Observations of a very thin collisionless shock

Abstract

The structure of collisionless shocks is governed by the upstream plasma conditions. The laminar collisionless shock is well studied and has been found to have a ramp thickness on the order of an ion inertial length. The behavior of the shock at higher Mach number and β is not as thoroughly studied. However, it has been speculated that an exactly perpendicular shock behaves like the soliton wave solution from classic plasma theory. This would result in a much thinner shock ramp with a thickness on the order of electron inertial lengths. No such thin shock has been observed in the laminar regime. In this study of ISEE‐1 and ‐2 crossings of supercritical, nearly‐perpendicular bow shocks (θBn > 80°), we find that the shock velocity is highly variable between spacecraft crossings and during observations. The fine structure of high velocity shocks is poorly resolved by the ISEE‐1 and ‐2 magnetometers due to filters used to prevent aliasing of the digitized data. When we restrict our attention to slowly moving shocks, we find one example of a very thin shock with a ramp thickness of only two electron inertial lengths. We suggest that this thin structure arises when the nearly perpendicular shock moves into the perpendicular regime, but cannot confirm this conjecture because of the uncertainty and temporal variability of the normal.