Multiscale-multifluid simulations of the 26 February 2008 substorm: Evidence for internal triggering of a substorm

Abstract

Multiscale-multifluid simulations are used to provide high-resolution (∼470 km) simulations of the 26 February 2008 substorm that was well observed by the THEMIS spacecraft to investigate whether the substorm was internally or externally triggered. This substorm occurred during an extended (1 h 45 min) period of weak (-2.5 to -1.5 nT) southward interplanetary magnetic field (IMF). Simulation results show that this substorm was internally triggered well before the IMF start to become more northerly. The southward IMF leads to an enhancement in the cross-polar cap potential, strong thinning of the current sheet, and pseudo-breakup in association with patchy reconnection. The southward IMF also produces an enhancement in the outflow of ionospheric oxygen into the current sheet. Because of the tilt of the magnetic field the strongest outflows originate from the southern polar cap. The arrival of these heavy ions in the tail occurs about 10 min prior to the observed substorm onset and leads to enhanced tail reconnection, including flux rope development and fast earthward flows. The interaction of these fast flows at the inner edge of the plasma sheet is closely associated with auroral onset, indicating that the substorm is internally triggered. Comparison with THEMIS data suggests that reconnection occurred earlier than THEMIS observations. The results show onset is associated with processes at the inner edge of the plasma sheet and is not directly related to the onset of reconnection. Comparison with THEMIS data indicates that processes at scale lengths less than 500 km are occurring. Copyright 2010 by the American Geophysical Union.