Workshop: Kinetic physics of astrophysical plasmas

18 mai 2026, 09:00 → 20 mai 2026, 21:30 Europe/Paris

Bâtiment 20 Amphithéâtre (Montpellier)

The mechanisms of energy dissipation (magnetic or kinetic) that lead to plasma heating or the generation of suprathermal particles occupy a central place in both the heliospheric plasma community and the high energy astrophysics community.  This workshop aims to bring these two communities together to discuss issues of common interest and to share the knowledge gained, both in terms of the physics studied and the numerical methods used. This workshop will leave plenty of room for discussion and exchange between participants. It will focus on three main topics: reconnection, turbulence and shock waves.

This workshop will take place on the campus of Université de Montpellier from May 18 to May 20. The scientific program will be divided into long, didactic review presentations (45 minutes) and shorter presentations on more specialized topics (30 minutes). 

For practical reasons, we will not be able to accommodate more than 60 participants. There will be no registration fee. 

This page will be updated on a regular basis.

Local organizing committee: C. Guépin (LUPM), A. Marcowith (LUPM), C. Sauty (LUPM), M. Lemoine (APC).

Organizing Committee: M. Lemoine (APC - Chair), O. Alexandrova (Obs. Paris), S. Cerri (OCA), B. Cerutti (IPAG), A. Ciardi (LUX), A. Grassi (LULI), A. Marcowith (LUPM), T. Passot (OCA), J. Pétri (Obs. Strasbourg), I. Plotnikov (IRAP), F. Sahraoui (LPP), A. Vanthieghem (LUX).

Zoom link : https://umontpellier-fr.zoom.us/j/91833675232

 

lun. 18 mai

09:15 → 09:45 Welcome

09:45 → 12:30 Turbulence & Transport

Invited speakers : Y.Dubois (IAP), G. Giacinti (APC), P.Lesaffre (ENS)

09:45 Magnetized turbulence and dynamos in compact objects (Chair: Sauty)

Turbulence plays a fundamental role in the transfer, amplification, and dissipation of magnetic and kinetic energy in astrophysical plasmas, from the solar corona to the vicinity of compact objects. In compact objects, including neutron stars, neutron star merger remnants and black-hole accretion flows, turbulent motions are crucial as they drive angular momentum transport, magnetic reconnection, particle acceleration, plasma heating and the generation of large-scale magnetic fields through dynamo action.
This talk will describe our current understanding of turbulence and dynamos in compact-object systems, with a particular emphasis on neutron stars in core-collapse supernova and neutron-star mergers and their remnants, but also on accretion disks around black holes. I will discuss how small-scale instabilities, such as convective instabilities, shear-driven turbulence the magneto-rotational instability, amplify magnetic fields and drive nonlinear turbulent cascades. I will show how these may lead to the generation of large-scale magnetic fields through mean-field dynamo processes and notably form the most magnetized neutron stars, magnetars. These large-scale magnetic fields can also launch relativistic jets in both core-collapse supernova and binary neutron star mergers. The goal of this talk is to provide a broad overview of how turbulence and dynamo action shape compact-object environments and to stimulate discussion on the common plasma processes that connect high-energy astrophysics and heliospheric science.

Orateur: Dr Alexis Reboul-Salze (Max-Planck Institute for Gravitational Physics)

Reboul_Salze_KPAP_presentation.pdf

10:45 Coffee break

11:30 Simulated galaxies with cosmic-ray magneto-hydrodynamics (Chair: Marcowith)

GeV cosmic rays, accelerated in supernova explosions, represent a significant energetic component of the interstellar medium. Through their coupling to magnetic fields, their ability to efficiently diffuse along field lines, and their long radiative loss timescales, they provide a non-thermal feedback capable of significantly influencing galaxy evolution. In particular, they can alter the properties of large-scale galactic winds and contribute to regulating the dense gas that forms stars. Magnetohydrodynamical simulations including cosmic ray transport have highlighted the potentially key role of this non-thermal component in galaxy self-regulation. However, significant uncertainties remain, especially regarding the cosmic ray diffusion in the interstellar medium, a poorly constrained parameter that critically determines their feedback efficiency over galactic scales. In this talk, I will review recent developments and results from galaxy simulations including cosmic rays, with a focus on modeling challenges and current uncertainties.

