Dependence of the cross-shock potential on the ion-to-electron mass and light speed to thermal velocity ratios

Abstract:

Collisionless shocks, shock-waves which steepen on scales much smaller than the collisional mean-free-path, have a myriad of means by which their upstream bulk kinetic energy is converted to other forms of energy, such as thermal and electromagnetic. Some of these mechanisms, such as shock-drift acceleration and diffusive shock acceleration, rely on the shock reflecting the incoming plasma population so that it gain energy, sometimes repeatedly, in the upstream plasma conditions before returning downstream. While these energization processes are robust once the plasma particles return upstream, the efficiency of this reflection process is a matter of some debate, with some studies invoking a cross-shock potential to generate an electric field parallel to the shock-normal and assist in reflecting protons and energizing electrons.

In this talk, we show the results of a systematic study of how simulation parameters, especially simulation parameters which are often artificially adjusted for computational cost reasons, affect the cross-shock potential in one dimensional perpendicular shocks. We focus in particular on how the ratio of the light speed to thermal velocity, along with other dimensionless ratios such as the electron to proton mass ratio, affect the strength of the cross-shock potential. All simulations employ the continuum Vlasov-Maxwell solver in the Gkeyll simulation framework to avoid the discrete particle noise of a PIC code and more carefully diagnose the size of the cross-shock potential, free of the computational quasi-thermal noise inherent in the sampling of PIC particles. Although the presented simulation are one-dimensional, they are the first simulations, to our knowledge, to use realistic values of both the electron to proton mass ratio and the light speed to thermal velocity ratio and will inform future computational studies of collisionless shocks, most especially simulations designed to compare to observations.