Speaker: Lorenzo-Maria Perrone, Leibniz Institute for Astrophysics Potsdam, Germany- Chaired by: Francesco Califano, Associate Editor, JPP
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Date/Time: Thursday 6th November 2025, 4PM GMT/11AM EST
Title: Heat transport in the intra-cluster medium: the impact of microphysics on the large-scale dynamics in the periphery of galaxy clusters
Abstract: The intracluster medium (ICM) is a high-beta weakly-collisional plasma that plays a central role in many key processes that shape the evolution of galaxy clusters. Recent work has focused on two interlinked aspects: the preferential transport of heat and momentum along magnetic field lines on macroscopic (fluid) scales, the impact on particle transport of plasma instabilities on kinetic scales. Anisotropic heat conduction renders the periphery of galaxy clusters buoyantly unstable. The ensuing instability, called magneto-thermal instability (MTI), can contribute to the observed turbulence in galaxy clusters. Our recent work on the MTI shows that the strength of the turbulence is linked to the plasma thermal conductivity. This property renders the MTI sensitive to the action of kinetic instabilities that can slow down the thermal electrons: in particular, particle-in-cell (PIC) simulations suggest that whistler waves may significantly suppress thermal conductivity below its nominal Spitzer value. This scenario, where the MTI responds to whistler-wave suppression, is further explored with idealized MHD simulations with a subgrid closure for the heat flux inspired from kinetic theory. MTI turbulence with whistler suppression exhibits a critical transition as the plasma collisionality decreases and turbulence dies out. In the ICM, however, turbulence can be excited through other means such as shocks and mergers. While usually thought of as deleterious for the MTI, external turbulence with whistler-suppressed thermal conductivity can revive a “dead” MTI, keeping the plasma buoyantly unstable.
Speaker: Tommaso Barberis, Princeton University, USA- Chaired by: Alex Schekochihin, Editor, JPP
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Date/Time: Thursday 13th November 2025, 4PM GMT/11AM EST
Title: Perturbative model for the saturation of energetic-particle-driven modes limited by self-generated zonal modes
Abstract: We present a simplified energy-conserving approach to incorporate wave-wave nonlinear effects within the framework commonly used to describe wave-particle nonlinearities. In particular, the effects of zonal mode generation on the determination of the saturation amplitude of energetic particle (EP)-driven Alfv´enic instabilities is studied. The model assumes that the zonal perturbations grow at a rate twice that of the original (pump) wave, consistent with a beat-driven (or force-driven) generation mechanism. The evolution and saturation of the mode amplitude are investigated both analytically and numerically within our reduced model assumptions, in both the collisionless and scattering-dominated regimes. These studies underscore the crucial role of sources and sinks in capturing the impact and the role of beat-driven zonal perturbations on mode evolution. In the realistic case of saturation set by sources and sinks, we discuss the role of a finite amplitude zonal mode in reducing microturbulent particle scattering, thus limiting the energy source for the resonant mode. We then discuss comparisons between the model’s predictions and simulation results. The model reproduces key features observed in gyrokinetic simulations as the reduction in saturated mode amplitude and the onset of wave–wave nonlinear effects as functions of mode growth rate and amplitude. Thanks to its simplicity, it can be readily implemented into codes based on reduced models, thereby improving their predictive capability for strongly driven instabilities.