THE ROLE OF PRECIPITATION LOSSES IN PRODUCING THE RAPID EARLY RECOVERY PHASE OF THE GREAT MAGNETIC STORM OF FEBRUARY 1986

The possible role of precipitation losses in eroding stormtime ring cu rrent is subject to debate. To explore this controversy, the recovery phase of the February 6-10, 1986, great magnetic storm is examined, wh en intense ion precipitation was observed at midlatitudes by NOAA-6 an d DMSP satellites, This storm period is particularly interesting becau se the ring current exhibits distinctive two-phase decay as seen in th e Dst index, the early rapid timescale decay corresponding to the inte nse ion precipitation period described above, Hamilton et al. [1988] c oncluded, from close agreement between the observed timescale for ring current decay and the theoretical timescale for O+ charge exchange lo ss, that rapid early recovery phase of this storm resulted from the ch arge exchange loss of high energy O+; the second and longer decay phas e was equated with H+ charge exchange loss. A model of the ring cut-re nt evolution during this great magnetic storm [Fok et al., 1995] faile d to reproduce the observed ring current decay rates, a puzzling resul t because charge exchange losses were well represented in the ring cur rent model and initial and boundary conditions were taken from the sam e data set used in the Hamilton et al. [1988] study. A simple energy b alance calculation for the global ring current is carried out using (1 ) either an energy input predicted from upstream solar wind parameters or one calculated from the drift-loss model output, (2) collisional l oss timescales extracted from the drift-loss model, and (3) precipitat ion losses estimated from NOAA-6 and DMSP observations. The energy bal ance model replicates the evolution of the ring current energy content derived from Active Magnetospheric Particle Tracer Explorers/Charge C omposition Explorer (AMPTE/CCE) observations when ion precipitation lo sses are included and model energy input function is reduced to agree with predictions based upon upstream solar wind parameters. The Of cha rge exchange losses and observed global precipitation losses were of e qual magnitude in early recovery of the ring current during this great magnetic storm. Later longer decay timescales in the model resulted f rom a combination of O+ and H+ charge exchange losses; O+ charge excha nge losses remained important throughout the model time interval. The present model produces agreement with the AMPTE/CCE estimates of ring current kinetic energy content versus time. Disagreement between the D st inferred from the AMPTE/CCE particle measurements and the observed Dst is an interesting issue needing further explanation.