Supercriticality of ICME and CIR shocks

Abstract Interplanetary coronal mass ejection (ICME) and corotating interaction region (CIR) shocks are characterized in terms of supercriticality introduced by Edmiston and Kennel (1984) to classify shocks based on whether dissipation is provided by electron resistivity alone or also requires ion viscosity. The condition for determining supercriticality is a critical Mach number, M C , a function of θ Bn , the angle between the upstream magnetic field, B , and the normal to the shock surface, n, and β , the ratio of the plasma and magnetic pressures. The criterion was subsequently revised by Kennel (1987) to include dissipation by electron thermal as well as electrical conductivity. Two early separate studies of ICME and CIR shocks motivated our investigation that included several improvements. We use Kennel (1987) and shocks identified by WIND near 1 AU and by Ulysses near 5 AU from the same solar cycle to provide Occurrence Probability Distributions and statistical information for all parameters. We answer three questions (1) Is the supercriticality of ICME and CIR shocks different? (2) If so, why? (3) Does the latter M C criterion change the answers? Our conclusions are (1) about two thirds of CIR shocks are supercritical as compared to one third of ICME shocks, (2) although ICME shock speeds are typically higher than CIR shocks, the fast‐mode wave speeds are even higher at 1 AU than that of CIR shocks at ~5 AU causing a reduction in Mach numbers, and (3) CIR shocks are also more supercritical than ICME shocks using both criteria with slight differences.