Numerical analysis of collisionless particle motion in an observed supercritical shock front

Abstract

The well‐accepted shock model assumes that the shock front is stationary and one dimensional, in the sense that ion and electron dynamics are determined primarily by their interaction with the quasi‐stationary electric and magnetic fields in the shock front. We study the applicability of this model to an observed high Mach number supercritical shock. For this purpose we numerically analyze ion and electron dynamics in the measured magnetic field and modeled electric field (on the basis of the measurements of electron heating) and check the consistency of the numerically determined particle features with the observed shock profile. We find that the shock must be narrow enough (for given Mach number MA, kinetic‐to‐magnetic pressure ratio β, angle θ between the shock normal and upstream magnetic field, and magnetic compression Bd/Bu) to ensure that ion reflection is not too strong. We also find that the small‐scale structure is an important part of the shock: the shock front would be grossly nonstationary if it were too wide or if small‐scale features were absent. The ion energization due to the reflection and gyration is qualitatively consistent with the ion dynamics in quasi‐stationary electric and magnetic fields. However, smoothing of the downstream ion distribution is insufficient, and deviations from one dimensionality and/or nonstationarity are necessary for the shock to be stable. Electron dynamics are weakly nonadiabatic, and evolution of the collisionless electrons follows the potential and magnetic field. Gap filling cannot be studied within this collisionless approach.