Wave-plasma interactions and energy transport are fundamental processes in stellar atmospheres, shaping elemental composition through the first ionization potential (FIP) and inverse FIP (IFIP) effects. Although stellar measurements provide global evidence of abundance anomalies, the Sun offers a unique local laboratory in which to resolve how these processes operate on small spatial and temporal scales. In this overview, we summarize the current state of knowledge of the IFIP effect, its observational signatures and the theoretical framework that underpins it. We then present new insights from a detailed case study of active region (AR) NOAA AR 11967, where combined Hinode/extreme-ultraviolet imaging spectrometer (EIS), IRIS, SDO/AIA and Fermi/GBM observations and IRIS2+ inversions indicate that torsional Alfvén waves generated below the chromospheric fractionation region can account for the highly localized IFIP effect plasma observed on the Sun. This analysis highlights the possible role of sub-chromospheric processes in the formation of the IFIP effect on the Sun and informs pathways for understanding similar mechanisms in the coronae of active M dwarfs. This article is part of the Theo Murphy meeting issue 'Solar atmospheric abundances in space and time'.

We investigate the collisionless kinetic structure of the upper solar atmosphere in the presence of an expanding magnetic field. We consider a stationary two-component plasma confined within an expanding magnetic flux tube and subject to gravity, self-electrostatic interactions, the Pannekoek-Rosseland electric field, and magnetic moment conservation. Starting from the Vlasov equation, we derive fully analytical expressions for the particle distribution functions, density profiles, and the parallel, perpendicular, and total temperature profiles. We show that the combined conservation of energy and magnetic moment generates a loss-cone distribution, reducing the density with respect to the corresponding unmagnetized atmosphere and producing a pronounced temperature anisotropy. For a single-temperature boundary condition, the competition between magnetic moment conservation and gravity causes the parallel temperature to develop a maximum. We derive analytical scaling laws for its location and amplitude and validate them against numerical calculations. We further show that the anisotropy persists independently of the temperature distribution at the lower boundary. In the regime of rare but intense heating events, gravitational filtering enhances the contribution of the hottest particle populations at coronal heights, while magnetic moment conservation further amplifies the resulting velocity-space anisotropy. This work provides a fully analytical kinetic description of the combined effects of gravitational filtering and magnetic moment conservation in an expanding coronal magnetic flux tube undergoing stochastic heating at its base. These results establish a theoretical framework for investigating the role of magnetic-field expansion in shaping the density and temperature structure of weakly collisional stellar coronae.

The properties of the solar wind, as measured in situ throughout the heliosphere, depend both on the characteristics of its coronal source and on the intrinsic processes governing its interplanetary evolution. Recently, radial and Parker spiral alignment techniques have been applied to Parker Solar Probe (PSP) and Solar Orbiter (SO) observations to investigate the radial evolution of the same solar wind parcel. These studies have shown that the solar wind can undergo significant acceleration even beyond its primary acceleration region (i.e., above 15 R circle dot). However, such radial and Parker spiral alignments are rare in practice, which limits the statistical significance and general applicability of the results. We introduce a new source alignment technique designed to overcome these limitations. Using magnetic backmapping, we associate similar solar wind streams observed by the two spacecraft based on the proximity of their photospheric footpoints, combined with additional in situ stream similarity criteria. Applying the source alignment method to PSP and SO observations, we identify a total of 548 alignment intervals, each lasting 30 minutes. By constructing statistics over all alignments, we find that the solar wind speed increases by an average of 45% per radial decade (approximately 147 km s-1) between the two probes. This result demonstrates that solar wind acceleration in the inner heliosphere remains significant compared to that occurring below 15 R circle dot. Among the different studied plasma parameters, the radial evolution of the electron temperature and plasma density shows the strongest anticorrelation with the increase in bulk velocity.

