Plasma Instabilities Driven by Earthward Mesoscale Convection

Abstract:

Mesoscale convection, i.e., discrete localized (to ~Earth radius in the azimuthal extent) enhancements of earthward plasma flow preceded by sharp dipolarizations of magnetic field, is a ubiquitous feature of Earth’s magnetotail during active geomagnetic conditions with in situ observational history going back by over 40 years (e.g., AMPTE, ISEE, SCATHA, GOES, LANL, Geotail, Cluster, THEMIS). With the apogee of 5.8 Earth radii and low inclination, the Van Allen Probes (RBSP) mission provided a factor of 10 increase in the sampling rate of the equatorial inner magnetosphere. It was shown that during storms mesoscale flows penetrate deep into the inner magnetosphere, injecting energetic ions can provide a major (approximately 30%) contribution to the plasma pressure and the buildup of the Earth’s ring current. It has recently been recognize that the inward mesoscale convection is also associated with a complex chain of processes cascading down to kinetic scales such as whistler chorus waves and nonlinear time-domain structures. In this paper we explore the mechanisms and the types of plasma instabilities that can be induced by the mesoscale flows. For this purpose we employ test-particle simulations using our Conservative Hamiltonian Integrator of Magnetospheric Particles (CHIMP) with high-resolution MHD simulations of plasma convection in the magnetotail. For the later we use Grid Agnostic MHD for Extended Research Applications (GAMERA) global magnetospheric model. Spatial and temporal evolution of the phase space density of energetic plasma in mesoscale flows are computed from test-particle trajectories and then used to assess the pitch-angle anisotropy parameters and search for phase-space structures such as beams and boundary layers.