Nonrelativistic Collisionless Shocks in Unmagnetized Electron-Ion Plasmas

Abstract

We show that the Weibel-mediated collisionless shocks are driven at\nnon-relativistic propagation speed (0.1c < V < 0.45c) in unmagnetized\nelectron-ion plasmas by performing two-dimensional particle-in-cell\nsimulations. It is shown that the profiles of the number density and the mean\nvelocity in the vicinity of the shock transition region, which are normalized\nby the respective upstream values, are almost independent of the upstream bulk\nvelocity, i.e., the shock velocity. In particular, the width of the shock\ntransition region is ~100 ion inertial length independent of the shock\nvelocity. For these shocks the energy density of the magnetic field generated\nby the Weibel-type instability within the shock transition region reaches\ntypically 1-2% of the upstream bulk kinetic energy density. This mechanism\nprobably explains the robust formation of collisionless shocks, for example,\ndriven by young supernova remnants, with no assumption of external magnetic\nfield in the universe.\n