The decay of suprathermal ion fluxes during the substorm recovery phase

Abstract

During a magnetospheric substorm, the energetic particle flux in the plasma sheet increases by as much as 2 orders of magnitude. During the recovery phase the ion flux decreases. The decay rate is energy dependent, with the decay time constant decreasing for increasing energies. By comparing the decay rates of different species, we are able to test whether the decay rate is organized by total energy, energy per charge, velocity, or rigidity. Using the suprathermal energetic ion charge analyzer (SULEICA) instrument on Active Magnetospheric Particle Tracer Explorers Ion Release Module (AMPTE IRM), we have determined the decay rates for the species H+, He+, O+, and He++ for individual cases and by using a superposed epoch analysis. We find that the decay rate is organized by energy per charge. We have calculated the decay rates predicted by two different models: adiabatic cooling in a radially expanding plasma sheet, and particle drift loss, or “leakage,” from the plasma sheet. Both models predict that the decay rates will be organized by energy per charge, and in both cases the predicted decay rate is faster than that actually observed. This indicates that the production of energetic particles continues at a reduced rate in the the tail during the recovery phase, as the neutral line retreats down the tail.