Discussions and synergies on astrophysical plasma with PIC simulations - Prague 2026 edition

29 Sept 2026, 09:00 → 7 Oct 2026, 18:00 Europe/Prague

Slov - "Sál V. Dvořáka: hlavní sál" - A/1.NP/10 (Slovanka)

• Face-to-face meeting between different groups working with particle-in-cell simulations in the context of astrophysical systems.

 

• Main event : from Tuesday September 29th afternoon up until Friday October 2nd early afternoon.

 

• This meeting is part of a long series of f2f workshops, with the past ones in the last three years occurring in Potsdam and Tokyo.

 

• This year we are organising it in Prague at the Institute of Physics of the Czech Academy of Sciences (FZU) and we aim to facilitate also remote attendance for colleagues abroad that are unable to attend in-person.

 

• The "hackathon" event will take place at FZU, in a different room, from Monday October 5th to Wednesday October 7th. For the hackathon, remote connection will be on-demand.

 

Tuesday 29 September

Caj a kava: afternoon break

Presentations & discussions: Session I.a.

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

1 Welcome notes & logistics

Simply setting the framework of this face-to-face meeting

Speaker: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

2 Preliminary simulations of astrophysical double-shock structures

TBD

Speaker: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

3 Caviton-Driven Bremsstrahlung: A novel mechanism for fast radio bursts from magnetars

Speaker: Anabella Teresa Araudo

16:00 Buffer break

Presentations & discussions: Session I.b.

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

4 The Role of Bell Turbulence Driven by Proton Cosmic Rays on Electron Acceleration at Oblique Shocks

Shock acceleration is generally a highly coupled problem involving nonlinear interactions between thermal plasma, waves, and nonthermal particles called cosmic rays (CRs). Upstream of the shock, the super-Alfvénic drift of CR protons excites the nonresonant streaming (Bell) instability. These unstable modes can grow to nonlinear levels, generating strong turbulence that further impacts electron dynamics. Although particle-in-cell (PIC) simulations provide a first-principle approach to shock acceleration physics, they do not always reproduce efficient electron energization. These simulations are limited to phenomena at electron kinetic scales, which makes investigating the impact of CR proton-driven turbulence on electron acceleration challenging. Using a recently developed simulation framework, we inject such turbulence upstream of non-relativistic high-Mach-number shocks. Our simulations reveal shock front corrugations that significantly influence the overall electron behavior. We observe a pronounced power law in the downstream electron energy spectrum and the onset of diffusive shock acceleration, both of which are absent in simulations with an initially homogeneous upstream medium.

Speaker: Karol Fulat (UW-Madison)

5 Nonthermal Particle Acceleration by Magnetic Pumping in Pulsating Plasmas

We present a new "pulsating box" setup to investigate particle acceleration in high-beta plasmas undergoing compression-expansion cycles. Our fully kinetic simulations show that particles are efficiently accelerated by magnetic pumping, producing nonthermal energy distributions with power-law tails. Numerical results are in excellent agreement with a generalized maximum entropy model that we derive, linking the power-law index of distributions to the injected energy. Our results are relevant for understanding the origin of high-energy particles in space and astrophysical plasmas.

Speaker: Dr Giuseppe Arrò (University of Wisconsin - Madison)

19:00 Casual dinner

Suggestion : Cobolis restaurant

• Gmap
• Website

Wednesday 30 September

Caj a kava: dobry den

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

Presentations & discussions: Session II.a

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

6 PIC simulations of shocks at novae

Preliminary results of PIC-simulation of shocks at novae:
structure, turbulence, instabilities, particle acceleration.

Speaker: Dr Oleh Kobzar (Astronomical Observatory, Jagiellonian University, Kraków, Poland)

7 Kinetic Study of Shock Formation in Protostellar Jets

Synchrotron radio emission has been detected from jet termination regions of young stars, indicating the presence of relativistic electrons. Non-thermal electrons are most probably accelerated through the Fermi I acceleration. However, thermal electrons in the jet are not energetic enough to traverse the shock. This brings up the need for an electron pre-acceleration mechanism. We perform Particle-in-Cell (PIC) simulations of these astrophysical shocks to study the Shock Surfing Acceleration (SSA) mechanism. We will present preliminary results of 1D simulations of perpendicular shocks in electron-proton weakly magnetised plasma.

Speaker: Jakub Gazdoš

11:45 Lunch

Presentations & discussions: Session II.b

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

8 Transition from Proton Cyclotron to Weibel Instabilities in Perpendicular Non-relativistic Shocks

Using 2D3V fully kinetic PIC simulations, we investigate how the dominant ion-scale instability regime evolves with Alfvénic Mach number ($M_A = 8.6-23$). At low $M_A$ ($\lesssim10$), the proton cyclotron instability (PCI), driven by ion temperature anisotropy, dominates and is accompanied by whistler-mode waves excited by electron temperature anisotropy in the shock ramp. At high $M_A$ ($\gtrsim20$), the Weibel filamentation instability, sustained by counterstreaming reflected ion beams, becomes decisively dominant. The transition occurs between $M_A = 8.6$ and $15$; the Weibel growth number and the transverse magnetic field amplification both rise, while whistler power drops, revealing a shift from wave-dominated to filamentation-dominated shock dissipation.

