The Proton Alpha Sensor (PAS), part of the Solar Wind Analyzer (SWA) onboard Solar Orbiter, has been designed to measure the full 3D ion velocity distribution function (VDF) at time cadence larger than one sample per second, thus, faster than the typical proton cyclotron period (Burst mode). Unfortunately, due to software difficulties, this capability to explore ion kinetic processes has been only activated during the first months of operation (2020-mid 2021) so that the ‘normal mode’ cadence (1 VDF each 4 s) was systematically used from 2021 to 2024. After in-depth analysis, a new software version has finally been implemented at the end of 2024, restoring in part the PAS burst mode capability. As a result, an impressive set of continuous full 3D VDF measurements at 1 s cadence has been obtained in 2025. More recently, we have fully restored the PAS burst mode. Since December 2025, PAS is then operated at a continuous 1 s cadence, with 8 bursts of 5 minutes per day during which full 3D measurements at 2 or 4 Hz, thus below the proton cyclotron period, are performed. As illustrated by examples (waves, sharp gradients, turbulence, shocks), this obviously re-opens a window to study various dynamical plasma phenomena at ion kinetic scales.

Solar energetic particle (SEP) events, particularly those of significant magnitude, are commonly associated with fast and wide coronal mass ejections (CMEs). These CMEs generate and drive shock waves in the solar corona, proving to be highly efficient in particle acceleration to high energies. Understanding the intricate connections between shock wave properties and SEP characteristics is crucial for advancing Space Weather forecasting.To achieve this objective, we employ a methodology to analyze a SEP event involving a coronal shock wave, observed by several spacecraft well distributed around the Sun. Initially, we reconstruct the 3D ellipsoidal shape of the expanding shock, enabling the extraction of its geometry and kinematic properties. Using magneto-hydrodynamics (MHD) cubes, we then reconstruct the magnetic connectivity of spacecrafts and retrieve the MHD properties of the shock wave at the intersections with these magnetic field lines. The temporal correlations between the shock properties and the SEPs recorded by individual spacecraft can finally be compared.Through the application of this methodology, we identify enhanced correlation coefficients between SEPs and shock parameters, such as speed, Alfvénic Mach Number, and theta_BN (the angle between the shock's normal and the magnetic field line). This work is funded by the H2020 SERPENTINE project.

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).

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).

Powerful solar eruptions are known to produce fast and wide shock waves in the solar corona and inner heliosphere. The relationship between the coronal shock waves, solar energetic particles and different types of radio emission is a subject of long-lasting research activity. In this work, we perform a case study of 31 December 2023 eruption that occurred near eastern limb of the Sun. It produced a X5.0 class X-ray flare, a global EUV wave, a fast ∼3000 km/s Coronal Mass Ejection, strong radio emissions (including several type III and type II bursts), solar energetic particles in-situ, and long duration high-energy gamma-ray emission. We employ a technique that combines the reconstructed coronal shock from observations with background coronal MHD simulations to produce shock-mediated synthetic radio spectrum, assuming local emission at plasma frequency. We show that transient high Mach number and quasi-perpendicular coronal shock region explains both a “hot flux tube” precursor seen in EUV observations and reverse drifting radio spectral features observed by ground-based facilities. The occurrence of this evanescent strong shock patch was observed when it propagated across pseudo-streamer's cusp where the magnetic field was particularly low. We also find evidence that, at higher coronal altitudes, the low-frequency type II radio burst detected by several spacecraft, is triggered by the interaction of the shock with the heliospheric current sheet. This study provides additional evidence that high-M_A regions of coronal shock surface are instrumental in energetic particle phenomenology.