This study focuses on energy conversion related to tearing-driven magnetic reconnection in the context of weakly collisional astrophysical plasmas. We present results from a two-dimensional hybrid particle-in-cell simulation employing novel periodic conditions with a topology akin to the M & ouml;bius strip, which doubles the computation efficiency as compared to regular periodic conditions. Evaluation of the ion electric work rate (ji & centerdot; E) and pressure-strain interaction (Pi : del ui) shows that most of the energy conversion occurs during the nonlinear phase of the instability, where magnetic energy is transferred toward ion kinetic energy (bulk outflows) and internal energy (heating). These energy conversion rates are of the same order but inhomogeneous. Heating predominantly occurs within the magnetic islands, while near the X-points, nearly the same amount of magnetic energy is transferred to the bulk plasma flow and heating. The reconnected plasma moreover exhibits an ion temperature higher parallel than perpendicular to the local magnetic field B. This temperature anisotropy is sustained by the islands' contraction, but eventually gets regulated by kinetic firehose instabilities (parallel and oblique), and/or firehose-like processes, whose main effect is to redistribute the internal energy from the parallel to the perpendicular direction.

We investigate stationary states of a collisionless, gravitationally stratified plasma atmosphere composed of electrons, protons, and alpha particles by extending Pannekoek–Rosseland theory to multispecies and multi-temperature plasmas. Starting from Liouville's theorem, we derive the self-consistent ambipolar electric field from kinetic equilibrium and charge neutrality. For a single-temperature atmosphere, we obtain analytical expressions for the ambipolar field, show its dependence on alpha-particle abundance, and determine the relative stratification of the three species. A first-order analytical approximation to the electrostatic potential accurately reproduces the numerical solution. We then generalize the formalism to multi-temperature plasmas generated by stochastic boundary heating, representing the stationary distribution as a superposition of Maxwellian populations. Gravitational filtering produces non-exponential density profiles and increasing temperatures with altitude, while preserving the relative species stratification, with alpha particles most strongly stratified and protons least. The ambipolar electric field contains a dominant gravitational contribution, corresponding to the generalized Pannekoek–Rosseland field, and a thermoelectric contribution arising from species-dependent temperature gradients, which accounts for its non-monotonic structure. These results provide a framework for studying the combined effects of plasma composition and stochastic heating in gravitationally stratified astrophysical plasmas.

This study focuses on the radial evolution of the solar wind proton adiabatic invariants and temperature anisotropies in the inner heliosphere. More specifically, we study in-situ measurements provided by the Parker Solar Probe, between 0.05 au and 0.25 au from the Sun, and Solar Orbiter spacecraft between 0.3 au and 1 au. Throughout the studied range of radial distances, we observe a significant average heating in the direction perpendicular to the local magnetic field for both fast and slow solar wind populations. On the other hand, there is no clear deviation from adiabaticity in the parallel direction regardless of the wind speed. The perpendicular heating is enough to significantly reduce the generation of the temperature anisotropy expected from a double adiabatic evolution. Despite the heating, an important portion of the solar wind (especially the slower wind streams) develops substantial anisotropies with higher parallel temperatures, which eventually become constrained by kinetic firehose instabilities.

The magneto-hydrodynamic (MHD) description of the solar wind has long been regarded as one of the most successful frameworks for investigating solar wind heating and acceleration. Under a set of simplifying assumptions, it provides a reduced large-scale description that is widely used to study the energetic role of electromagnetic fluctuations in the solar wind energy budget. In practice, applying simplification to MHD can result in the loss of some physics compared to an exact set of equations. However, an observational energy budget can help us to verify whether any energetic information has been lost in the process of simplification. To do so, we directly test the validity of the large-scale non-linear ideal MHD energy budget with solar wind observations from Parker Solar Probe (PSP) and Solar Orbiter (SO). After verifying the reliability of the measurements through comparison with previous studies of similar solar wind properties, we test the conservation of the total energy predicted by the MHD theory. We find that the average total energy increases with radial distance by 56% (±9%) between 14 and 203 solar radii. The increase in kinetic energy is not sufficiently compensated by the decrease of the thermal and electromagnetic contributions. We investigate the major sources of observational uncertainty and find that they cannot account for the observed energy increase. These results suggest that the large-scale theoretical description may neglect an energetically significant contribution to solar wind acceleration. Assuming that such a contribution exists, we predict that this extra energy would scale with radial distance as r^-0.54±0.38.

