Nicolas Wijsen

Context . To enable timely protective actions against severe space weather, it is essential to develop advanced Sun-to-Earth magnetohydrodynamic (MHD) models to deliver faster-than-real-time, high-fidelity, and comprehensive space weather forecasts. Aims . For this study, we modelled the evolution of the solar corona and wind from the Sun to Earth using a stand-alone, time-evolving MHD model to show that an implicit MHD method can significantly simplify and improve the overall Sun-to-Earth modelling pipeline. Methods . We extended the implicit time-evolving coronal MHD model COCONUT up to 1 AU, and utilised it to investigate solar coronal and wind evolutions around a solar maximum Carrington rotation (CR). We compared quasi-steady-state and time-evolving Sun-to-Earth simulations to evaluate the impact of the inner-boundary magnetic field evolution, which is neglected in steady-state simulations, on background plasma parameters. Comparisons with commonly used coupled Sun-to-Earth simulations were also conducted to further validate and assess the Sun-to-Earth model COCONUT. Results . The results show that the time-evolving implicit MHD modelling approach yields noticeable differences compared with oversimplified steady-state simulations, while remaining sufficiently computationally efficient and numerically stable to model the entire Sun-to-Earth domain with a single model. This approach greatly simplifies the simulation pipeline and avoids uncertainties associated with coupling separate coronal and interplanetary heliospheric (IH) models. The noticeable differences in the temporal evolution of solutions at the L1 and L5 points highlight the need to use continuously evolving, synchronised magnetic field observations to improve global solar coronal and wind simulations. Meanwhile, the overall consistency in the temporal evolution of the simulated radial velocity and northward magnetic field, with lag-correlation coefficients of approximately 0.94 and 0.80 at lags of 108 and 100 hours, respectively, demonstrates the feasibility of using L5 observations to forecast background solar-wind conditions near Earth approximately 4–5 days in advance.

Collisionless shocks confine the particles they accelerate with the upstream wave field they themselves drive. Streaming particles resonantly amplify Alfvén waves, and the waves then confine the same particles by pitch-angle scattering. The rigidity a shock precursor can hold is therefore capped by how fast those waves grow, and that coupling has not previously been constrained in situ at any interplanetary shock. Here, we report Parker Solar Probe measurements of a fast, near-parallel coronal mass ejection-driven shock at 0.24 au on 2023 March 13 that constrain this wave–particle coupling through the energetic-particle population it accelerates. We measure the spectral folding energy, the boundary between the confined and the upstream-escaping protons, and find that it falls from ∼6 to ∼1.5 MeV as E _fold ∝ Δ t ^α with α ≃ 0.34, where Δ t is the time remaining before the shock arrival. One precursor convective time before the shock arrival, this boundary is at E _c ≃ 2.46 MeV, defining a characteristic confinement energy for the precursor of scale L _EP . Read through the quasi-linear marginal-stability condition; the same evolution implies a resonant wave-growth timescale τ _growth ( E ) ∝ E ^{1/ α} ≃ E ^2.92 that exceeds the precursor convective time above E _c . The postshock pitch-angle anisotropy reverses its sign across the same confinement–escape transition at 2–5 MeV. The rigidity to which a shock precursor can confine particles is therefore set, in situ, by how quickly the streaming population builds its own confining waves.

Context . Counterstreaming suprathermal electron beams are frequently observed within interplanetary coronal mass ejections (CMEs) and are commonly interpreted as signatures of particle trapping along closed magnetic flux ropes. Their properties are expected to depend on the internal magnetic structure of the CME flux rope and on the level of particle scattering. Aims . We investigate how the internal magnetic configuration of flux-rope CMEs, particularly magnetic twists, influences the propagation, focusing, and mirroring of trapped electrons. Our objective is to determine the conditions that influence counterstreaming electron beam formation at 1 au. Methods . We employed the Flux Rope in 3D (FRi3D) CME model implemented in EUHFORIA to simulate CMEs with a realistic magnetic field structure. Electron transport was modelled with the test-particle code PARADISE by solving the focused transport equation along CME field lines. We varied the magnetic twist of the flux rope and the parallel mean free path of the electrons to quantify their influence on beam formation and evolution. Results . The simulations produce pronounced counterstreaming electron signatures at 1 au. The modelled flux ropes contain three mirroring regions, arising from the enhanced magnetic field strength near the flux rope’s footpoints and apex. Clear counterstreaming beams are reproduced for λ _∥ ≳ 10 au. As the magnetic twist increases, the signatures remain but become broader and more asymmetric. The values of λ _∥ required to reproduce these signatures are substantially larger than typical values for the ambient solar wind but remain consistent with the comparatively ordered large-scale magnetic structure expected inside flux ropes. Conclusions . Our results show that the properties of suprathermal counterstreaming electron beams are highly sensitive to the internal structure of flux-rope CMEs. In particular, magnetic twist and pitch-angle scattering jointly control the degree of focusing, mirroring, and asymmetry observed at 1 au. This supports the use of suprathermal electron pitch-angle distributions as diagnostics of flux-rope structure in CMEs.

