Rippled quasi‐perpendicular collisionless shocks: Local and global normals

Abstract

Proper determination of the shock normal is necessary for reliable determination of observed heliospheric shock parameters and comparison of observations with theory. The existing methods work sufficiently well for low and moderate Mach numbers one‐dimensional stationary shocks. Higher‐Mach‐number shocks are no longer planar at the scales of the ion convective gyroradius or smaller. In rippled shock fronts, the local shock normal may differ substantially from the global normal. The former is determined by the local direction of the fastest variation of the magnetic field, while the latter is determined by the far upstream and far downstream plasma conditions. Here we use 2‐D hybrid modeling of quasi‐perpendicular collisionless shocks with moderate and high Mach numbers to quantify the difference between the directions of the two normals. We find that the angle between the local normal and the global normal may be as large as 40° within the front of a rippled heliospheric shock. The coplanarity method of the shock normal determination is sensitive to the choice of the region for the magnetic field averaging. We also find that the usage of the sliding averaging region in the close vicinity of the shock transition provides satisfactory estimates of the global normal.