A Study of Tearing Instability Onset with Particle In Cell Simulations

A comprehensive theory of tearing instability onset in collisionless plasmas is still missing. However, studies conducted in specific regimes, e.g. DelSarto et al 2016, provide critical aspect ratios (current sheet thickness over length) below which tearing instability is expected to grow on a "faster" time scale.

We explore tearing instability onset with fully kinetic Particle In Cell (PIC) simulations of current sheets with aspect ratio below and above the critical aspect ratio, using different mass ratios and different initial equilibria (Harris equilibrium and force free).

Such simulations are typically extremely computationally expensive, especially when performed with an explicit PIC code: the domain needs to be quite large and instability onset is observed over tens to hundreds of inverse ion cyclotron frequencies, depending on current sheet parameters and number of particles per cell. This last parameter is particularly important, due to the known correlation, in PIC simulations, of particle number per unit volume and the numerical noise that seeds instability onset.

To overcome this challenge, we use the semi-implicit, energy conserving ECsim code (Lapenta et al, 2017), which allows us to step over the smallest and fastest spatial and temporal scales in the system in order to simulate domains of the order of the hundreds of ion skin depths for hundreds of inverse ion cyclotron frequencies. Careful convergence studies are then needed to validate results.