arXiv subjects

Pascal Démoulin

Publications and source records attributed to Pascal Démoulin.

At least 19 recordsLinked to original sources

arXiv2026

Statistical Energy Budget of the Solar Wind with Parker Solar Probe and Solar Orbiter : 1 - Unbalanced Total Energy Radial Evolution

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 $56\%$ ($\pm9\%$) 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 $r^{-0.54\pm0.38}$.

astro-ph.SR↗

arXiv2026

The Ambipolar electric field in multispecies plasma atmospheres: effects of alpha particles and stochastic heating

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.

physics.plasm-ph↗

arXiv2026

Solar Wind Proton Heating and its Effect on Temperature Anisotropy Evolution between 0.05 and 1 au

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.

astro-ph.SR↗

arXiv2026

Collisionless stationary states of a stratified plasma in an expanding magnetic tube with stochastic heating

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.

astro-ph.SR↗

arXiv2026

Collisionless and collisional kinetics of a plasma atmosphere with spatially and temporally intermittent heating at its base

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.

astro-ph.SR↗

arXiv2026

On the Radial Evolution of the Solar Wind : The Source Alignment Method Applied to Parker Solar Probe and Solar Orbiter Observations

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 solar radii). 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) between the two probes. This result demonstrates that solar wind acceleration in the inner heliosphere remains significant compared to that occurring below 15 solar radii. Among the different studied plasma parameters, the radial evolution of the electron temperature and plasma density, show the strongest anti-correlation with the increase in bulk velocity.

astro-ph.SR↗

arXiv2026

Tearing Driven Reconnection: Energy Conversion Involving Firehose Kinetic Instabilities (2D Hybrid M\"obius Simulations)

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\"obius strip, which double the computation efficiency as compared to regular periodic conditions. Evaluation of the ion electric work rate ($\mathbf{j}_i \cdot \mathbf{E}$) and pressure strain interaction ($\mathbf{P}_i : \mathbf{\nabla u}_i)$ shows that most of the energy conversion occurs during the nonlinear phase of the instability, where magnetic energy is transferred towards 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 bulk plasma flow and heating. The reconnected plasma moreover exhibits an ion temperature higher parallel than perpendicular to the local magnetic field $\mathbf{B}$. 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.

astro-ph.SR↗

arXiv2026

Elongation of a Solar Filament and its Three-Dimensional Numerical Reconstruction for Magnetic Structures

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 from 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$\alpha$ and Mg II k are used from the T\'elescope H\'eliographique pour l'Etude du Magn\'etisme et des Instabilit\'es Solaires (THEMIS) and the Interface Region Imaging Spectrograph (IRIS), respectively. To track changes in the filament, we utilise long-term data from the Atmospheric Imaging Assembly (AIA) 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 (SOT) onboard Hinode, as well as SDO/Helioseismic and Magnetic Imager (HMI) data. In addition to this, we use GONG H$\alpha$ 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 is growing up. Moreover, the longitudinal oscillations in the extended filament are attributed to heating from flux cancellation in underlying bright points.

astro-ph.SR↗

arXiv2025

Investigating the impact of the dynamic solar wind on the propagation of a coronal mass ejection with two models and multi-spacecraft measurements

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. 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. 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. 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. 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.

astro-ph.SR↗

arXiv2025

Partial Null Point Reconnection of an Eruptive Filament

Solar filaments are cool and dense plasma structures suspended in the solar corona against gravity. We present observations of a quiescent filament eruption that occurs on 13 July 2015. The eruption is associated with a two-ribbon GOES B8.9 class flare. Photospheric magnetic flux cancellation is present below the filament during days. This builds up a flux rope which progressively rises until it gets unstable, first leading to a confined eruption and pre-flare brightenings, then to an ejection which starts $\approx$ 20 min later with the flare onset. An interesting feature of this event is the presence of a large circular brightening formed around the erupting region. This brightening is produced due to interchange reconnection of the ejected magnetic configuration with the surrounding open magnetic field. This null-point topology is confirmed by a potential-field extrapolation. The EUV loops located on the southern side of the filament eruption first contract during the null-point reconnection, then expand as the flux rope is ejected. The associated CME has both a classical flux rope shape and plasma ejected along open field lines on the flux rope side (a trace of interchange reconnection). Finally, we set all this disparate observations within a coherent framework where magnetic reconnection occurs both below and above the erupting filament.

