Escaping Back-Streaming Ions at Interplanetary Shocks

It is well known that at intense interplanetary shocks, ions can be reflected and can contribute to the generation of the "magnetic foot" directly in front of the shock ramp and "magnetic overshoot" at the ramp transition to downstream. Those reflected ions stay at the foot and typically have energies in the ~2-4 keV range. Such situations occur in supercritical or marginal supercritical shocks when the ratio of the shock Mach number over the corresponding critical Mach number is greater than one. Recent studies using the kinetic theory and observational analyses have found that there is another group of ion reflection, energetic ion reflection (EIR), which also occurs in supercritical shocks, especially those determined by the critical Mach number defined by Kennel [1987], at which ion viscosity is required in addition to the electron resistivity and thermal conduction to satisfy the shock dissipation mechanism. In ARTEMIS observations, energies of the ions in EIRs are in the ~4-25 keV range, which is apparently higher than those that generate the foot and overshoot, and are able to escape and backstream into the shock upstream in minutes or more. For shock angles below ~60 degrees, the ion escape into the upstream region may be explained by multiple reflections. But for the quasi-perpendicular supercritical shocks with a shock angle greater than 60 degrees, there have not been analytical descriptions using the kinetic theory, although plenty EIRs are observed by the ARTEMIS/ESA instrument and are well consistent with the criticality defined by Kennel [1987], which requires viscosity (ion-related dissipation processes). How to depict the viscosity in the dissipative MHD using the kinetic theory is still an open question.