Orateur: Dr Yohan Dubois (IAP)

Talk_YD.pdf

12:30 → 14:00 Lunch break

14:00 → 17:30 Turbulence & Transport

Invited speakers : Y.Dubois (IAP), G. Giacinti (APC), P.Lesaffre (ENS)

14:00 Cosmic-ray propagation in our Galaxy: Insights from the diffuse emission, TeV halos and the CR anisotropy (Chair: Lemoine)

Gamma-ray observations provide important information on cosmic-ray (CR) propagation in our Galaxy.
First, we present a new model of CR propagation in the Milky Way, where CRs are injected at discrete transient sources in the disc. We then calculate the corresponding diffuse Galactic gamma-ray emission. We find that the diffuse gamma-ray emission at >~ 100 TeV is very clumpy, and does not correlate with the gas density along the line of sight. It is substantially different from the relatively smoother emission detected by Fermi at ~ GeV energies. We also discuss how many hadronic and leptonic PeVatrons would be detectable in our simulations, and compare with LHAASO data. We show that this allows to place new constraints on PeV CR sources.
Second, we discuss TeV gamma-ray halos around old pulsars. We show that current gamma-ray measurements place interesting constraints on the turbulent magnetic fields around these pulsars, and we examine the implications for CR transport. In particular, we study the case of LHAASO’s “peanut-shaped” source.
Finally, we discuss the implications of the observed TeV-PeV cosmic-ray anisotropies on CR propagation in the ISM.

Orateur: Dr Gwenael Giacinti (APC Paris)

KPAP_2026_Giacinti.pdf

15:00 Magnetic field line and cosmic-ray transport in 3D polarised synthetic turbulence (Chair Lemoine)

Field line random walk (FLRW) is an essential process involved in charged particle transport, especially cosmic rays (CRs). In general, CR can jump from one FL to another due to scattering processes or because of the finite size of their gyro-motion. However, when the Larmor radius is sufficiently small, CRs tend to move along magnetic FLs for a certain amount of time between scattering events. Thus, how turbulence shapes magnetic field transport strongly constrains the propagation of CRs.
Using high precision polarized synthetic turbulence simulations, we study FL transport in Alfvén-like and magnetosonic-like polarized turbulence where polarisation configurations are inspired by linearized ideal MHD modes. We show remarkably different transport behaviors with respect to random polarisation. It consequently influences CR diffu

Orateur: M. Matthieu Bouchet (LAPTH)

KPAP2026_Matthieu_BOUCHET.pdf

15:20 Coffee break

16:05 Interstellar medium turbulence from numerical experiments (Chair: Guillet)

I will very briefly review the basics of the legacy of Kolmogorov
turbulence statistics. Interstellar turbulence is very far from the
simplicity of incompressible flows where Kolmogorov ideas
apply. Indeed, the interstellar medium is very dilute, extremely
compressible, multiphase, partially ionised, magnetised, and both dust
and cosmic rays travel are coupled with it. Nevertheless, we still use
the tools Kolmogorov devised to probe a variety of flows in
astrophysics. I will show how statistical laws (such as the extended
self-similarity) unexpectedly apply in isothermal
magnetohydrodynamical (MHD) simulations, and how this might be used to
detect the coherent discontinuities which quickly form in supersonic
turbulence. One of the suprising result lies in the lack of
sensitivity of the macroscopic parameters of these discontinuities to
the dissipation coefficients. This is also supported by recent work
with JB Durrive on analytical solutions of Burgers equations. I will
finally present our attempts to generate realistic (in the sense that
coherent structures are reproduced) turbulent fields at a much lower
cost than full fledge numerical simulations.

Orateur: Dr Pierre Lesaffre (CNRS/LEPNS)

kpap-2026-final.pdf

17:05 Modelling global plasma transport in the magnetospheres of gas giant planets (Chair : Guillet)

The magnetospheres of giant planets are governed by the interplay of these planets’ fast rotation,
the solar wind and inner plasma sources. In the Saturn and Jupiter magnetospheres, plasma is mainly
produced by the ionization of neutral gas tori at the radial location of active moons: Io at Jupiter
and Enceladus at Saturn. The mechanisms by which these moon-associated plasma sources are re-
distributed throughout these magnetospheres involve both plasma motions and magnetic flux tube
exchanges which can be modeled as fairly axisymmetrical in the inner regions of the system. These motions are coupled to the rotation of the planets through electric current systems
originating in the equatorial plasma disk and closing into their upper atmosphere and ionosphere. Plasma additionally undergo a second type of large-scale convection due to the interaction between the magnetosphere and the Solar Wind, inherently asymmetrical in azimuth.