Active regions (ARs) are the photospheric manifestation of magnetic flux ropes (FRs) emerging from the solar interior. A key parameter in their evolution is the tilt angle, defined as the inclination of the AR polarity axis relative to the solar equator. Despite its central role in flux-transport dynamo models, tilt measurements are affected by systematic biases—most notably magnetic tongues—particularly during the emergence phase. In this work we analyze tilt estimates for 108 bipolar active regions from Solar Cycle 23 using two methodologies: (1) the conventional magnetic barycenter method and (2) a Bayesian FR model that fits observed magnetograms with a synthetic model of an emerging twisted toroidal flux tube. We find notable differences between the two estimates, ranging from about 30^∘ at early stages of emergence to ∼ 10^∘ near the end. Our analysis shows that Joy’s law is already present at the earliest stages of AR emergence, although the tilt dispersion is significantly larger at these times. Tilt rotations are strongest during the early emergence phase, with no preferred sense of rotation, and decrease as ARs approach their maximum magnetic flux, suggesting an important role of near-surface convective motions. We also find that the quantitative characterization of Joy’s law depends on both the magnetic flux of ARs and the method used to estimate their tilt. The Bayesian framework adopted here provides a robust basis for future analyses by allowing known sources of scatter to be explicitly modeled within a consistent statistical formalism.

Upflows at coronal temperatures are typically present at the edges of active regions, where plasma escapes into the upper atmosphere. This study focuses on NOAA Active Regions 12665 and 12685 which are in different evolutionary stages, they display varying activity levels, and have a large difference in their magnetic field structure. Our targets were followed by Hinode/EIS as the ARs crossed the solar disk, and we used Fe XII maps to track their evolution. We separated time periods with nearly stationary flows from the non-stationary ones, in order to isolate the effects of solar rotation and activity on both the line-of-sight and non-thermal velocities. We confirmed the presence of a strong positive correlation between the Doppler and non-thermal velocity during both time periods and with different levels of activity. These results help us differentiate the physical origin of non-thermal velocities, such as Alfvén waves, MHD turbulence, magnetic reconnection, and multi-stream components. Our results favour the latter.

Quiescent filaments are prominent features of the solar atmosphere, and their evolution reflects the coronal magnetic field’s response to photospheric magnetic activity. Here, we report on a quiescent filament observed on 2023 September 28–29, aiming to understand how the magnetic configuration shapes its feet and drives its extension. For this purpose, high-resolution spectral data in H α and Mg II k are used from the Télescope Héliographique pour l’Etude du Magnétisme et des Instabilités Solaires and the Interface Region Imaging Spectrograph, respectively. To track changes in the filament, we utilise long-term data from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory (SDO) and from the Global Oscillation Network Group (GONG). We analyse the longitudinal magnetic field in the photosphere using the Solar Optical Telescope on board Hinode, as well as SDO/Helioseismic and Magnetic Imager data. In addition to this, we use GONG H α data to analyze the longitudinal oscillations in the filament. Observations show that parasitic polarities and canceling flux play a key role in forming and reorganizing the filament feet and in lengthening the filament. A 3D MHD reconstruction using vector magnetograms reveals that its magnetic configuration evolves into a full flux rope (FR), whose extension on the second day matches the observed filament growth. The FR is separated from the surrounding nearly potential field by quasi-separatrix layers, which in turn are separated by current layers. They get more organized around the FR as it grows. Moreover, the longitudinal oscillations in the extended filament are attributed to heating from flux cancellation at underlying bright points.

The solar corona exhibits a pronounced temperature inversion, with plasma temperatures increasing by nearly two orders of magnitude from the chromosphere to the corona. We investigate how spatially sparse and temporally intermittent stochastic heating at the base of the transition region shapes the temperature and density structure of coronal loops within a kinetic framework. Stochastic thermal boundary conditions and surface coarse graining are introduced. Analytical solutions are derived in the collisionless limit for heating-event time scales shorter or longer than the particle crossing time, and Coulomb collisions are incorporated through a reduced kinetic model describing the thermalization of suprathermal particles. In the short-time-scale regime, spatial filling factor and temporal intermittency combine into a single effective parameter controlling the suprathermal population, producing a transition region and a hot corona both within individual loops and after coarse graining. Collisions preserve this thermal structure while reducing the coronal density through progressive thermalization. In the long-time-scale regime, individual loops are nearly isothermal and the temperature inversion emerges only after coarse graining, depending solely on the spatial filling factor. Here, Coulomb collisions and optically thin radiative losses have only minor effects, while density and temperature profiles remain broadly consistent with coronal observations. These results show that sparse, intermittent heating naturally generates suprathermal particle distributions and reproduces the observed thermal structure of the solar corona within a kinetic framework, highlighting the different sensitivity of the two regimes to collisional effects.

