The Influence of Temperature Anisotropies in Controlling the Development of Magnetospheric Substorms

Abstract

Ion anisotropies can affect a host of processes within the magnetosphere, from modifying the growth rate of various instabilities to the energization and mass transport within the magnetosphere. Global multi-fluid simulations using a full treatment of the pressure tensor are used to evaluate the influence of temperature anisotropies on the magnetospheric dynamics for an idealized substorm. The simulations are able to resolve the development of conics over the polar cap which eventually turn into beams in the lobes. Entry of this plasma, particularly heavy ions with high Tparallel, leads to a faster reconnection rate, additional turbulence within the reconnection region, and the substorm onset time occurs approximately 5 min earlier relative to isotropic simulations. The anisotropic treatment yields much more intense auroral currents on the nightside associated with onset and a faster expansion phase of the substorm. The plasma entering into the current sheet experiences stronger heating in the anisotropic simulations, which increases plasma transport into the equatorial dayside. Hence, the convection of the plasmasphere towards the dayside magnetosphere during the growth phase is accompanied by a relative increase in Tparallel. This leads to a very distinct temperature profile on the dayside. The injection of the energetic particles from the substorm leads to an overall increase Tperpendicular on the dayside, but only O+ reaches sufficient energies for the bulk of these ions to be able to cross noon local time during the expansion phase to participate in the formation of symmetric ring current. The net effect from these processes is to decrease the cross-polar cap potential, modify the timing of the development of reconnection and the development of the auroral currents and enhance the injection of energetic particles into the inner magnetosphere.