Based on the magnetosphere-ionosphere coupling approach, we build an analytical model of axisymmetrical plasma transport and solar-wind-driven convection in the gas giant magnetospheres. The specificity of each system is discussed as well as the way forward for modelling work.

Orateur: Mlle Marie Devinat (IRAP)

KPAP_DEVINAT_2026_compressed.pdf

mar. 19 mai

09:25 → 12:30 Shocks / High-energy processes /

Invited speakers: A.R. Bell (Univ Oxford, UK), G Fichet de ClaireFontaine (Univ Valencia, Spain), P. Cristofari (LUX)

09:25 Cosmic ray PeVatrons (Chair: Giacinti)

Recent observations have opened a new window on the long-standing question of the origin of Galactic cosmic rays up to PeV energies. In particular, the LHAASO experiment has revealed a population of ultra-high-energy gamma-ray sources extending beyond 100 TeV, providing compelling evidence for Galactic pevatrons (accelerating CRs up to 10$^{15}$ eV). These results challenge the traditional paradigm in which supernova remnants (SNRs) are the dominant sources of PeV cosmic rays, as most of the sources detected sources seem to not be associated with any SNR.

Additional candidates such as microquasars and X-ray binaries—where powerful jets and strong shocks can potentially accelerate particles to extreme energies—are gaining increasing attention alongside young massive stellar clusters, pulsar wind nebulae, and superbubbles. The latest observational breakthroughs will be reviewed, along with their implications and open questions on particle acceleration and escape.

Orateur: Dr Pierre Cristofari (Observatoire de Paris)

KPAP_2026_pevatrons_Cristofari.pdf

10:25 coffee break

11:10 Does the Bohm limit determine the maximum of energy of shock-accelerated cosmic rays? (Chair: Cristofari)

Ever since the classic papers of Lagage & Cesarsky (1983) and Hillas (1984), the Bohm limit uBR has been taken as the measure of the maximum energy to which cosmic rays can be accelerated by shocks. The obvious problem in the 1980s was that the Bohm limit for supernova remnants fell well short of the knee in the spectrum at a few PeV. This problem appeared to be solved when fields of a few hundred microGauss fields were observed in young supernova remnants and the theory of non-resonant magnetic field amplification said this should be so. But the problem was not solved. Gamma-ray observations imply a turnover in the cosmic ray spectrum at a few hundred TeV. Furthermore, the time-dependent theory of non-resonant amplification said again that this should be so since there is insufficient time to amplify the field on the Larmor scale of PeV cosmic rays.
The Bohm limit is based on the assumption that cosmic ray transport is diffusive and that the minimum mean free path is the Larmor radius. As early as 1966 Jokipii pointed out the possibility that cosmic rays could be confined by mirrors ahead of the shock, but he did not integrate this into a broader model in which cosmic rays cross the shock many times. Mirroring has recently been receiving attention in cosmic ray transport in general. Here I examine the possibility that non-diffusive transport with mirroring may explain cosmic ray acceleration beyond the Bohm limit (Bell et al 2025).

Orateur: Prof. Tony Bell (University of Oxford)

Montepellier.pdf

Montepellier.pptx

12:10 Generation and characterization of scaled magnetic turbulence in the laboratory (Chair: Cristofari)

