Evolution of plasma sheet particle content under different interplanetary magnetic field conditions

Abstract

We have statistically analyzed Geotail data to investigate the processes that result in a plasma sheet that is denser under a prolonged northward than southward interplanetary magnetic field (IMF) period. The observations show that the change of number density with the IMF conditions is mainly due to the changes of particle number per unit magnetic flux (particle content N), with N increasing (decreasing) as the period of northward (southward) IMF extends. The changes are quicker in the first ∼2 to 4 h then substantially slow down. The Y profiles show that N is always lowest around midnight and becomes higher toward the flanks. The observed plasma velocities suggest that plasma sheet particles undergo earthward and flankward drift transport, as well as diffusive transport resulting from drift fluctuations. The diffusion coefficients associated with fluctuating drift are estimated to be ∼105 to 106 km2/s. We have simulated evolution of N resulting from drift and diffusive transport with particle sources along the flanks. The simulation results show that the observed temporal and Y variations of N under different IMF conditions can be accounted for by the competition between the particle increase owing to particles diffusing toward midnight from the flank sources and the particle decrease owing to particles drifting away from midnight. As the IMF turns northward (southward), it is mainly the strengthening (weakening) of diffusive transport owing to the increase (decrease) of the flank source that results in the increase (decrease) of N.