Context.In the solar wind (SW), the particle distribution functions are generally not Gaussian. They present nonthermal features that are related to underlying acceleration and heating processes. These processes are critical in the overall dynamics of this expanding astrophysical fluid.Aims.The Proton Alpha Sensor (PAS) on board Solar Orbiter commonly observes skewed proton distributions, with a more populated high-energy side in the magnetic field direction than the Gaussian distribution. Our objectives are: (1) to identify a theoretical statistical function that adequately models the observed distributions and (2) to use its statistical interpretation to constrain the acceleration and heating processes.Methods.We analyzed the 3D velocity distribution functions (VDFs) measured by PAS and compared them to model statistical functions.Results.We show that the normal inverse Gaussian (NIG), a type of hyperbolic statistical distribution, provides excellent fits of skewed and leptokurtic proton distributions. NIG can model both the core distribution and the beam, if present. We propose an interpretation that is inspired by the mathematical formulation of the NIG. It assumes that the acceleration or heating mechanism can be modeled as a drifting diffusion process in velocity space, controlled (or subordinated) by the time of interaction of the particles with “accelerating structures”. The probability function of the interaction time is an inverse Gaussian (IG), obtained by considering a random drift across structures of a given size. The control of the diffusion by interaction times that follow an IG probability function formally defines the NIG distribution. Following this model, we show that skewness and kurtosis can be used to estimate the kinetic and thermal energy gains provided by the interaction with structures. For example, in the case studies presented here, the analyzed populations would have gained kinetic energy representing approximately two to four times their thermal energy, with an increase in velocity – due to acceleration – of from one-tenth to one-third of the observed flow velocity. We also show that the model constrains the initial temperature of the populations.Conclusions.Overall, the NIG model offers excellent fits of the observed proton distributions. Combining the skewness and the kurtosis, it also leads to constraints in the part of acceleration and heating due to the interactions with structures in the formation of the proton populations. We suggest that these effects add to the classical thermal evolution of the bulk velocity and temperature resulting from SW expansion.

The Parker Solar Probe's discovery that magnetic switchbacks and velocity spikes in the young solar wind are abundant has prompted intensive research into their origin(s) and formation mechanism(s) in the solar atmosphere. Recent studies, based on in situ measurements and numerical simulations, argue that velocity spikes are produced through interchange magnetic reconnection. Our work studies the relationship between interplanetary velocity spikes and coronal brightenings induced by changes in the photospheric magnetic field. Our analysis focuses on the characteristic periodicities of velocity spikes detected by the Proton Alpha Sensor on the Solar Orbiter during its fifth perihelion pass. Throughout the time period analyzed here, we estimate their origin along the boundary of a coronal hole. Around the boundary region, we identify periodic variations in coronal brightening activity observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. The spectral characteristics of the time series of in situ velocity spikes, remote coronal brightenings, and remote photospheric magnetic flux exhibit correspondence in their periodicities. Therefore, we suggest that the localized small-scale magnetic flux within coronal holes fuels a magnetic reconnection process that can be observed as slight brightness augmentations and outward fluctuations or jets. These dynamic elements may act as mediators, bonding magnetic reconnection with the genesis of velocity spikes and magnetic switchbacks.