astro-ph.SR↗

arXiv2025

Kinetic collisionless model of the solar transition region and corona with spatially intermittent heating

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.

astro-ph.SR↗

arXiv2025

Radial evolution of a density structure within a solar wind magnetic sector boundary

This study focuses on a radial alignment between Parker Solar Probe (PSP) and Solar Orbiter (SolO) on the 29$^{\text{th}}$ of April 2021 (during a solar minimum), when the two spacecraft were respectively located at $\sim 0.075$ and $\sim 0.9$~au from the Sun. A previous study of this alignment allowed the identification of the same density enhancement (with a time scale of $\sim$1.5~h), and substructures ($\sim$20-30~min timescale), passing first by PSP, and then SolO after a $\sim 138$~h 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$_R \sim 10^6$~km, while its longitudinal and latitudinal sizes, are estimated to expand from L$_{φ, θ} \sim 10^4$-$10^5$~km in the high solar corona, to L$_{φ, θ} \sim 10^5$-$10^6$~km at PSP, and L$_{φ, θ} \sim 10^6$-$10^7$~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 $\sim$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.

astro-ph.SR↗

arXiv2024

Coronal magnetic field and emission properties of small-scale bright and faint loops in the quiet Sun

The present study provides statistical information on the coronal magnetic field and intensity properties of small-scale bright and faint loops in the quiet Sun. We aim to quantitatively investigate the morphological and topological properties of the coronal magnetic field in bright and faint small-scale loops, with the former known as coronal bright points (CBPs). We analyse 126 small-scale loops using quasi-temporal imaging and line-of-sight magnetic field observations. We employ a recently developed automatic tool that uses a linear magneto-hydro-static model to compute the magnetic field in the solar atmosphere and automatically match individual magnetic field lines with small-scale loops. For most of the loops, we automatically obtain an excellent agreement of the magnetic field lines from the LMHS model and the loops seen in AIA 193 A. One stand-out result is that the magnetic field is non-potential. We obtain the typical ranges of loop heights, lengths, intensities, mean magnetic field strength along the loops and at loop tops, and magnetic field strength at loop footpoints. We find that loops below the classic chromospheric height of 1.5 Mm are flatter suggesting that non-magnetic forces (one of which is the plasma pressure) play an important role below this height. We find a strong correlation (Pearson coefficient of 0.9) between loop heights and lengths. The average intensity along the loops correlates stronger with the average magnetic field along the loops than with the field strength at loop tops. The latter correlation indicates that the energy release in the loops is more likely linked to the average magnetic field along the loops than the field strength on the loop tops. In other words, the energy is probably released all along the loops, but not just at the loop top. This result is consistent with the recent benchmarking radiative 3D MHD model of Nóbrega-Siberio etal.

astro-ph.SR↗

arXiv2024

Embedded coherent structures from MHD to sub-ion scales in turbulent solar wind at 0.17 au

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 $τ\in (1, 10^{2} )$ 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, $τ\in (0.08, 1)$ s, and a compressible vortices at sub-ion scales, $τ\in (8,80)$ ms. The number of coherent structures grows toward smaller scales: we observe about $\sim 200$ events during 5 h time interval at MHD scales, $\sim 10^{3}$ events ai ion scales and $\sim 10^{4}$ 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.

astro-ph.SR↗

arXiv2024

Modeling Magnetic Flux Emergence in Bipolar Active Regions

Active regions (ARs) appear in the solar atmosphere as a consequence of the emergence of magnetic flux-ropes (FR). In this study, we use Bayesian methods to analyze line-of-sight magnetograms of emerging ARs. We employ a FR model consisting of a half-torus field structure based on eight parameters. The goal is to derive constrained physical parameters of the originating FR which are consistent with the observations. Specifically, we aim to obtain a precise estimation of the AR tilt angle and magnetic twist at different stages of the emergence process. To achieve this, we propose four temporal methods that correlate the field parameter evolutions with a single coherent FR. These methods differ from each other in the size of the explored parameter space. We test the methods on four bipolar ARs observed with the Michelson Doppler Imager on board the Solar and Heliospheric Observatory. We find that tilt angles are typically consistent between the temporal methods, improving previous estimations at all stages of the emergence. The twist sign derived from the temporal methods is consistent with previous estimations. The standard errors of all the methods used are similar, indicating that they model the observations equally well. These results indicate that the proposed methods can be used to obtain global magnetic parameters of ARs during their early evolution. The derived parameters contribute to a better understanding of the formation of FRs, and the role of ARs in the magnetic recycling process along the solar cycle.

