Theoretical astrophysics and plasma physics at RPC

Abstract:

How momentum, energy, and magnetic fields are transported in the presence of macroscopic gradients is a fundamental question in plasma physics. Answering this question is especially challenging for weakly collisional, magnetized plasmas, where macroscopic gradients influence the plasma's microphysical structure. In this paper, we introduce thermodynamic forcing, a new method for systematically modeling how macroscopic gradients in magnetized or unmagnetized plasmas shape the distribution functions of constituent particles. In this method, we propose to apply an anomalous force to those particles inducing the anisotropy that would naturally emerge due to macroscopic gradients in weakly collisional plasmas in which thermal pressure is much larger than magnetic pressure. We implement thermodynamic forcing in particle-in-cell (TF-PIC) simulations using a modified Vay particle pusher and validate it against analytic solutions of the equations of motion. We then carry out a series of simulations of electron-proton plasmas with periodic boundary conditions using TF-PIC. First, we confirm that the properties of two electron-scale kinetic instabilities—one driven by a temperature gradient and the other by bulk-velocity gradient—are consistent with previous results. Then, we demonstrate that in the presence of both macroscopic gradients, heat-flux saturation is mediated by the bulk-velocity-gradient-driven electron firehose instability rather than the temperature-gradient-driven whistler instability. This suggests that saturation mechanisms may differ from our current understanding in the presence of multiple free energy sources. This work enables, for the first time, systematic and self-consistent transport modeling in weakly collisional plasmas, with broad applications in astrophysics, laser-plasma physics, and inertial confinement fusion.

Abstract:

Abstract The reconfiguration of the magnetotail current sheet during substorms often includes the formation of a thin current sheet (TCS) with a strong magnetic field line tension force. This force cannot be balanced by isotropic plasma pressure gradients, and force balance in such a TCS requires ion anisotropy and/or agyrotropy of plasma pressure. A statistical investigation of these plasma properties in the magnetotail is challenging because a significant contribution to ion anisotropy or agyrotropy stems from the suprathermal ion population, which often has energies beyond the upper limit of electrostatic analyzer measurements. In this study, we compare ion measurements from Cluster, MMS, and THEMIS to investigate the ion anisotropy in the magnetotail current sheet. We show that the central region of the magnetotail is characterized by transverse anisotropy of suprathermal ions, whereas subthermal field‐aligned anisotropic ions populate the magnetotail current sheet boundaries. Our results indicate that different ion populations (subthermal, thermal, and suprathermal ions) exhibit distinct behaviors within the current sheet, showing that different combinations of these populations contribute differently to the stability of current sheets.