Context. Shock waves driven by fast and wide coronal mass ejections (CMEs) are considered to be very efficient particle accelerators and are involved in the production of solar energetic particle (SEP) events. These events cause space weather phenomena by disturbing the near-Earth radiation environment. In past studies, we analysed statistically the relation between the maximum intensity of energetic electrons and protons and the properties of coronal shocks inferred at the point of magnetic connectivity. The present study focuses on a gradual SEP event measured by STEREO-A and -B on 11 October 2013. This event had the interesting properties that it (1) occurred in isolation with very low background particle intensities measured before the event, (2) was associated with a clear onset of SEPs measured in situ allowing detailed timing analyses, and (3) was associated with a fast CME event that was magnetically connected with STEREO-A and -B. These three properties allowed us to investigate at a high cadence the temporal connection between the rapidly evolving shock properties and the SEPs measured in situ. Aims. The aim of the present study is to investigate the relative roles of fundamental shock parameters such as the compression ratio, Mach number and geometry, in the intensity and composition of the associated SEP event measured in situ. Methods. We used shock reconstruction techniques and multi-viewpoint imaging data obtained by the STEREO-A and -B, SOHO, and SDO spacecraft to determine the kinematic evolution of the expanding shock wave. We then exploited 3D magneto-hydrodynamic modelling to model the geometry and Mach number of the shock wave along an ensemble of magnetic field lines connected to STEREO-A and -B, also estimating the uncertainties of the shock parameters. Using a velocity dispersion analysis of the available SEP data we time-shifted the SEP time series and analysed the relations between observed SEP properties and the modelled shock properties. We also studied the energy dependence of these relations. Results. We find a very good temporal agreement between the formation of the modelled shock wave and the estimated release times for both electrons and protons. The simultaneous release of protons and electrons suggests a common acceleration process. This early phase is marked at both STEREOs by elevated electron-to-proton ratios that coincide with the highly quasi-perpendicular phase of the shock. These findings suggest that the rapid evolution of the shock as it transits from the low to the high corona modifies the conditions under which particles are accelerated. We discuss these findings in terms of basic geometry and acceleration processes.

Context. The possibility of slow diffusion regions as the origin for extended TeV emission halos around some pulsars (such as PSR J0633+1746 and PSR B0656+14) challenges the standard scaling of the electron diffusion coefficient in the interstellar medium. Aims. Self-generated turbulence by electron–positron pairs streaming out of the pulsar wind nebula was proposed as a possible mechanism to produce the enhanced turbulence required to explain the morphology and brightness of these TeV halos. Methods. We perform fully kinetic 1D3V particle-in-cell simulations of this instability, considering the case where streaming electrons and positrons have the same density. This implies purely resonant instability as the beam does not carry any current. Results. We compare the linear phase of the instability with analytical theory and find very reasonable agreement. The non-linear phase of the instability is also studied, which reveals that the intensity of saturated waves is consistent with a momentum exchange criterion between a decelerating beam and growing magnetic waves. With the adopted parameters, the instability-driven wavemodes cover both the Alfvénic (fluid) and kinetic scales. The spectrum of the produced waves is non-symmetric, with left-handed circular polarisation waves being strongly damped when entering the ion-cyclotron branch, while right-handed waves are suppressed at smaller wavelength when entering the Whistler branch. The low-wavenumber part of the spectrum remains symmetric when in the Alfvénic branch. As a result, positrons behave dynamically differently compared to electrons. The final drift velocity of positrons can maintain a larger value than the ambient Alfvén speed V A while the drift of electrons can drop below V A . We also observed a second harmonic plasma emission in the wave spectrum. An MHD-PIC approach is warranted to probe hotter beams and investigate the Alfvén branch physics. We provide a few such test simulations to support this assertion. Conclusions. This work confirms that the self-confinement scenario develops essentially according to analytical expectations, but some of the adopted approximations (like the distribution of non-thermal particles in the beam) need to be revised and other complementary numerical techniques should be used to get closer to more realistic configuration.

Coronal and interplanetary shock waves produced by coronal mass ejections (CMEs) are major drivers of space-weather phenomena, inducing major changes in the heliospheric radiation environment and directly perturbing the near-Earth environment, including its magnetosphere. A better understanding of how these shock waves evolve from the corona to the interplanetary medium can therefore contribute to improving nowcasting and forecasting of space weather. Early warnings from these shock waves can come from radio measurements as well as coronagraphic observations that can be exploited to characterise the dynamical evolution of these structures. Our aim is to analyse the geometrical and kinematic properties of 32 CME shock waves derived from multi-point white-light and ultraviolet imagery taken by the Solar Dynamics Observatory (SDO), Solar and Heliospheric Observatory (SoHO), and Solar-Terrestrial Relations Observatory (STEREO) to improve our understanding of how shock waves evolve in 3D during the eruption of a CME. We use our catalogue to search for relations between the shock wave's kinematic properties and the flaring activity associated with the underlying genesis of the CME piston. Past studies have shown that shock waves observed from multiple vantage points can be aptly reproduced geometrically by simple ellipsoids. The catalogue of reconstructed shock waves provides the time-dependent evolution of these ellipsoidal parameters. From these parameters, we deduced the lateral and radial expansion speeds of the shocks evolving over time. We compared these kinematic properties with those obtained from a single viewpoint by SoHO in order to evaluate projection effects. Finally, we examined the relationships between the shock wave and the associated flare when the latter was observed on the disc by considering the measurements of soft and hard X-rays.

