Monday, October 05, 2020 at 12:40

https://cuboulder.zoom.us/j/96981439846

Lynn Wilson, NASA/Goddard

"Particle Energization at Collisionless Shock Waves"

Abstract:

Shock waves are a discontinuous transition between supersonic and subsonic flows, characterized by an abrupt change in pressure, temperature, velocity, and density in the medium. Shock waves can arise from the nonlinear steepening of compressional waves when the steepening is balanced by some form of irreversible energy dissipation. Nonlinear wave steepening can result when a wave has an amplitude dependence in the phase speed. That is, the larger amplitude parts of a wave will "out run" the lower amplitude parts. If irreversible energy dissipation is insufficient, the steepening wave will reach a "gradient catastrophe" and undergo breaking (e.g., think of breaking water waves). In Earth's atmosphere shock wave energy dissipation is mediated by particle- particle collisions. Space plasmas, however, are ionized gases, mediated by long-range forces exhibiting a collective behavior. They are often weakly collisional to collisionless which begged the question of whether shocks could exist in such plasmas. Yet spacecraft have been observing phenomena that appear to be magnetized shocks since the 1960s despite the fact that they cannot be mediated by collisions. This led to the name collisionless shock wave. Collisionless shocks are an ubiquitous phenomena in astrophysical plasmas in the form of bow shocks upstream of magnetized planetary bodies (and unmagnetized objects like comets and Venus), interplanetary shocks, supernova blast waves, and stellar astrospheres. Despite their ubiquity, there are still numerous processes associated with collisionless shocks that are not well understood. We will focus on one topic that is of particular interest, particle energization, because collisionless shocks are thought to generate some of the most energetic particles in the universe. We will discuss some of the known and observed energization mechanisms and discuss unknowns that motivate future work.

Back to Speakers