Active regions (ARs) are the photospheric manifestations of emerging magnetic flux ropes (FRs) formed in the solar interior. We analyze the emergence of 126 bipolar ARs during Solar Cycle 23 using a flux rope model, whose parameters are inferred through a Bayesian inference method. This approach allows us to estimate key sub-photospheric properties of FRs. We find that the Bayesian method effectively captures the global magnetic characteristics of ARs, with discrepancies primarily arising in the later stages of emergence. We examine the ability of a flux-balanced FR model with a symmetric circular cross-section to reproduce polarity shapes during these late stages. Additionally, we analyze how the inclination of the FR legs provides insight into the emergence stage. We propose an improved method for estimating the separation of polarities, which decreases projection effects and flux distribution biases. Furthermore, we confirm a strong correlation between the AR flux and the distance between the main polarities, as well as the evolution of their separation speed. Finally, we identify a characteristic ratio between the thickness of the FR and its curvature radius, suggesting an underlying physical mechanism governing this ratio.

Filaments/prominences are cold plasma ( ≈ 10^4 K) embedded in the solar corona, two orders of magnitude hotter. Filament plasma is structured by the magnetic field in thin elongated threads. Counter-streaming flows have been observed. The aim of this paper is to characterize these flows. For that, we use high spatial resolution observations of spectral data obtained with THEMIS in H α and with IRIS in Mg II k lines on 29 September 2023. We best detect counter-streaming flows in both the blue and red wings of these spectral lines. They are forming long Doppler shifted strands slightly inclined on the filament axis. The blue/red shift alternates across the strands at the arc second scale. H α spectral profiles with large widths are interpreted as formed by multi-strands with opposite velocity directions. The absorption in the core of Mg II k line is also broader than in the chromosphere. This corresponds also to counter-streaming velocities. We derive that a fraction of the filament plasma is moving at supersonic speed (of the order of 20 km s−1) with the assumption that the filament is optically thick. We conclude that the counter-directed Doppler shifts might not be magnetic field aligned flows but rather correspond to kink transverse oscillations of the magnetic field with independent motions in nearby strands.

We develop a three-dimensional kinetic model of the solar transition region and corona in which the plasma above the chromosphere is collisionless and embedded in a uniform magnetic field. Heating occurs intermittently at discrete locations on the chromospheric surface, modeled through a surface coarse-graining procedure that produces non-thermal boundary conditions for the Vlasov equation. The resulting stationary distribution functions generate suprathermal particle populations and naturally lead to a temperature inversion via gravitational filtering, without any local coronal heating. The model reproduces realistic temperature and density profiles with a thin transition region and a hot corona, consistent with solar observations. These results demonstrate that the spatial intermittency of heating at the chromospheric interface is sufficient to account for the formation of the transition region and the high-temperature corona.

Context. The solar corona exhibits a striking temperature inversion, with plasma temperatures exceeding 10(6) K above a much cooler chromosphere. How the coronal plasma reaches such extreme temperatures remains a fundamental open question in solar and plasma physics, known as the coronal heating problem. Aims. We investigate whether localized heating events, spatially distributed across the upper chromosphere and base of the transition region, combined with a collisionless corona, can self-consistently generate realistic temperature and density profiles without requiring direct energy deposition within the corona itself. Models. We develop a 3D kinetic model of a collisionless stellar atmosphere embedded in a uniform magnetic field, where heating occurs intermittently at the chromosphere-transition region interface. A surface coarse-graining procedure is introduced to capture the spatial intermittency of heating, leading to non-thermal boundary conditions for the Vlasov equation. We derive analytical expressions for the stationary distribution functions and compute the corresponding macroscopic profiles. Results. We show that spatially intermittent heating, when coarse-grained over a surface containing many localized events, produces suprathermal particle distributions and a temperature inversion via velocity filtration. The resulting density and temperature profiles feature a transition region followed by a hot corona, provided that heating events are spatially sparse, consistently with solar observations. This result holds independently of the specific statistical distribution of temperature increments. Importantly, no local heating is applied within the corona. Conclusions. The model demonstrates that spatial intermittency alone, i.e. a sparse distribution of heated regions at the chromospheric interface, is sufficient to explain the formation of the transition region and the high-temperature corona.