Interplanetary (IP) shocks are fundamental building blocks of the heliosphere, and the possibility to observe them in-situ is crucial to address important aspects of energy conversion for a variety of astrophysical systems. Steepened waves known as shocklets are known to be important structures of planetary bow shocks, but they are very rarely observed related to IP shocks. We present here the first multi-spacecraft observations of shocklets observed by upstream of an unusually strong IP shock observed on November 3rd 2021 by several spacecraft at L1 and near-Earth solar wind. The same shock was detected also by radially aligned Solar Orbiter at 0.8 AU from the Sun, but no shocklets were identified from its data, introducing the possibility to study the environment in which shocklets developed. The Wind spacecraft has been used to characterise the shocklets, associated with pre-conditioning of the shock upstream by decelerating incoming plasma in the shock normal direction. Finally, using the Wind observations together with ACE and DSCOVR spacecraft at L1, as well as THEMIS B and THEMIS C in the near-Earth solar wind, the portion of interplanetary space filled with shocklets is addressed, and a lower limit for its extent is estimated to be of about 110 R_E in the shock normal direction and 25 R_E in the directions transverse to the shock normal. Using multiple spacecraft also reveals that for this strong IP shock, shocklets are observed for a large range of local obliquity estimates (9-64 degrees).

The mechanisms that produce solar energetic particles (SEPs) are still highly debated but coronal shock waves have been proposed as efficient particle accelerators that may be implicated in the production of SEPs.An analysis of 32 Coronal Mass Ejections (CMEs) that produced strong pressure waves in the solar corona during their eruption has been done. For each event Kouloumvakos et al. (2019) exploited remote-sensing observations from multiple vantage points to reconstruct their 3-D ellipsoidal shapes. This catalogue of shock waves provides important statistical information on their kinematic evolution that we report in Jarry et al. (2023) together with their relation to X-ray flaring activitiy.Different SEPs exhibit significant spectral and compositional variability. We looked for links between the composition of SEPs including abundance ratios (such as Fe/O) and shock parameters (Mach number, shock geometry, ..) that typically evolves rapidly along the magnetic field lines connected to the spacecraft recording SEPs.This work was funded by the H2020 SERPENTINE project.