We will present the characterization of magnetized turbulence generated in a laser-driven plasma. The work was performed at the LULI2000 laser facility (France). Starting from an homogenous magnetized plasma, produced by having a high-strength pulsed magnetic field [1] embedded in a gas jet, we randomly perturbed the gas using a speckled laser beam [2]. Using proton radiography [3], we will show quantitatively that the produced turbulence is intermittent and that its power distribution is akin that recorded in space [4,5,6]. We will also discuss of the applications for laboratory tests of astrophysical mechanisms involving turbulence, whether particle acceleration at high-velocity shock fronts [7].
[1] B. Albertazzi, et al., “Production of large volume, strongly magnetized laser-produced plasmas by use of pulsed external magnetic fields,” Review of Scientific Instruments, 84, 043505 (2013).
[2] W. Yao, et al., “Triggering and probing electromagnetic stochasticity in a low-density magnetized plasma irradiated by a multi-speckled laser beam,” CLF Annual Report 2021-22,
2022. https://www.clf.stfc.ac.uk/Gallery/13%20-%20Yao_CLF%20AR%2021-22.pdf
[3] D.B. Schaeffer, et al., “Proton imaging of high-energy-density laboratory plasmas,” Reviews of Modern Physics, v. 95(4), p. 045007 (2023).
[4] L. Sorriso-Valvo et al., “Observation of Inertial Energy Cascade in Interplanetary Space Plasma”, Phys. Rev. Lett. 99, 115001 (2007).
[5] V. Carbone et al., “Scaling Laws of Turbulence and Heating of Fast Solar Wind: The Role of Density Fluctuations”, Phys. Rev. Lett. 103, 061102 (2009).
[6] O. Alexandrova et al., “Universality of Solar-Wind Turbulent Spectrum from MHD to Electron Scales”, Phys. Rev. Lett. 103, 165003 (2009).
[7] A. R. Bell, “Cosmic ray acceleration”, Astroparticle Physics 43, 56 (2013).

Orateur: M. Itamar Cohen (Technion Israel Institute of Technology)

B turbulence Itamar Cohen KPAP26.pdf

12:30 → 14:00 Lunch break

14:00 → 14:20 Presentation of the COST action

Orateurs: Dr Anna Grassi (LULI), Arno Vanthieghem (Observatoire de Paris)

Presentation_Expand.pdf

14:20 → 14:30 Flash presentations

Short 2-3 minutes presentation of master student internship

G. Bilosi, X. Gong, TBC

DSA_presentation_Quentin.pdf

FLASH PRESENTATION - BILOSI (2).pdf

XingweiGongFlashTalk.pdf

14:30 → 16:35 Shocks / High-energy processes /

Invited speakers: A.R. Bell (Univ Oxford, UK), G Fichet de ClaireFontaine (Univ Valencia, Spain), P. Cristofari (LUX)

Président de session: Dr Guépin

14:30 Multimessenger emissions from active galactic nuclei (Chair: Guépin)

Jets of active galactic nuclei (AGN) rank among the most promising candidates for the sources of extragalactic high-energy neutrinos. In this presentation, I will begin by briefly building a broad picture of these jets first from recent observations, then by covering our current theoretical interpretations of their dynamics and multi-wavelength emissions, which will define a set of open questions. I will then turn to recent theoretical advances, ranging from analytical models to relativistic magnetohydrodynamic simulations of jets, which together shed light on some of these open questions. In particular, I will present a scenario in which the interaction of an AGN jet with red giant stars provides a site for particle acceleration and, potentially, high-energy neutrino production. I will close by discussing what these results imply for the future of multi-messenger astronomy.

Orateur: Dr Gaëtan Fichet de Clairfontaine (University of Würzburg)

MPL_GFDC.pdf

MPL_GFDC.pptx

15:30 coffee break

16:15 Impact of photon-photon pair creation on extreme gamma-ray plasma interaction (Chair: Guépin)

