The deacy of suprathermal ion fluxes during the substorm recovery phase

L. M. Kistler, D. J. Larson, E. Mobius, and W. Baumjohann

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 dereases. 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 fot the species H+, He+, O+, and He++ for individual cases and by using
a superposed epoch analysis. We find that the decay rates predicted by two different models:
adiabatic cooling in a radially expanding plasma sheet, and particle drift loss, or
"leakage," fromthe 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 tail during the recovery phase, as
the neutral line retreats down the tail. 
Paper number 93JA03180