Automated flux rope identification and magnetic helicity diagnostics were used to characterize magnetic flux ropes. Results. Both events showed two clear velocity dispersion signatures with opposite particle anisotropies during their onset phase. The sunward streaming protons, characterized by a delayed release time, a harder spectral index, and higher intensities, may be due to coronal mass ejection-driven shock acceleration, while the promptly released anti-sunward streaming protons are likely linked to flare acceleration. Notably, in both cases, small-scale flux ropes were identified in situ during the time intervals corresponding to the bidirectional particle streaming. Path lengths derived for sunward and anti-sunward injections were substantially greater than nominal values of the Parker field lines, further evidence of the role of the flux rope in shaping particle trajectories. Conclusions. These observations demonstrate that magnetic flux ropes can significantly affect magnetic connectivity to the source region and SEP propagation in the inner heliosphere, and that simultaneous velocity dispersion from two distinct particle sources can be used to place direct constraints on the topology of the flux rope. Our results highlight the value of combining particle anisotropy, release time, source spectra, and magnetic structure diagnostics to unravel SEP transport in complex transient magnetic structures, and also present new challenges for the current SEP transport model.

Space weather forecasting aims at predicting the impacts of the Sun, interplanetary space, and the planetary environment on biological and technological systems. To monitor space weather, the European Space Agency is developing the Vigil mission. Vigil will carry the Plasma Analyser (PLA) instrument. We investigate the expected impact of Solar Energetic Particles (SEPs) on PLA. We analyse previous measurements from Solar Orbiter’s Solar Wind Analyser (SWA) Electron Analyser System (EAS) that, like PLA, uses a microchannel plate (MCP) as its detector. Using a fitting algorithm, we extract the number of erroneous counts created by the interaction between SEPs and the MCP. Using SEP flux measurements from Solar Orbiter’s Energetic Particle Detector (EPD) and multilinear regression, we establish a linear relationship between the number of erroneous counts and SEP fluxes. We define an SEP index, characterizing the impact of SEP events on EAS measurements. We then simulate PLA measurements during SEP events characterized by their SEP index. We show that moderate SEP events cause significant discrepancies in PLA plasma moment estimations under common solar-wind conditions. Based on our results, we propose a correction method for erroneous counts created by SEPs in instruments such as EAS and PLA.

The dosimeter Liulin-MO for measuring the radiation environment on board the ExoMars Trace Gas Orbiter (TGO) is a module in the Fine Resolution Epithermal Neutron Detector (FREND). A number of solar energetic particle (SEP) events were observed in Mars orbit from July 2021 to 2024 during the increasing phase and close to the maximum of the 25th solar cycle activity. The results from the SEPs measurements obtained in 2021-2023 by Liulin-MO have been previously reported. Here we present the Liulin-MO results from the observation of the radiation parameters of the SEP events during January- October 2024. The most powerful SEP event registered up to now in TGO orbit started on 20 May 2024. The maximum dose rate during this SEP event has been 2800 ± 280 µGy h-1 and the maximum particle flux - 383 ± 19 cm-2 s-1. The total event lasted for about 64 hours up to 24 May with a long tail of increased dose rates and fluxes. The total dose from SEPs for the 64 hours of the main phase of the SEP event was 24.7 ± 2.5 mGy. The total dose from SEPs during this event is equal to the dose from the galactic cosmic rays (GCR) received for about 200 days at this phase of solar cycle 25. The total dose from all SEPs during January - September 2024 is 36.6 mGy (in Si), which is approximately equal to the dose received from GCR for the same period. The observations of SEPs in Mars orbit are compared to the observations during the same periods of proton fluxes measured by the GOES satellite in Earth orbit. The results show that some of the SEPs observed in Mars orbit, excluding the biggest SEP events of 20-24 May and 05-07 September, are also seen in the GOES proton fluxes data. SEP events recorded both in Mars and Earth orbits are related to coronal mass ejections (CMEs) observed by the SOHO and STEREO A coronagraphs. The paper shows that responsible for most of the SEP events registered both in the Liulin-MO data and in the GOES proton fluxes data are halo CMEs. The paper also shows that the sources of the three most powerful SEP events in Mars orbit - those of 20 May, 23 July and 05 September - are halo CMEs from the far side of the Sun. Some of these CMEs are associated with major X class far-side flares. Long-term investigations of the GCRs radiation parameters in Mars orbit show that in August 2024 (the last month of our data with no recorded SEP events) the dose rate was 6.5 ± 0.65 µGy h-1 and the particle flux - 1.4 ± 0.07 cm-2 s-1. These values are about 40 % of the corresponding maximal values measured by Liulin-MO during the solar cycle 24 minimum in March 2020. The above results show the importance of long-term measurements (at least during a full solar cycle) of the radiation conditions in Mars vicinity. Such measurements will make it possible to obtain the data necessary for the planning of future manned and robotic missions, as well as for the selection of the best time interval in the solar cycle for a manned flight to the planet.