Compton-driven interactions between ultraintense gamma-ray fluxes and background plasmas are expected to arise in various high-energy astrophysical settings. Self-consistent kinetic investigations into this problem are now accessible via particle-in-cell (PIC) simulations [1-3], which have recently uncovered a rich phenomenology of acceleration processes and plasma instabilities. Here, we report on the first PIC simulation study of such events allowing for electron-positron pair creation through photon-photon collisions (the linear Breit-Wheeler process).
First, we detail the Monte Carlo scheme incorporated in the CALDER code to describe the linear Breit-Wheeler process and benchmark it against the theory of Ref. [4]. We then focus on a reference configuration characterized by an electron-ion plasma of density 𝑛𝑝=1021cm−3, a photon energy of 𝜀𝛾=1.8𝑚𝑒𝑐2, and a photon density profile linearly rising up to 𝑛𝛾∼5×107𝑛𝑝. The Compton-scattered photons rapidly accelerate most of the plasma electrons to relativistic speeds and subsequently interact with the incoming photons. This creates pairs in quantities exceeding the initial plasma density by a factor of ~30, thereby drastically altering the system dynamics.
Regarding pair production, we show that, although the first scattered photons are sub-threshold, backward- accelerated electrons via charge separation scatter photons above the threshold. This initiates a cascade where the resulting pairs further emit above-threshold photons, triggering a snowballing of the pair yield.
Due to their low inertia, positrons act to screen the charge-separation field, gaining significant energy (up to ~20× the incident photon energy) at the expense of the background ions. While this mitigates ion acceleration compared to pair-free scenarios [3], pair creation enhances ion heating. To explain this, we have solved the linear electrostatic dispersion equation for the three-interacting species (electrons, positrons, protons) using the simulated particle distribution functions. We find that, without pair creation, ion heating stems from a kinetic ion acoustic instability triggered by the electrons drifting against the slower protons. Theoretical predictions match the simulated wave Fourier spectra. However, when enabling pair creation, the leading instability grows faster, and operates in an intermediate regime between the hydrodynamic Buneman and kinetic ion-acoustic instabilities.
Finally, we examine how the global dynamics of the system scales with the photon beam density and energy.
References
[1] F. Del Gaudio et al., Phys. Rev. Let. 125, 265001 (2020).
[2] B. Martinez et al., J. Plasma Phys. 87, 905870313 (2021).
[3] J. C. Faure et al., Phys. Rev. E 109, 015203 (2024).
[4] R. Schlickeiser et al., Astrophys. J. 758, 101 (2012

Orateur: M. Léo Molinier (CEA)

16:35 → 17:15 Gamma-Ray Emission from Protostellar Jets: From Candidates to Sources (Chair: Brunn)

Protostellar jets, reaching velocities of up to 1000 km/s, are a ubiquitous feature of the late stages of star formation. In several systems, these jets exhibit synchrotron radio emission, providing clear evidence for the presence of relativistic electrons and ongoing particle acceleration. Models of non-thermal emission from individual jets indicate that only the most powerful objects -such as HH80-81, recently associated with the Fermi source 4FGL J1818.5-2036- are expected to produce detectable gamma-ray fluxes. However, massive stars typically form in clustered environments, where multiple protostellar jets coexist. In this context, the collective contribution of an entire population of jets within a star-forming region may significantly enhance the prospects for gamma-ray detection. In this talk, I will present an overview of the non-thermal processes operating in protostellar jets, along with recent observational developments linking these systems to gamma-ray sources. Although protostellar jets were only considered candidate gamma-ray emitters until recently, growing observational evidence now suggests that they are beginning to emerge as a new class of high-energy sources. This opens a promising window for studying particle acceleration under conditions of relatively low-velocity shocks and high-density plasma, complementing more extreme astrophysical accelerators.

Orateur: Dr Anabella Araudo (Institute of Physics, Czech Academy of Sciences)

YSO_Workshop_Montpellier-2.pdf

19:30 → 22:30 Dinner

mer. 20 mai

09:00 → 12:30 Magnetic Reconnection & shear acceleration

Invited Speakers: N. Aunai (LULI), V. Brunn (Obs Arcetri, Italy), S. Masson (LPP)

09:25 Interactions between the turbulent solar wind and planets (Chair: Marcowith)

This review synthesizes global hybrid Particle-in-Cell (PIC) simulations using the Menura code to investigate how solar wind turbulence dynamically reshapes the plasma environments of magnetized and unmagnetized bodies. We employ a two-step dynamical approach: decaying turbulence simulations to generate a fully developed turbulent solar wind, followed by global simulations of the time-dependent interaction between turbulent solar wind and obstacles. We focus on the time-evolving morphology of bow shocks, magnetosheaths, and ion foreshocks, including the generation of transient structures and their propagation. We also explore how the turbulent nature of the solar wind affects kinetic instabilities in the magnetosheath, as well as the formation of magnetic holes, and alter particle acceleration and energy dissipation across plasma boundaries.

