The role of upstream waves in supercritical quasi‐parallel shock re‐formation

Abstract

It has been demonstrated by hybrid simulations of collisionless shocks that the shock itself is not stationary but exhibits a cyclic behavior. We have performed a number of one‐dimensional hybrid simulations of collisionless shocks and of the interaction of two plasma streams in order to assess the role of upstream waves in the re‐formation process. We found that when upstream waves and reflected ions are present, the waves inevitably steepen up and lead to shock re‐formation. To investigate the interaction of the reflected ions with upstream waves, the hybrid code is then used in two stages. In the first stage a shock together with upstream diffuse particles and waves is generated. In the second stage, upstream wave trains are isolated, and their subsequent interaction with a finite length ion beam as well as with ions emitted from a moving point source is investigated. The results show that the original wave amplitude grows and the wave steepens. This is due to a deceleration and deflection of the beam ions once they encounter a wave crest with a large local value of the angle ΘBn between the magnetic field and the direction of beam propagation: the deflection leads to a local density increase and due to the compressibility to a corresponding magnetic field increase. This sets up a positive feedback loop during which the background ions are also decelerated and can even get reflected. It is concluded that shock re‐formation can be caused by the interaction of reflected ions with low‐frequency upstream waves.