Can the tail wag the dog? The influence of micro-instabilities on collisionless plasma shocks.

Abstract: Collisionless shocks are ubiquitous and hugely important in space and astrophysical plasmas. In hot and rarified (weakly collisional) plasma, the energy dissipation necessary for such a shock to exist must be provided by kinetic processes, among which plasma instabilities have long been considered primary candidates. In this presentation, we will discuss results from a simulation study utilizing state-of-the-art fully kinetic simulations to assess the role of micro-instabilities on energy partition at quasi-perpendicular shocks. The study is motivated by a recent (surprising) realization that high-amplitude, high-frequency electric field fluctuations are present in spacecraft data in virtually every shock traversal where the required resolution is available, almost independent of shock parameters. Yet, it currently remains unclear if these fluctuations play an essential role in shock dynamics. The simulations exhibit many of the instabilities present in the data, such as ion-acoustic and lower-hybrid oscillations, solitary structures (ion and electron holes), and whistler-branch modes. For the cases considered, the instabilities are not seen to significantly affect energy partition in the average sense but do produce significant local variations in particle energization. Further, we show that the non-Maxwellian features in upstream electron distributions (halo and strahl), commonly present in the solar wind, do not significantly affect the amplitude or types of the instabilities developing in the shock transition region.

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