We find that the turbulent nature of the solar wind amplifies quasi-perpendicular shock oscillations and disrupts both foreshock and magnetosheath coherence. Solar wind turbulence significantly modifies the onset of the mirror instability in the magnetosheath, with growth timescales comparable to magnetosheath transit times, resulting in persistent unstable plasma in a small magnetosphere such as Mercury. Magnetic holes form via nonlinear mirror mode evolution in turbulent conditions, stabilized by time-dependent ion trapping. Large-scale solar wind structures are transmitted and compressed across the bow shock. Mercury’s magnetosheath exhibits faster dynamical responses than Earth’s, leading to high fractions of unstable plasma and short relaxation timescales. For comets, turbulence alters the structure and the dynamics of induced magnetospheres, generating bursty cometary plasma escaping structures due to forced magnetic reconnection, modifying the transport of particles and fields.

These simulations provide a global, time-resolved framework for studying the dynamical interplay between a supersonic turbulent plasma flow, such as the solar wind, and obstacles, such as planets and comets, offering predictive insights for future space missions like BepiColombo and Comet Interceptor.

Orateur: Dr Pierre Henri ((1) Laboratoire Lagrange, Observatoire Côte d'Azur, Université Côte d'Azur, CNRS, Nice, France & (2) LPC2E, CNRS, Univ. Orléans, CNES, Orléans, France)

10:25 Flaring activity in kinetic simulations of an accreting black hole (Chair: Marcowith)

The dynamics of black hole magnetospheres critically depend on the black hole spin and on the structure of the accretion flow. In the limit of a Schwarzschild black hole immersed in a zero-net angular momentum flow, accretion is spherical. However, in the presence of a large-scale vertical magnetic field, the classical Bondi accretion model is significantly altered. The frozen-in field is stretched radially as the plasma is pulled inward by gravity. This continues until the restoring force from the magnetic tension suddenly expels the material and resets the field, allowing a new cycle to begin.

Although this scenario has been well depicted in previous studies, it remains incomplete as the issues of dissipation and particle acceleration are not yet fully resolved. In this work, we aim to revisit these issues with a first-principles kinetic plasma model. We perform two-dimensional global general relativistic particle-in-cell simulations of magnetized spherical accretion onto a Schwarzschild black hole, for both pair and electron-ion plasmas. The simulations are evolved over long timescales to capture multiple flux eruption events and establish a quasi-steady state.

For each accretion cycle, we find that the system goes through three main stages: (i) an ideal advection phase where magnetic flux through the horizon increases quasi-linearly with time; (ii) a reconnection-regulated phase where the net increase of the flux is slowed down by intermittent reconnection events near the horizon; and (iii) a flaring phase when a major, large-scale reconnection event expels the flux, leading to efficient particle acceleration. The emergence of large-amplitude quasi-periodic flux eruptions and concomitant particle acceleration is reminiscent of Sgr A* flaring activity. This phenomenon could also be applicable to quiescent black holes, especially isolated black holes accreting the interstellar medium.

Orateur: M. Enzo Figueiredo (IPAG)

10:45 coffee break

11:30 Solar energetic particles : observations and modelling (Chair: Plotnikov)

Solar eruptions are initiated and develop in the solar corona and are one of the most extreme manifestation of the sun impulsive release of energy. Solar particles are accelerated during solar eruption. We observe their radiative signatures in the solar atmosphere over a large range of wavelengths from EUV to gamma rays and measure them in-situ in the heliosphere. Solar energetic particles provide a unique opportunity to study particle acceleration and transport processes in astrophysical plasmas. I will present a comprehensive overview of the current understanding of Solar Energetic Particles, focusing on their observational diagnostics and theoretical modeling.
I will first describe how complementary multi-wavelength and multi-messenger diagnostics offer powerful constraints on the physical processes governing particle acceleration, injection, and propagation. Then I will focus on the main physical mechanisms proposed to explain solar particle acceleration such as acceleration at CME-driven shocks, magnetic reconnection in solar flares, and stochastic acceleration in turbulent environments. I will highlight the strengths and limitations of existing models, as well as the challenges in reconciling them with the observations. Finally, while a wide range of observational diagnostics and theoretical modelling have provided valuable insights, they are often interpreted within simplified and/or localized frameworks. In many cases, the global dynamics of the eruptive event are not taken into account. This can lead to ambiguities or even misconceptions regarding the timing, location, and efficiency of particle acceleration and injection. I will discuss how developing multi-scale models to simulate the global context of solar eruptions is essential to interpret SEP observations and to go beyond our current understanding of their acceleration, injection and propagation.

