Towards understanding particle energy partition in magnetized quasi-perpendicular collisionless shocks in the lab

A seminar by Vicente Valenzuela

Abstract: Collisionless shocks are ubiquitous structures in the universe that can be found in planetary magnetospheres, supernova remnants, and when galaxies merge inside a cluster. In a collisionless shock, the ion-ion mean-free-path for Coulomb collisions far exceeds the density gradient scale-length, implying that collective electromagnetic fields in the plasma provide dissipation, rather than collisional viscosity. These collective fields provide complex non-linear and kinetic interactions within the plasma that can support collisionless/anomalous heating mechanisms. Consequently, there is no generally accepted theory of particle energy partitioning and heating in collisionless shocks. Electron and ion temperatures are obtained by in situ spacecraft measurement of planetary bow-shocks and analysis of Balmer breaks in supernova remnant spectra. However, large statistical variance of the inferred energy partition have prevented detailed benchmarking of theory. Thus, controlled laboratory experiments can provide independent measurements of energy partition that can be contrasted with theory, astrophysical, and heliospheric data.

In this talk, I will present results from supercritical, magnetized collisionless shock laboratory experiments at conditions relevant to planetary bow-shocks (MA~ 10, ~ 1). We report the first observation of fully developed shocks (×4 compression ratio and a downstream region decoupled from the piston). The data indicate the presence of a foot ahead of the density discontinuity, where both electrons and ions exhibit significant super-adiabatic heating. Surprisingly, the downstream exhibits significant super-adiabatic heating of both species with electrons and ions in thermal equipartition, with an order-unity electron-ion downstream temperature ratio Te/Ti ~ 1.2. This temperature ratio is inconsistent with a combination of adiabatic compression and electron-ion collisional equilibration; thus, it provides the first laboratory evidence of collisionless anomalous heating in a fully developed perpendicular magnetized collisionless shock.

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