We study intermittent coherent structures in solar wind magnetic turbulence from MHD to kinetic plasma scales using Parker Solar Probe data during its first perihelion (at 0.17 au), when the satellite was in the Alfvénic slow wind of 340 km/s. The coherent structures are energetic events localized in time and covering wide range of scales. We detect them using Morlet wavelets. For the first time, we apply a multi-scale analyses in physical space to study these structures. At MHD scales within the inertial range, times scales τ∈ (1, 10^2 ) s, we find (i) current sheets including switchback boundaries and (ii) Alfvén vortices. Within these events, there are embedded structures at smaller scales: typically Alfvén vortices at ion scales, τ∈ (0.08, 1) s, and a compressible vortices at sub-ion scales, τ∈ (8,80) ms. The number of coherent structures grows toward smaller scales: we observe about ∼ 200 events during 5 h time interval at MHD scales, ∼ 10^3 events ai ion scales and ∼ 10^4 events at sub-ion scales. In general, there are multiple structures of ion and sub-ion scales embedded within one MHD structure. There are also examples of ion and sub-ion scales structures outside of MHD structures. To quantify the relative importance of different type of structures, we do a statistical comparison of the observed structures with the expectations of models of the current sheets and vortices. This comparison is based on amplitude anisotropy of magnetic fluctuations within the structures. The results show the dominance of Alfvén vortices at all scales in contrast to the widespread view of dominance of current sheets. This means that Alfvén vortices are important building blocs of solar wind turbulence.

Context. Coronal mass ejections (CMEs) are the main drivers of disturbances in the solar heliosphere because they propagate and interact with the magnetic field of the solar wind. It is crucial to investigate the evolution of CMEs and their deformation for understanding the interaction between the solar wind and CMEs. Aims. We quantify the effect of the dynamic solar wind on the propagation of a CME in the heliosphere with a hydrodynamic plasma cloud-cone model and a linear force-free spheromak model at various locations in the heliosphere. Methods. We chose a CME event that launched on SOL2021-09-23T04:39:45 and was observed by multiple spacecraft, namely BepiColombo, Parker Solar Probe, Solar Orbiter, Stereo A and ACE. The solar wind was modelled in the steady and dynamic regimes in the Icarus model. The CME parameters were approximated for the selected event, and two CME models (spheromak and cone) were launched from the inner heliosphere boundary. The obtained synthetic in situ measurements were compared to the observed in situ measurements at all spacecraft. Results. The internal magnetic field of the flux rope was better reconstructed by the spheromak model than by the cone CME model. The cone CME model maintained a nearly constant longitudinal angular extension while somewhat contracting in the radial direction. In contrast, the spheromak model contracted in the longitudinal direction while expanding in the radial direction. Conclusions. The CME sheath and magnetic cloud signatures were better reproduced at the four spacecraft clustered around the CME nose by the spheromak CME model. The dynamic solar wind caused a greater deceleration of the modelled CME than the steady-state solar wind solution. Because the background was homogeneous, the modelled CME properties were only mildly affected by the solar wind regime, however.

