Magnetotail Convection: from Mesocales to Microscales

Abstract

Plasma convection across Earth’s magnetotail is largely comprised of mesoscale activations localized in the azimuthal extent to several Earth radii preceded by sharp dipolarizations of the magnetic field. According to spacecraft observations mesoscale injections can be accompanied by a complex chain of interactions cascading down to microscales, from kinetic Alfven waves to whistler chorus waves and time-domain structures. The goal of this study is to investigate how the mesoscale flows sculpt microscale instabilities. 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 the Lyon-Fedder-Mobarry (LFM) global magnetospheric model. Test-particles are initialized inside and around an isolated flow channel with a substantial dipolarization front with the use of the kappa distribution function and temperature and number density informed by the MDH simulations. The simulations reveal rapid growth of interchanging regions of parallel and perpendicular temperature anisotropies conforming around the magnetic island configuration associated with the dipolarization front. The instabilities are attributed to polarization in energetic particle distribution produced by particle perpendicular drift motion along the magnetic terrain boundaries and subsequent adiabatic heating and cooling (betatron effect). The results are compared with plasma and wave observations from the Magnetospheric Multiscale Mission (MMS) that show interchanging regions of broad-band electrostatic noise and whistler wave activity induced around dipolarization fronts at the spatial scales that closely resemble the patches of temperature anisotropy produced in our test particle simulations. To determine whether the simulated anisotropic energetic electron distributions can account for the observed wave activity we compute and compare growth rates of parallel whistler and firehose instabilities.