Understanding the near Earth plasma sheet instability responsible for substorm onset

Abstract:

Substorms are one of the most dramatic disturbances of the global magnetotail-ionosphere coupling system that release large amounts of solar wind energy accumulated in the magnetotail and are associated with auroral activations. A critical, long-standing problem in substorm research is to identify what physical mechanism leads to substorm auroral onset. It has been suggested that pre-onset waves with ~1-2 min periodicity are an important facet of the instability in the near-Earth region that leads to substorm onset and auroral beads. It is thus critical to determine if, and if so how, pre-onset waves in the near-Earth plasma sheet evolve to substorm onset waves. We found using the THEMIS satellite-imager conjunction events that westward-propagating pre-onset waves evolve to the beginning part of onset waves soon after auroral streamers, indicating an importance of the coupling process between incoming flows and pre-existing waves to substorm onset. On the other hand, a portion of the onset waves propagates eastward in contrast to westward propagation of the pre-onset waves and part of the onset waves. These unique propagation characteristics as well as the wavelength and periodicity can be used to identify a possible mechanism of near-Earth plasma sheet instability. We have compared the observation results with 3-D kinetic simulations with a localized tailward entropy gradient. This initial condition induces weak waves propagating westward that can be interpreted as due to kinetic ballooning/interchange instability and electromagnetic current-driven ion cyclotron instability. Part of those waves grows much larger in amplitude and propagated eastward and earthward due to charge separations between magnetized electrons and unmagnetized ions, indicating that kinetic effects are responsible for the onset wave propagation. These propagation features of simulated waves are consistent with the observations, and therefore suggest that these kinetic waves may play an important role in initiating the instability responsible for substorm expansion phase onset.