Wavelength and decay length of density overshoot structure in supercritical, collisionless bow shocks

Abstract

Above some critical Mach number, quasiperpendicular collisionless shocks are known to exhibit “overshoot” and “undershoot” structure thought to be associated with the motion of ions trapped at the shock front. Using spacecraft potential data from the Cluster spacecraft, the overshoot/undershoot density structure at 56 crossings of the quasiperpendicular bow shock is studied. The envelope of the absolute value of the density, in most cases, decays exponentially and these envelopes are fitted to a decaying function from which we calculate the decay length scale. The overshoot/undershoot wavelength is also estimated using the zero crossings of the density profile and a good correlation between the average wavelength and the convected ion gyroradius is found: the wavelength is approximately two to three times the ion gyroradius. There is no evidence of a strong correlation between the wavelength and the ion inertial length. Similar results are found for the decay length, which also seems to be ordered by the convected ion gyroradius.