Context. On 2020 November 29, an eruptive event occurred in an active region located behind the eastern solar limb as seen from Earth. The event consisted of an M4.4 class flare, a coronal mass ejection, an extreme ultraviolet (EUV) wave, and a white-light (WL) shock wave. The eruption gave rise to the first widespread solar energetic particle (SEP) event of solar cycle 25, which was observed at four widely separated heliospheric locations (∼230°). Aims. Our aim is to better understand the source of this widespread SEP event, examine the role of the coronal shock wave in the wide distribution of SEPs, and investigate the shock wave properties at the field lines magnetically connected to the spacecraft. Methods. Using EUV and WL data, we reconstructed the global three-dimensional structure of the shock in the corona and computed its kinematics. We determined the magnetic field configurations in the corona and interplanetary space, inferred the magnetic connectivity of the spacecraft with the shock surface, and derived the evolution of the shock parameters at the connecting field lines. Results. Remote sensing observations show formation of the coronal shock wave occurring early during the eruption, and its rapid propagation to distant locations. The results of the shock wave modelling show multiple regions where a strong shock has formed and efficient particle acceleration is expected to take place. The pressure/shock wave is magnetically connected to all spacecraft locations before or during the estimated SEP release times. The release of the observed near-relativistic electrons occurs predominantly close to the time when the pressure/shock wave connects to the magnetic field lines or when the shock wave becomes supercritical, whereas the proton release is significantly delayed with respect to the time when the shock wave becomes supercritical, with the only exception being the proton release at the Parker Solar Probe. Conclusions. Our results suggest that the shock wave plays an important role in the spread of SEPs. Supercritical shock regions are connected to most of the spacecraft. The particle increase at Earth, which is barely connected to the wave, also suggests that the cross-field transport cannot be ignored. The release of energetic electrons seems to occur close to the time when the shock wave connects to, or becomes supercritical at, the field lines connecting to the spacecraft. Energetic protons are released with a time-delay relative to the time when the pressure/shock wave connects to the spacecraft locations. We attribute this delay to the time that it takes for the shock wave to accelerate protons efficiently.

Context.The Alfvénic slow solar wind is of particular interest, as it is often characterized by intense magnetic turbulence, complex proton 3D velocity distribution functions (VDF), and an ensuing richness of kinetic and dynamic processes.Aims.We take advantage of the fast time cadence of measurements taken by the Proton-Alpha Sensor (PAS) on board Solar Orbiter to analyze the kinetic properties of the proton population, the variability of their VDFs, and the possible link with propagating magnetic structures. We also study the magnetic (B) and velocity (V) correlation that characterizes this type of wind down to the ion gyroperiod.Methods.We analyzed the VDFs measured by PAS, a novelty that take advantages of the capability of 3D measurements at a 4 Hz cadence. In addition, we considered MAG observations.Results.We first show that there is a remarkable correlation between theBandVcomponents observed down to timescales approaching the ion gyrofrequency. This concerns a wide variety of fluctuations, such as waves, isolated peaks, and discontinuities. The great variability of the proton VDFs is also documented. The juxtaposition of a core and a field-aligned beam is the norm but the relative density of the beam, drift speed, and temperatures can considerably change on scales as short as as a few seconds. The characteristics of the core are comparatively more stable. These variations in the beam characteristics mostly explain the variations in the total parallel temperature and, therefore, in the total anisotropy of the proton VDFs. Two magnetic structures that are associated with significant changes in the shape of VDFs, one corresponding to relaxation of total anisotropy and the other to its strong increase, are analyzed here. Our statistical analysis shows a clear link between total anisotropy (and, thus, beam characteristics) and the direction ofBwith respect to the Parker spiral. In the present case, flux tubes aligned with Parker spiral contain an average proton VDF with a much more developed beam (thus, with larger total anisotropy) than those that are inclined, perpendicular, or even reverse with regard to the outward direction.Conclusions.These observations document the variability of the proton VDF shape in relation to the propagation of magnetic structures. This is a key area of interest for understanding of the effect of turbulence on solar wind dynamics.

Context.Spacecraft data revealed early on the frequent observation of multiple near-thermal proton populations in the solar wind. Decades of research on their origin have focused on processes such as magnetic reconnection in the low corona and wave-particle interactions in the corona and locally in the solar wind.Aims.This study aims to highlight the fact that such multiple thermal proton populations and beams are also produced by magnetic reconnection occurring locally in the solar wind.Methods.We used high-resolution Solar Orbiter proton velocity distribution function measurements, complemented by electron and magnetic field data, to analyze the association of multiple thermal proton populations and beams with magnetic reconnection during a period of slow Alfvénic solar wind on 16 July 2020.Results.At least six reconnecting current sheets with associated multiple thermal proton populations and beams, including a case of magnetic reconnection at a switchback boundary, were found on this day. This represents 2% of the measured distribution functions. We discuss how this proportion may be underestimated, and how it may depend on solar wind type and distance from the Sun.Conclusions.Although suggesting a likely small contribution, but which remains to be quantitatively assessed, Solar Orbiter observations show that magnetic reconnection must be considered as one of the mechanisms that produce multiple thermal proton populations and beams locally in the solar wind.

