Venus express observes a new type of shock with pure kinematic relaxation

Abstract

Collisionless shocks are present in the vicinity of many astrophysical objects such as supemova remnants, space jets, stars and planets immersed in the supersonic flow of stellar winds. Understanding the shock structure is crucial for understanding the processes of the redistribution of the upstream flow energy into accelerated particles and formation of downstream thermalized distribution. We report first observations (by Venus Express) of subcritical shocks that do not fit into the well-established classical structure classification. It is shown that its abnormal structure is due to kinematical collisionless relaxation of downstream ions. The spatial gyrophase mixing leads to formation of a downstream thermalized distribution, instead of various instabilities. This type of subcritical shock with kinematic relaxation has never been discussed before in theoretical models (e.g., C. F. Kennel et al., 1985). Copyright 2008 by the American Geophysical Union.