We examine the angular distributions of He, O, and Fe in large solar energetic particle (SEP) events measured on the Wind spacecraft. We report, for the first time, that in a fixed velocity interval, Fe/O is often larger for particles flowing sunward along the magnetic field than for particles flowing outward from the Sun in many SEP events. This occurs because the anisotropy for O exceeds that for Fe, even though both species are streaming outward. There are no examples of events for which the outward Fe/O dominates. The behavior of Fe and O conflicts with the expectations of simple diffusion theory, that angular distributions should be independent of species. It also seems to conflict with the idea that energetic Fe scatters less than O of the same velocity. However, preliminary modeling suggests that the presence of a reflecting magnetic boundary beyond 1 AU, together with the increased scattering of O over Fe due to proton-generated Alfvén waves, can explain the direction and magnitude of the effect. These observations add a new dimension to the study of SEP transport.

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

Large solar energetic particles (SEPs) can cause adverse space weather hazards to human technology, and such events are especially associated with halo coronal mass ejections (CMEs). But in turn, a significant portion of halo CMEs are not associated with large SEPs. The objective of this study is to gain an understanding of the source region distinctions between halo CMEs in SEP and non-SEP events. Among the 176 halo CMEs observed from 2010 to 2024, we screen out 45 large SEP events and 131 non-SEP events from this data set. It is revealed that CME speed is a good discriminator between SEP and non-SEP events. Through classifying the source regions of all the halo CMEs, we find that 53% of SEP events originate from “single AR (active region),” and 47% from “multiple ARs” or “outside of ARs.” The corresponding proportion for non-SEP events is 70% and 30%. This suggests that SEP source regions are more likely to originate from large-scale sources. We have also calculated the relevant magnetic parameters of the source regions and found that SEP source regions have higher magnetic free energy and reconnection flux compared to non-SEP source regions. However, SEP source regions are smaller in terms of the intensive magnetic parameters such as mean characteristic magnetic twist α and mean shear angles. Our statistical results can provide new potential variables for forecasting SEPs.

Transient energetic (E> 10 MeV) particle events from the Sun have been observed at the Earth for half a century using several detection techniques. We review these observations and the change in focus from solar flares to fast coronal mass ejections (CMEs) and shocks as the causal agents of solar energetic particle (SEP) events. We review the properties of SEP events that are important for space weather: 1) the number distributions of peak SEP intensities and of SEP fluences; 2) the relationship of SEP intensities to CME speeds; 3) when SEP events occur during the solar cycle; and 4) where SEP events occur relative to the CME-driven shocks. We conclude with a discussion of two cases of direct negative effects of SEPs on space experiments.

A heliospheric simulation-based approach to SEP source and transport modeling

This study is based on proton flux data from ESA’s Solar Energetic Particle Environment Modeling database, analyzing solar energetic particle (SEP) events observed near Earth from 1976 to 2017. We have matched these events with corresponding solar flares and coronal mass ejections (CMEs) to create a comprehensive list of SEP events spanning almost four solar cycles. Using this list, we conduct a statistical analysis of the flare–SEP correlation and SEP timing information. Our findings confirm that SEP events recorded at 1 au are more commonly associated with flares occurring in the western hemisphere of the Sun’s surface. Moreover, we develop an empirical model relating the SEP proton peak flux to the characteristics of flares and CMEs, which works particularly effectively for SEP events triggered by eastern flares. We also verify that flares with higher intensity are more likely to produce SEP events up to higher energies, such as those detected above 100 MeV. In this energy range, more than half of the SEP events are triggered by X-class flares. Finally, we introduce an empirical model that uses the flare longitude to predict the timing of the SEP peak flux, especially for intense flare events. These models can aid in mitigating particle radiation risks for space missions.

To improve the forecasting capability of impactful solar energetic particle (SEP) events, the relation between coronal mass ejections (CMEs) and SEP events needs to be better understood. Here we present a statistical study of SEP occurrences and timescales with respect to the CME source locations and speeds, considering all 257 fast (v CME ≥ 900 km s−1) and wide (angular width ≥60°) CMEs that occurred between 2006 December and 2017 October. We associate them with SEP events at energies above 10 MeV. Examination of the source region of each CME reveals that CMEs more often accompany a SEP event if they originate from the longitude of E20–W100 relative to the observer. However, an SEP event could still be absent if the CME is <2000 km s−1. For the associated CME–SEP pairs, we compute three timescales for each of the SEP events, namely the timescale of the onset (TO), the rise time (TR), and the duration (TD). They are correlated with the longitude of the CME source region relative to the footpoint of the Parker spiral (ΔΦ) and v CME. The TO tends to be short for 60°. This trend is weaker for TR and TD. The SEP timescales are only weakly correlated with v CME. Positive correlations of both TR and TD with v CME are seen in poorly connected (large ) events. Additionally, TO appears to be negatively correlated with v CME for events with small .