The origin of the gamma-ray halo around pulsars is associated with the reduced diffusivity of energetic particles responsible for gamma-ray emission with respect to the mean-free path they adopt in the interstellar medium. A possible explanation for this behaviour is that the energetic particles released from the pulsarwind termination shock themselves trigger the turbulence necessary to explain this reduced diffusivity. In order to test the ability of the electron-positron beam to trigger an efficient streaming instability we are in the process of conducting a series of b simulations using both the PIC-MHD technique to follow the evolution of both the electron-positron beam and the thermal background plasma and determine whether the beam can trigger the necessary instabilities. We find that the passage of the electron-positron beam through the thermal plasma triggers streaming instabilities that lead to local amplification of the magnetic field.

The origin of hard X-rays and γ -rays emitted from the solar atmosphere during occulted solar flares is still debated. The hard X-ray emissions could come from flaring loop tops rising above the limb or coronal mass ejection shock waves, two by-products of energetic solar storms. For the shock scenario to work, accelerated particles must be released on magnetic field lines rooted on the visible disk and precipitate. We present a new Monte Carlo code that computes particle acceleration at shocks propagating along large coronal magnetic loops. A first implementation of the model is carried out for the 2014 September 1 event, and the modeled electron spectra are compared with those inferred from Fermi Gamma-ray Burst Monitor (GBM) measurements. When particle diffusion processes are invoked, our model can reproduce the hard electron spectra measured by GBM nearly 10 minutes after the estimated on-disk hard X-rays appear to have ceased from the flare site.

Energetic non-thermal particles, or cosmic rays, are a major component of astrophysical plasmas next to magnetic fields, radiation, and thermal gas. Cosmic rays are usually sub-dominant in density but carry as much pressure as the thermal plasma background. In some cases, cosmic rays drift at faster speeds with respect to the normal modes' phase speeds of the background plasma. Because of this, cosmic rays are a strong source of free energy that causes new classes of kinetic or convective instabilities. Recent years have seen the development of intense analytical and numerical efforts to analyze the onset of an instability produced by the motion of these particles at fast bulk speeds: this is the streaming instability. The streaming instability has been applied to different space plasmas and astrophysical contexts like strong shocks, jets, or in interstellar and intergalactic medium studies. Streaming instabilities participate in the production of magnetic turbulence at scales corresponding to the gyroradius of the particles. By scattering off their self-generated waves, cosmic rays are coupled to the background thermal plasma. This mechanism is able to self-confine cosmic rays around sources and launch winds out of the disk of the galaxy, hence impacting galactic matter dynamics and ultimately the galactic star formation rate. We discuss a few science cases, which should be accessible in the near future for analytical calculations and numerical simulations.

Accessing, visualizing and analyzing heterogeneous plasma datasets has always been a tedious task that hindered students and senior researchers as well. Offering user friendly and versatile tools to perform basic research tasks is therefore pivotal for data centres including the Centre de Données de la Physique des Plasmas (CDPP http://www.cdpp.eu/) which holds a large variety of plasma data from various Earth, planetary and heliophysics missions and observatories in plasma physics. This clearly helps gaining increased attention, relevant feedback, and enhanced science return on data. These are the key ideas that crystallized at CDPP more than 15 years ago and resulted in the lay-out of the concepts, and then development, of AMDA, the Automated Multi-Dataset Analysis software (http://amda.cdpp.eu/). This paper gives a description of the architecture of AMDA, describes its functionalities, presents some use cases taken from the literature or fruitful collaborations and shows how it offers unique capabilities for educational purposes.