Diffusive shock acceleration, at shocks from coronal mass ejections to supernova remnant blast waves, presupposes a scattering wave field that the accelerated particles themselves maintain. This self-regulation has not been resolved in situ. We report Parker Solar Probe observations of a fast (≃2800 km s ⁻¹ ), near-parallel interplanetary shock at 0.24 au on 2023 March 13 and separate its upstream wave field into four families, a classification not made before at a fast shock near the Sun. Right-hand and left-hand circularly polarized families over a common wavenumber band, with a field-aligned linearly polarized family, are cyclotron resonant with the suprathermal-to-MeV protons streaming from the shock: the beam drives the field that scatters it, and the measured mean free path, half the precursor scale, leaves the beam anisotropic enough to sustain the drive. Outside this loop lies a weak, oblique, linearly polarized component, a few percent of the wave power, resolved here for the first time at an in situ foreshock. Its in-phase density and field-magnitude fluctuations identify the compressive part as fast magnetosonic and shift the cyclotron resonance energies of the resonant families by up to 13% along the precursor. Acceleration at shocks inside 0.3 au is governed upstream, in a foreshock the shock builds for itself.

The propagation of solar energetic particles (SEPs) through the heliosphere is primarily guided by the interplanetary magnetic field (IMF), which is embedded in the solar wind plasma. Large-scale IMF structures can drive transient variations in SEP intensities. Using Solar Orbiter observations, we identify a distinct class of SEP variations: SEP flux deflections (SFDs), which are commonly detected in SEP events and frequently recur multiple times within a single event. SFDs are characterized by a sudden change in SEP flux directions where the intensities drop in one direction and increase in another direction, without a significant net change in total flux magnitude. These deflections occur dispersionlessly across a broad energy range—from tens of keV to over 100 MeV—and exhibit steep intensity gradients. SFDs appear to be associated with magnetic flux tubes whose boundaries exhibit features consistent with tangential discontinuities. We further show that the solar wind inside these structures exhibits distinct plasma properties, and that the SEP streaming direction within SFDs aligns closely to the flux-tube axis. These observations are consistent with magnetic flux tubes being a prevalent structure of the solar wind, and suggest that SEPs can serve as an effective diagnostic tool for probing the topology and dynamics of solar wind structures.

Context. Suprathermal electrons significantly contribute to the heat flux in the solar wind (SW), making their treatment essential for improved SW modelling. The still-significant presence of suprathermal electron populations (halo and strahl), recently revealed by the Parker Solar Probe in the young SW, makes their involvement in energy transfer between plasma populations highly plausible. Aims. We aim to quantify the impact of suprathermal electrons on steady-state SW solutions by incorporating their contribution to thermal conduction in a macroscopic magneto-hydrodynamic (MHD) framework. Methods. The Icarus heliosphere modelling tool implemented within the MPI-AMRVAC framework was used to simulate a steady-state SW. The effect of heat flux carried by free-streaming electrons was included through a source term in the energy equation. We adopted the collisionless heat flux prescription of Hollweg (1974, J. Geophys. Res., 79, 3845), ordered by a structural parameter ( α _H ) that has now been adjusted ( α _H → α _κ ) to incorporate the influence of kinetic processes that shape the electron velocity distribution. The new, still simplified version proposed here ( α _κ ) accounts for not only the skewness determined by the strahl but also the Kappa suprathermal tails of halo populations. The SW is driven with the semi-empirical Wang-Sheeley-Arge (WSA) coronal model. Our computations use the WSA maps as the inner boundary (IB) in Icarus, but also apply modified-IB conditions to reduce interactions between fast and slow wind streams and to adjust the IB pressure (and hence temperature) to the data recently reported by the Parker Solar Probe (PSP). Results. Including collisionless heat flux can considerably reduce the rate of temperature decrease with heliocentric distance, preventing cooling as rapidly as that dictated by adiabatic expansion. As expected, heat is deposited primarily in lower-temperature regions, tending to level out the structured temperature(-velocity) distribution. Furthermore, heat flux contributes to accelerating the SW, exerting a less significant yet concrete influence on radial velocity compared to temperature variations. These effects persist for both the original WSA and modified-IB conditions and can be markedly amplified by an enhanced heat flux associated with suprathermal Kappa-distributed electrons. Consistent with PSP observations, higher pressure at the IB further enhances SW acceleration, magnifying the impact of the heat flux. Conclusions. The implications of collisionless heat flux depend, to a large extent, on the IB conditions, particularly on the pressure and bulk speed of the young SW. For example, a lower thermal pressure (or temperature) at the IB can be compensated for by a heat flux input. The improved agreement between modelled and observed radial profiles in fast SWs is consistent with the enhanced electron strahl that is more specific to higher bulk speeds.