astro-ph.SR↗

arXiv2024

Identification of a Single Plasma Parcel during a Parker Solar Probe-Solar Orbiter radial alignment

Configurations where two spacecraft, such as Parker Solar Probe (PSP) and Solar Orbiter (SolO), are radially aligned provide opportunities to study the evolution of a single solar wind parcel during so called plasma line-ups. The most critical part of such studies is arguably the identification of what can be considered a same plasma crossing both spacecraft. We present here a method that allowed us to find what we believe to be the same plasma parcel passing through PSP ($\sim 0.075$ au) and SolO ($\sim 0.9$ au) after their radial alignment the 29/04/2021. We started by modeling the plasma propagation in order to get a first estimation of the plasma line-up intervals. The identification of the same density structure (with a crossing duration $\sim 1.5$ h) passing through the two spacecraft allows to precise and confirm this estimation. Our main finding is how stable the density structure is, remaining well recognizable from PSP to SolO despite its $\sim 137$ hours journey in the inner heliosphere. We moreover found that the studied slow solar wind plasma parcel undergone a significant acceleration (from $\sim 200$ to $\sim 300$ km/s) during its propagation.

astro-ph.SR↗

arXiv2023

Impact of the solar activity on the propagation of ICMEs: Simulations of hydro, magnetic and median ICMEs at minimum and maximum of activity

The propagation of Interplanetary Coronal Mass Ejections (ICMEs) in the heliosphere is influenced by many physical phenomena, related to the internal structure of the ICME and its interaction with the ambient solar wind and magnetic field. As the solar magnetic field is modulated by the 11-year dynamo cycle, our goal is to perform a theoretical exploratory study to assess the difference of propagation of an ICME in typical minimum and maximum activity backgrounds. We define a median representative CME at 0.1~au, using both observations and numerical simulations, and describe it using a spheromak model. We use the heliospheric propagator European Heliospheric FORecasting Information Asset (EUHFORIA) to inject the same ICME in two different background wind environments. We then study how the environment and the internal CME structure impact the propagation of the ICME towards Earth, by comparison with an unmagnetized CME. At minimum of activity, the structure of the heliosphere around the ecliptic causes the ICME to slow down, creating a delay with the polar parts of the ejecta. This delay is more important if the ICME is faster. At maximum of activity, a southern coronal hole causes a northward deflection. For these cases, we always find that the ICME at maximum of activity arrives first, while the ICME at minimum of activity is actually more geo-effective. The helicity sign of the ICME is also a crucial parameter but at minimum of activity only, since it affects the magnetic profile and the arrival time of up to 8 hours.

astro-ph.SR↗

arXiv2023

Statistical Analysis of Interplanetary Shocks from Mercury to Jupiter

In situ observations of interplanetary (IP) coronal mass ejections (ICMEs) and IP shocks are important to study as they are the main components of the solar activity. Hundreds of IP shocks have been detected by various space missions at different times and heliocentric distances. Some of these are followed by clearly identified drivers, while some others are not. In this study, we carry out a statistical analysis of the distributions of plasma and magnetic parameters of the IP shocks recorded at various distances to the Sun. We classify the shocks according to the heliocentric distance, namely from 0.29 to 0.99 AU (Helios-1/2); near 1 AU (Wind, ACE and STEREO-A/B); and from 1.35 to 5.4 AU (Ulysses). We also differentiate the IP shocks into two populations, those with a detected ICME and those without one. We find, as expected, that there are no significant differences in the results from spacecraft positioned at 1 AU. Moreover, the distributions of shock parameters, as well as the shock normal have no significant variations with the heliocentric distance. Additionally, we investigate how the number of shocks associated to stream-interaction regions (SIRs) increases with distance in proportion of ICME/shocks. From 1 to 5 AU, SIRs/ shock occurrence increases slightly from 21% to 34%, in contrast ICME/shocks occurrence decreases from 47% to 17%. We find also indication of an asymmetry induced by the Parker spiral for SIRs and none for ICMEs.

astro-ph.SR↗