Context. It has been recently accepted that the standard classification of the solar wind solely according to flow speed is outdated, and particular interest has been devoted to the study of the origin and evolution of so-called Alfvenic slow solar wind streams and to what extent such streams resemble or differ from fast wind. Aims. In March 2022, Solar Orbiter completed its first nominal phase perihelion passage. During this interval, it observed several Alfvenic streams, allowing for characterization of fluctuations in three slow wind intervals (AS1-AS3) and comparison with a fast wind stream (F) at almost the same heliocentric distance. Methods. This work makes use of Solar Orbiter plasma parameters from the Solar Wind Analyzer (SWA) and magnetic field measurements from the magnetometer (MAG). The magnetic connectivity to the solar sources of selected solar wind intervals was reconstructed using a ballistic extrapolation based on measured solar wind speed down to the (spherical) source surface at 2.5 Rs below which a potential field extrapolation was used to map back to the Sun. The source regions were identified using SDO/AIA observations. A spectral analysis of in situ measured magnetic field and velocity fluctuations was performed to characterize correlations, Alfvenicity, normalized cross-helicity, and residual energy in the frequency domain as well as intermittency of the fluctuations and spectral energy transfer rate estimated via mixed third-order moments. A machine learning technique was used to separate proton core, proton beam, and alpha particles and to study v-b correlations for the different ion populations in order to evaluate the role played by each population in determining the Alfvenic content of solar wind fluctuations. Results. The comparison between fast wind and Alfvenic slow wind intervals highlights the differences between the two solar wind regimes: The fast wind is characterized by larger amplitude fluctuations, and magnetic and velocity fluctuations are closer to equipartition of energy. In fact the Alfvenic slow wind streams appear to be on a spectrum of wind types, with AS1, originating from open field lines neighboring active regions and displaying similarities with the fast wind in terms of fluctuation amplitude and turbulence characteristics, but not with respect to the alpha particles and proton beams. The other two slow streams differed both in their sources as well as plasma characteristics, with AS2 coming from the expansion of a narrow coronal hole corridor and AS3 from a region straddling a pseudostreamer. The latter displayed the coldest and highest density but the slowest stream with the smallest fluctuation amplitude and greatest magnetic energy excess. It also showed the largest scatter in proton beam speeds and the greatest difference in speed between proton beam and alpha particles. Conclusions. This study shows how the old fast-slow solar wind dichotomy, already called into question by the observations of slower Alfvenic solar wind streams, should further be refined, as the Alfvenic slow wind, originating in different solar wind regions, show significant differences in density, temperature, and proton and alpha-particle properties in the inner heliosphere. The observations presented here provide the starting point for a better understanding of the origin and evolution of different solar wind streams as well as the evolving turbulence contained within.

This study focuses on a radial alignment between Parker Solar Probe (PSP) and Solar Orbiter (SolO) on 2021 April 29 (during a solar minimum), when the two spacecraft were, respectively, located at ∼0.075 and ∼0.9 au from the Sun. A previous study of this alignment allowed the identification of the same density enhancement (with a timescale of ∼1.5 hr) and substructures (timescales of ∼20–30 minutes), passing first by PSP and then by SolO after a ∼138 hr propagation time in the inner heliosphere. We show here that this structure belongs to the large-scale heliospheric magnetic sector boundary. In this region, the density is dominated by radial gradients, whereas the magnetic field reversal is consistent with longitudinal gradients in the Carrington reference frame. We estimate the density structure radial size to remain of the order L _R ∼ 10 ^6 km, while its longitudinal and latitudinal sizes are estimated to expand from L _φ _, _θ ∼ 10 ^4 –10 ^5 km in the high solar corona to L _φ _, _θ ∼ 10 ^5 –10 ^6 km at PSP and L _φ _, _θ ∼ 10 ^6 –10 ^7 km at SolO. This implies a strong evolution of the structure’s aspect ratio during the propagation, due to the plasma’s nearly spherical expansion. The structure’s shape is therefore inferred to evolve from elongated in the radial direction at ∼2–3 solar radii (high corona) to sizes of nearly the same order in all directions at PSP and then becoming elongated in the directions transverse to the radial at SolO. Measurements are not concordant with local reconnection of open solar wind field lines, so we propose that the structure has been generated through interchange reconnection near the tip of a coronal streamer.

Stream interaction regions (SIRs) are created from the interaction between fast and slow interplanetary plasma. SIRs at 1 au can cause significant perturbations in geospace, including geomagnetic and ionospheric storms. In this paper we present a case study involving an additional structure in the solar wind before the typical SIR signatures. We find that this first preceding structure corresponds to the flank of an interplanetary coronal mass ejection while the second one is associated to an SIR. Both structures cause significant perturbations in the magnetosphere and ionosphere.