Large solar energetic particle (SEP) events are thought to originate from the shocks driven by fast coronal mass ejections (CMEs) and thus are generally accompanied by type II radio bursts. However, a significant proportion of type II radio bursts is not accompanied by SEP events. To study the relationship between SEPs and type II radio bursts and the associated physical mechanisms, we statistically analyze 43 SEP halo CMEs and 131 non-SEP halo CMEs observed from 2010 to 2024, and check the related properties of type II radio bursts and the solar source region. We find that nearly all SEP events and approximately two-thirds of non-SEP events are accompanied by type II radio bursts. Type II radio bursts associated with SEP events usually have longer durations and lower ending frequencies. The starting frequency exhibits a clear source region dependence, being highest for “single active region (AR),” intermediate for “multiple ARs,” and lowest for “outside of ARs.” Furthermore, the spectra of both protons and electrons exhibit a similar softening trend in the three types of source regions. Joint analysis of spectra and type II radio bursts reveals that the proton spectra index has a good anticorrelation with the starting frequency of the type II radio bursts. Our statistical results have important implications for the mechanisms behind SEP acceleration.

We present a detailed investigation of the shock properties associated with solar energetic particle (SEP) events that exhibit a concave (“nose-like”) shape in their energy spectrogram, characterized by inverse velocity arrival (IVA) of the particles, where high-energy particles arrive later than mid-energy ones. Using measurements from Solar Orbiter and Parker Solar Probe between 2018 and 2025, we identify 26 such SEP events and reconstruct the observed shock fronts in three dimensions. We derive shock parameters along the magnetic field lines connected to each spacecraft using kinematic modeling and coronal magnetohydrodynamic simulations. Our analysis indicates that IVA-SEP events arise due to the spatial and temporal evolution of the shock properties and magnetic connectivity. In most of the cases analyzed here, the magnetic connectivity starts on the flanks of coronal mass ejection-driven shocks, where shocks tend to be weak, and shifts toward the shock apex, sampling stronger portions of the shock front. This evolution of the shock properties at the connected field lines likely leads to the delayed arrival of high-energy particles and the progressive hardening of the SEP energy spectrum, observed in some of the events. We find a correlation between the transition energy at which the IVA begins and the shock speed along the connected field lines, consistent with expectations from time-dependent diffusive shock acceleration. Our results underscore the importance of the evolving shock properties, magnetic connectivity, and instrumental sensitivity in shaping SEP intensity profiles and the formation of IVA signatures.

Collisionless shocks are often modelled as smooth, planar surfaces - but many show organized corrugations that steer how particles get accelerated and how they radiate. We present a simple, linear magnetohydrodynamic (MHD) model that treats the shock as an evolving interface. This lets us separate two things: (1) the shock’s own properties and geometry, and (2) the statistics of the upstream turbulence that hits it. With this separation, we obtain adirect map from incoming fluctuations to the corrugation patterns they create, including their drift speed and coherence. In our model, the interface acts like an “impedance” that focuses broad-band turbulent power into fast-mode waves that skim along the shock. The shock responds most strongly when the wave’s normal group speed is small (a sharp, single-peaked response). Corrugation strength increases with compression, while the shock geometry and plasma beta control how long these patterns persist. The framework makes testable predictions linking upstream turbulence and shock shape to finestructure in electromagnetic signals from heliospheric and supernova-remnant shocks.

Context. Technetium (Tc)-rich M stars have been known for more than 45 years. However, the origin of Tc in these stars, particularly its detection without the concomitant detection of other s -process elements, which are typically produced during asymptotic giant branch (AGB) s -process nucleosynthesis, remains poorly understood. Technetium was first identified in the spectra of S stars (which exhibit prominent ZrO bands) in 1952. The simultaneous enrichment of both Zr and Tc is well understood within the framework of s -process nucleosynthesis, which occurs during the AGB phase. However, Tc-rich M stars remain an enigma, as M stars are typically not enriched in heavy elements. Aims. This study aims to analyse high-resolution spectra of a large sample of M stars to examine their spectral characteristics, and to compare their spectral properties with those of Tc-rich S stars in an attempt to understand the origin of their difference. Methods. We defined a robust classification scheme to assign M stars to the Tc-rich or Tc-poor class. We computed nucleosynthesis models to trace the evolution of Zr and Tc abundances across successive thermal pulses during the AGB phase. We further analysed spectral indices measuring the depth of the TiO and ZrO bands, as well as the wavelength of the Tc blend, on both synthetic and observed spectra. Results. The Tc lines in Tc-rich M stars are similar to those in S stars. However, Tc-rich M stars exhibit stronger TiO bands than S stars, while displaying similarly strong ZrO bands. Spectral synthesis, together with location in the Hertzsprung–Russell (HR) diagram and spatial properties, suggests that Tc-rich M stars may have slightly lower metallicity and lower masses than Tc-rich S stars.