SPRG Seminars - Archive

September 13, 2005:

Self-reformation of perpendicular shocks: a simulationists' figment?
Laurent Muschietti, UCB/SSL

It is argued that collisionless plasma shocks are a multi-scale structure. From a macroscopic viewpoint a shock is an interface between two different plasmas. An undisturbed plasma flows in at high speed, a hot and denser plasma flows out at reduced speed. The transition is assumed to be at rest in some inertial frame, the so-called shock frame. Applied in the latter, conservation laws of MHD define the properties of the downstream plasma in terms of the upstream plasma. On the other hand, from the microscopic viewpoint afforded by particle-in-cell simulations, the mere shock transition is a complex structure, even in the simple case of perpendicular geometry. First, a significant fraction of the incoming upstream ions are reflected by the electrostatic field at the shock front. As a result, the magnetic field profile is characterized by (a) a FOOT in front of the shock due to the accumulation of reflected ions, (b) a RAMP with strong magnetic gradient, (c) an OVERSHOOT followed by an UNDERSHOOT, which are due to the large gyration of the ions that succeed in crossing the front on their second approach, and (d) downstream oscillations. It is now accepted that the reflected ions provide the main source of free energy for the purpose of dissipating the streaming energy at the shock. Second, simulations with full particle codes demonstrate that the shock structure can be nonstationary. These codes treat both ions and electrons as macro-particles, and enable the access to small electron scales. In these simulations, the foot is an evolving feature which builds up as the reflected ions accumulate to eventually turn into a new shock front which starts reflecting new upstream ions, and so on. The process is described as cyclic self-reformation of the shock front and occurs on a timescale on the order of the ion cyclotron period. In this zoomed-in view, the shock propagates upstream in a stepwise fashion. I will show examples, explain the physics of the self-reformation, and discuss its parameter regime.

The second part of the talk will briefly present the Darwin electromagnetic model, which is implemented in my own shock simulations.

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