Cross-scale energy cascade powered by magnetospheric convection

Plasma convection in the Earth's magnetosphere from the distant magnetotail to the inner magnetosphere occurs largely in the form of mesoscale flows, i.e., discrete enhancements in the plasma flow with sharp dipolarizations of magnetic field. Recent spacecraft observations suggest that the dipolarization flows are associated with a wide range of kinetic processes such as kinetic Alfvén waves, whistler-mode waves, and nonlinear time-domain structures. In this paper we explore how mesoscale dipolarization flows produce suprathermal electron instabilities, thus providing free energy for the generation of the observed kinetic waves and structures. We employ three-dimensional test-particle simulations of electron dynamics one-way coupled to a global magnetospheric model. The simulations show rapid growth of interchanging regions of parallel and perpendicular electron temperature anisotropies distributed along the magnetic terrain formed around the dipolarization flows. Unencumbered in test-particle simulations, a rapid growth of velocity-space anisotropies in the collisionless magnetotail plasma is expected to be curbed by the generation of plasma waves. The results are compared with in situ observations of an isolated dipolarization flow at one of the Magnetospheric Multiscale Mission spacecraft. The observations show strong wave activity alternating between broad-band wave activity and whistler waves. With estimated spatial extent being similar to the characteristic size of the temperature anisotropy patches in our test-particle simulations, the observed bursts of the wave activity are likely to be produced by the parallel and perpendicular electron energy anisotropies driven by the dipolarization flow, as suggested by our modeling results.