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

The well-accepted shock model assumes that the shock front is stationary an d one dimensional, in the sense that ion and electron dynamics are determin ed primarily by their interaction with the quasi-stationary electric and ma gnetic fields in the shock front. We study the applicability of this model to an observed high Mach number supercritical shock. For this purpose we nu merically analyze ion and electron dynamics in the measured magnetic field and modeled electric field (on the basis of the measurements of electron he ating) and check the consistency of the numerically determined particle fea tures with the observed shock profile. We find that the shock must be narro w enough (for given Mach number M-A, kinetic-to-magnetic pressure ratio bet a, angle theta between the shock normal and upstream magnetic field, and ma gnetic compression B-d/B-u) to ensure that ion reflection is not too strong . We also find that the small-scale structure is an important part of the s hock: 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 refle ction and gyration is qualitatively consistent with the ion dynamics in qua si-stationary electric and magnetic fields. However, smoothing of the downs tream ion distribution is insufficient, and deviations from one dimensional ity and/or nonstationarity are necessary for the shock to be stable. Electr on dynamics are weakly nonadiabatic, and evolution of the collisionless ele ctrons follows the potential and magnetic field. Gap filling cannot be stud ied within this collisionless approach.