Relative timing of substorm processes as derived from multifluid/multiscale simulations: Internally driven substorms

Abstract

The role of tail reconnection in initiating substorm onset remains highly controversial. Multifluid/multiscale simulations are used to examine this problem with the unique capability of being able to resolve ion skin depth and ion cyclotron processes within a very thin tail current sheet within a global magnetosphere. It is shown that, consistent with the near‐Earth neutral line model, a thin current sheet forms during the growth phase with the eventual formation of an X line and the ejection of a plasmoid. However, these processes do not produce substorm onset but are precursors to onset. The modeling indicates that in the wake of the plasmoid, a Y line forms within which there is the intermittent generation of flux ropes that can propagate either earthward or tailward with equal probability. The intensity and size of the flux ropes are seen to increase in time up to substorm onset. Depending on the size of the earthward moving flux rope, pseudobreakup or substorm onset occurs when the energy within the earthward moving flux rope is dissipated against the inner edge of the plasma sheet. This interaction leads to rapid (1 min) dipolarization, magnetospheric injection of energetic particles, and intensification of the nightside auroral currents that expand in local time and to higher latitudes. These processes are consistent with the current disruption model where onset occurs as a direct consequence of processes occurring at the inner edge of the plasma sheet. Ionospheric outflows and their energization with the thin current sheet play important roles in the buildup of energy in the plasma sheet that eventually produces substorm onset and drives breakup.