Title:	PIC simulations of subcritical shocks in collisionless plasma
DNr:	NAISS 2025/6-338
Project Type:	NAISS Medium Storage
Principal Investigator:	Mark Eric Dieckmann <Mark.E.Dieckmann@liu.se>
Affiliation:	Linköpings universitet
Duration:	2025-10-01 – 2026-10-01
Classification:	10303
Homepage:	https://liu.se/en/employee/mardi06
Keywords:

Abstract

Shocks in collisionless plasma are mediated by macroscopic electromagnetic fields rather than by binary particle collisions. As a result, they differ in several fundamental ways from shocks in dense gases. First, macroscopic fields cannot fully randomize the trajectories of particles crossing the shock, which is necessary for entropy increase. Second, the cross-shock potential of a subcritical shock can only heat ions along one direction, leading to thermal anisotropy in the downstream plasma. Third, the differing compressibility of plasma particles and magnetic fields gives rise to two distinct shock transition layers. Instabilities can develop within the broader magnetic transition layer, generating internal structures driven by plasma waves. These phenomena are governed by collisionless instabilities that either lead to wave growth or a nonstationary shock front. Particle-in-cell (PIC) simulations are uniquely suited to capture all of these effects and are therefore the method of choice for studying collisionless shocks. We will continue our investigations of waves and instabilities in the transition layers of subcritical collisionless shocks. Our previous studies focused on plasma conditions representative of laser-generated plasmas. We now aim to extend our work to plasma conditions relevant to shocks in the solar system. We will also study curved shock fronts. Two key scenarios will be explored. In the first, a radially symmetric rarefaction wave expands into an ambient, magnetized plasma. In the second, a uniform plasma flow is stopped by a spatially localized, static dipole-type magnetic field. Most simulations are performed in two spatial dimensions. The simulation plane resolves the shock propagation direction and one perpendicular direction. As a result, only one wave vector component perpendicular to the shock normal is captured, limiting the physical realism of the simulation. However, this restriction enables the isolated study of specific growing wave modes. A more comprehensive picture can be obtained either by performing a series of 2D simulations with varying magnetic field orientations relative to the simulation plane or by conducting small-scale 3D simulations. Each planned particle-in-cell (PIC) simulation produces data sets of several terabytes in size. Since we must compare data sets from more than one simulation, we need sufficient mass storage capacity. We currently have an allocation of 15 TB at the NSC. Given that the simulation domain sizes will be larger in 2025-2026 than in the previous year, we request an increase of the disk capacity to 20 TB.