Orateur: Dr Sophie Masson (LPP - Observatoire de Paris)

KPAP_2026_Masson.pdf

12:30 → 14:00 Lunch break

14:00 → 16:50 Magnetic Reconnection & shear acceleration

Invited Speakers: N. Aunai (LULI), V. Brunn (Obs Arcetri, Italy), S. Masson (LPP)

14:00 A new view on magnetic reconnection in the Earth’s magnetosphere (Chair: Henri)

Magnetic reconnection at Earth’s magnetopause is the key process controlling the coupling between the solar wind and the magnetosphere, enabling plasma and magnetic flux from the interplanetary medium to enter an otherwise largely closed cavity. A central question is where reconnection develops along the magnetopause, since its location strongly influences the efficiency of this coupling. Despite decades of work, predicting the reconnection line remains difficult because the process is inherently multiscale, while spacecraft observations provide only local, in situ measurements and kinetic numerical modeling at large scale is extremely costly.

Recent progress has become possible by combining decades of observations from multiple missions with innovative machine-learning-based analysis. These approaches have yielded new insight into the environmental and physical constraints that govern both the location of the dayside reconnection line and the reconnection efficiency. This presentation will review these advances, from large-scale statistical results based on in situ measurements to new models of the X-line and recent developments in multiscale modeling to offer a complementary perspective on magnetopause reconnection.

Orateur: Dr Nicolas Aunai

KPAP26_AUNAI.pdf

15:00 A generalized Fermi model of shear acceleration (Chair: Henri)

Shearing flows give rise to a Fermi-type acceleration and have been proposed to explain e.g. the presence of high-energy electrons along astrophysical jets and to contribute to the acceleration of ultra-high energy cosmic rays.
Particles diffusing along the shearing direction experience a change in the flow speed, which translates to a change in momentum. In a Fokker-Planck picture, shear acceleration can be described by a diffusion and drift in momentum, where assumptions on the particle scattering and flow profile go into the corresponding Fokker-Planck coefficients.
We present a stochastic model, following Lemoine, 2025, where those assumptions can be relaxed, allowing to study non-gradual shear profiles and the influence of anisotropic spatial diffusion. The momentum evolution of a particle is tracked in a comoving frame that moves with the plasma velocity, assuming that perturbations in the magnetic field are carried with the flow. We find that shear acceleration is most efficient in mildly-relativistic jets while high-relativistic jet speeds lead to particle trapping in space and momentum. The latter results in a subdiffusive evolution of the particle momentum leading to power-law spectra that go beyond Fokker-Planck expectations.

Orateur: Dr Sophie Aerdker (APC Paris)

Aerdker-Montpellier_2026 .pdf

15:20 short break

15:50 In-situ particle acceleration by magnetic reconnection in young stellar objects (Chair: Araudo)

Young stellar objects host strongly magnetised and highly variable plasma environments in which several acceleration processes may generate suprathermal and non-thermal particles. In this talk, I will discuss two complementary regimes of particle acceleration in the star-disc system. The first is associated with large-scale magnetic reconnection during flares in the innermost regions, where impulsive energy release can produce intense bursts of energetic particles. The second is related to turbulence-driven reconnection operating from the inner to intermediate disc regions, where particle acceleration may occur in a more distributed and persistent way.

I will present the physical conditions under which these two mechanisms can operate and the expected properties of the accelerated particle populations. This comparison provides a framework to distinguish between impulsive and continuous sources of non-thermal ionisation in young stellar environments.

I will then show how these particles can affect the chemistry of the inner disc by enhancing ionisation in layers that are important for molecular processing and line formation in the JWST band. In particular, I will discuss how flare-accelerated and turbulence-accelerated particles may modify the abundances of key molecules in the warm inner disc, and how these changes could translate into observable signatures in infrared molecular tracers. More broadly, the aim is to connect kinetic plasma processes in young stellar objects with their chemical and observational consequences, and to assess whether energetic particles accelerated by flares and turbulent reconnection should be considered as an important ingredient of inner-disc physics.

Orateur: Dr Valentin Brunn (Arcetri Observatory)

BrunnKPAP2026.pdf

16:50 → 16:55 End of the workshop