Speaker: Valentine Devos (Max Planck Institute for Plasma Physics)

9 Kinetic Simulations of moderate Relativistic Pair Shocks with Trace Ion Admixture

Relativistic collisionless shocks accelerate particles through collective wave-particle interactions. In pair plasmas, turbulence is self-generated via the Weibel instability, but its structure and efficiency depend sensitively on plasma composition and magnetization. We perform large-scale Particle-in-Cell (PIC) simulations to study relativistic pair shocks with a sub-percent ion admixture, a regime that has not been explored before and is directly relevant for pair-enriched astrophysical environments such as GRB afterglows. Even a small fraction of ions can introduce new wave–particle interaction channels, potentially modifying magnetic turbulence, shock structure, and the efficiency of non-thermal particle acceleration.

Speaker: Mahmoud Alawashra (DESY)

Caj a kava: afternoon break

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

Presentations & discussions: Session II.c

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

10 Nonthermal particle acceleration in shock-mediated relativistic plasma turbulence

Turbulence with sufficiently strong and compressive driving develop shocks, which influence the energy cascade and dissipation. The properties of collisionless shock-mediated turbulence remain poorly understood. Using PIC simulations, we find efficient particle acceleration, which may be relevant for modeling nonthermal emission from relativistic jets.

Speaker: Karol Fulat (UW-Madison)

11 Kinetic simulations of oblique high-Mach number shocks propagating in Alfvenic turbulence

Astrophysical collisionless shocks like that of supernova remants are efficient particle accelerators that require some pre-acceleration mechanism in order for electrons to participate in diffusive shock acceleration. Using the PIC code, THATMPI, we perform novel simulations of oblique non-relativistic high-Mach-number shocks propagating into an upstream containing pre-existing decaying turbulence similar to Alfvenic turbulence. We investigate how the amplitude of turbulent fluctuations impact the shock structure, plasma instabilities, and electron pre-acceleration

Speaker: Eloise Moore (Uni Potsdam)

19:00 Dinner

Suggestion : Cafe Louvre

• Gmap
• Website

Thursday 1 October

Caj a kava: dobry den

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

Presentations & discussions: Session III.a.

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

12 Signatures of Weibel instability on Earth`s bow shock: MMS observations

Collisionless shocks dissipate the incoming flow’s kinetic energy through collective plasma processes rather than particle collisions, and at supercritical Mach numbers this dissipation relies on the reflection of a fraction of the incoming ions back into the upstream region. The resulting counter-streaming of reflected and incoming ion populations can drive ion Weibel instability, characterized by growing quasi-static filamentary currents. We study signatures of this instability at Earth’s bow shock using burst-mode MMS observations of quasi-perpendicular crossings, and compare them to particle-in-cell simulations spanning the same range of Alfvénic Mach numbers. We calculate magnetic field amplification across the shock ramp and overshoot for each crossing and show that it follows the Mach-number dependence predicted by simulations, consistent with a Weibel driven origin. We further corroborate this identification individually through wave analysis, confirming additional individual crossings that show signatures of this instability.

Speaker: Valentina Villaflor (Max Planck Institute for Plasma Physics)

11:45 Lunch

Presentations & discussions: Session III.b

13 Charge Accumulation in Rotating Black Hole Magnetospheres: A First-Principles Study

Whether a rotating black hole in an external magnetic field actually charges up to the theoretical Wald value has remained an open question with major implications for magnetospheric structure and particle acceleration. Using general-relativistic particle-in-cell simulations with self-consistent pair production, we evolve black holes of different spins from two opposite starting charges and find that both converge to the same steady-state value within a few tens of gravitational times, showing the equilibrium is a true dynamical attractor rather than a relic of initial conditions. This equilibrium charge sits well below the Wald prediction — around 30% of it for moderate spins, dropping toward zero as the spin approaches its maximum value. We show this behavior matches a simple analytic formula derived by balancing the magnetic flux carried by positive and negative charges through the horizon, confirming that charge transport along field lines governs the result. The findings establish that a black hole's charge state is set by kinetic plasma physics rather than the Wald formula alone, and that since the equilibrium charge remains below the Wald value, it does not shut down the potential drop that powers energy-extraction mechanisms like Blandford-Znajek.