Understanding the transport of solar energetic particles (SEPs) from acceleration sites at the Sun into interplanetary space and to the Earth is an important question for forecasting space weather. The interplanetary magnetic field (IMF), with two distinct polarities and a complex structure, governs energetic particle transport and drifts. We analyze for the first time the effect of a wavy heliospheric current sheet (HCS) on the propagation of SEPs. We inject protons close to the Sun and propagate them by integrating fully 3D trajectories within the inner heliosphere in the presence of weak scattering. We model the HCS position using fits based on neutral lines of magnetic field source surface maps (SSMs). We map 1 au proton crossings, which show efficient transport in longitude via HCS, depending on the location of the injection region with respect to the HCS. For HCS tilt angles around 30°–40°, we find significant qualitative differences between A+ and A− configurations of the IMF, with stronger fluences along the HCS in the former case but with a distribution of particles across a wider range of longitudes and latitudes in the latter. We show how a wavy current sheet leads to longitudinally periodic enhancements in particle fluence. We show that for an A+ IMF configuration, a wavy HCS allows for more proton deceleration than a flat HCS. We find that A− IMF configurations result in larger average fluences than A+ IMF configurations, due to a radial drift component at the current sheet.

It has been inferred from theory that the spatial diffusion coefficient ( κ ) upstream of shocks is anticorrelated with the intensity of solar energetic particles (SEPs) at the shock ( j shock ) motivated by quasi-linear theory (QLT). This is because a lower κ along the magnetic field ( κ ∥ ) implies that particles are trapped for longer, providing more acceleration and resulting in a higher j shock . However, the simplest version of DSA predicts that j shock is determined by the source of the injected population at the shock and plasma density jump with no relation to κ for low-energy SEPs. Here, we identify the relationship between κ and j shock , whose form is unknown, using Parker Solar Probe observations of eight shocks within 1 au. We estimate a characteristic κ fit along the shock normal by fitting the upstream SEP intensity profiles with a 1D steady-state transport model for acceleration and escape assuming pitch-angle isotropy in the plasma frame. Also, we estimate κ ∥ based on the magnetic power spectral density using QLT for comparison with κ fit . Our results show that both quantities are anticorrelated with j shock . Instead of a uniform relationship between κ and j shock , we find distinct relationships appearing as potential power laws manifested across SEP events with no obvious radial dependence from 0.07 to 0.74 au. These relationships may be grouped by similar shock parameters (in terms of speed, strength, and orientation). Our findings raise questions about SEP transport and its radial dependence within 1 au and provide important observational constraints for models of shock-accelerated particles.

The martian atmosphere is sensitive to disturbances in interplanetary space due to the absence of a strong planetary magnetic field. Solar energetic particle (SEP) events comprise high-energy, electrically-charged sub-atomic particles and are produced during solar flares and coronal mass ejections. Previous work has shown that SEPs result in diffuse aurorae, disruption of radio propagation, the dispersion of atmospheric compounds, and the ionisation of atmospheric layers. In this study, we explore the relationship between SEP events and lower-atmospheric heating at Mars. Five SEP events with durations of four days or longer were identified in the years 2020-2021. Measurements from the Mars Atmosphere And Volatile EvolutioN (MAVEN) mission and the Trace Gas Orbiter (TGO) spacecraft are compared to atmospheric temperature profiles derived from the Mars Climate Database. Specifically, Mars’ lower-atmospheric temperature profiles before, during and after the SEP events are analysed. No strong evidence is found that indicates SEP events lead to the heating of Mars’ atmosphere. However, in the one case, a SEP event occurred concurrently with an expanding global dust storm. In this case, a clear heating effect is observed, but further research is required to attribute atmospheric temperature variations as a result of the global dust storms and SEP events where the two occur simultaneously.

Solar energetic particle (SEP) events are related to flares and coronal mass ejections (CMEs). This work is a new investigation of statistical relationships between SEP peak intensities – deka-MeV protons and near-relativistic electrons – and characteristic quantities of the associated solar activity. We consider the speed of the CME and quantities describing the flare-related energy release: peak flux and fluence of soft X-ray (SXR) emission and the fluence of microwave emission. The sample comprises 38 SEP events associated with strong SXR bursts (classes M and X) in the western solar hemisphere between 1997 and 2006, in which the flare-related particle acceleration was accompanied by radio bursts indicating electron escape into the interplanetary space. The main distinction of the present statistical analysis from earlier work is that in addition to the classical Pearson correlation coefficient, the partial correlation coefficients are calculated to remove the correlation effects between the solar parameters themselves. The classical correlation analysis shows the usual picture of correlations with broad scatter between SEP peak intensities and the different parameters of solar activity and strong correlations between the solar activity parameters themselves. The partial correlation analysis shows that the only parameters that significantly affect the SEP intensity are the CME speed and the SXR fluence. The SXR peak flux and the microwave fluence make no additional contribution. We conclude that these findings bring statistical evidence that both flare acceleration and CME shock acceleration contribute to the deka-MeV proton and near-relativistic electron populations in large SEP events.