3D kinetic picture of magnetotail explosions and characteristic auroral features prior to and after substorm onset

Abstract:

Recent findings in theory, observations and 3D particle-in-cell simulations of magnetotail explosions reveal a complex picture of reconnection, buoyancy and flapping motions, which have interesting correlations with the auroral morphology. First, the formation of the tailward Bz gradient as a theoretical prerequisite for tearing, ballooning/interchange and flapping instabilities is consistent with the structure of the pre-onset quiet arc and the associated deep minimum of Bz. Another distinctive pre-onset feature, equatorward extension of the auroral oval in the late growth phase, is conventionally associated with earthward motion of the inner edge of the plasma sheet. However, if open magnetic flux saturates in the late growth phase, it may also be treated as a signature of magnetic flux accumulation tailward of the Bz minimum, which is also favorable for the tail plasma sheet instabilities. 3D PIC simulations of similar magnetotail equilibria with a tailward Bz gradient show spontaneous formation of earthward flows led by dipolarization fronts. They are structured in the dawn-dusk direction on the ion inertial scale, consistent with the minimum scales of the observed auroral beads. At the same time, simulations show the formation of a new X-line in the wake of the dipolarization front with no significant spatial modulation in the dawn-dusk direction suggesting smooth profiles of the substorm current wedge as well as poleward parts of auroral streamers. Flapping motions, which also grow at the dipolarization front, extend beyond it, up to the new X-line region. To understand auroral manifestations of tail structures in our simulations we investigate the plasma moments at the plasma sheet boundary.