Speaker: Farukh Abdulkhamidov (Silesian University in Opava)

14 Kinetic modeling of binary neutron star magnetospheres

Presentation

Speaker: Jan Benacek

Caj a kava: afternoon break

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

Presentations & discussions: Session III.c

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

15 Energy partition in collisionless counter-streaming plasmas

Speaker: Frederico Fiuza

16 Mapping particle acceleration across collisionless shock parameter space

Speaker: Marco Costa

17 Data-driven anomalous resistivity models from kinetic simulations

Speaker: João Biu

18 Analytical time-dependent distribution functions for driven PIC simulations

We present analytical expressions for the evolution of a general, gyrotropic, relativistic distribution function (VDF) of a collisionless, uniform, magnetized plasma subject to external forcings in the form of density and magnetic field variations. By analytically solving the relativistic, time-dependent drift kinetic equation for a collisionless, uniform plasma, a solution is obtained in terms of an initial condition. When considering an initial nonrelativistic Maxwellian VDF, a Bi-Maxwellian form is naturally recovered. Interestingly, when considering an initial relativistic Maxwell-Jüttner VDF, we obtain a new time-dependent anisotropic form of the Maxwell-Jüttner. By taking pressure moments of this distribution, relativistic double-adiabatic equations are obtained, extending the well-known CGL equations. We validate these double-adiabatic equations and our new time-dependent VDF with PIC simulations of shearing and compressing boxes.
These distributions can be used in linear stability analysis for accurately predict instabilities beyond the bi-Maxwellian approximation and quantification of distortions in the VDF by wave-particle interactions. Similar studies can be performed for other initial VDFs like power-laws and kappa distributions.

Speaker: Francisco Ley (University of Chile)

19:00 Casual dinner

Suggestion : Oishi

• Gmap
• Website

Friday 2 October

Caj a kava: dobry den

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

Presentations & discussions: Session IV.a

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

19 Particle acceleration and heating in a highly magnetized mildly relativistic shock

We demonstrate that magnetized mildly relativistic shocks may be efficient particle accelerators capable of accounting for the intense X-ray and gamma-ray emission observed in jets of active galactic nuclei (AGN).

We employ the particle-in-cell (PIC) method to investigate a mildly relativistic shock in a magnetized electron-ion plasma under two magnetic field configurations: an oblique shock with the magnetic field oriented just below the critical angle and a quasi-parallel shock. While the only difference between both simulations is the obliquity of the magnetic field, the results show notable difference between both configurations.

In the oblique configuration, a portion of electrons, which move predominantly along magnetic field lines, is efficiently trapped by the cross-shock potential. Trapped cannot readily escape and instead oscillate within the potential peak at the shock. During this motion, the motional electric field, aided by the ExB drift, accelerates trapped electrons to very high energies. Ions are likewise trapped, albeit within potential troughs, and attain comparable maximum kinetic energies to that of electrons. A portion of trapped ions may escape upstream, undergoing shock surfing acceleration along the shock front.

In the quasi-parallel configuration, elliptically polarized whistler waves are self-generated at the shock and propagate upstream. A fraction of ions escape from the non-stationary downstream region into the upstream, where they are trapped by longitudinal electric waves generated by the oblique propagation of whistler waves. This results in efficient heating to relativistic temperatures. Additionally, several plasma instabilities develop, including the modified two-stream instability (MTSI) and the parametric decay instability (PDI), further contributing to enhanced turbulence and heating.

Speaker: Gabriel Torralba Paz (Max Planck Institute for Plasma Physics)

Prague_Meeting_2026_Gabriel.pdf

20 Kinetic simulations of mildly relativistic weakly magnetized shocks in AGN jets

Plasma shock waves are among the most promising sites for efficient particle acceleration in extragalactic jets. In mildly relativistic electron–ion shocks, electrons can be heated to high Lorentz factors, providing a natural explanation for the large minimum electron energies often required to model the emission of BL Lac objects (Arbet-Engels et al. 2025). In this work, we present results from particle-in-cell (PIC) simulations of mildly relativistic weakly magnetized shocks, covering a broad range of shock Lorentz factors (γ = 1.5–16), magnetizations (σ = 10⁻⁴–0.1), and shock obliquities. We investigate ion-to-electron energy transfer, magnetic field amplification, and particle acceleration, and their dependence on upstream shock parameters. Finally, we discuss the implications of these results for multiwavelength emission modeling.

Speaker: Artem Bohdan (Max Planck Institute for Plasma Physics)

12:00 Lunch

Presentations & discussions: Session IV.b

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

21 THATMPI : the road to an updated main branch

Technical internal contribution of current and past THATMPI developpers

Speaker: Martin Pohl

Presentations & discussions: Farewells + information for the hackathon

Convener: Michelle Tsirou (Institute of Physics of the Czech Academy of Sciences (FZU))

Monday 5 October

Hackathon

Convener: Jan Benacek

12:00 Lunch

Hackathon

Convener: Jan Benacek

Tuesday 6 October

Hackathon

Convener: Jan Benacek

12:00 Lunch

Hackathon

Convener: Jan Benacek

Wednesday 7 October

Hackathon

Convener: Jan Benacek

12:00 Farewell lunch

Hackathon

Convener: Jan Benacek