Outflow of energetic ions from the magnetosphere and its contribution to the decay of the storm time ring current

摘要

We statistically and quantitatively examine the outflow of energetic ions from the magnetosphere during magnetic storms. We also evaluate the contribution of the outflow to the decay of the ring current. We use energetic ion (9–210 keV) data obtained by the energetic particle and ion composition (EPIC) instrument and magnetic field data obtained by the magnetic field measurements (MGF) system, both on board the Geotail spacecraft. The outflowing energy flux, that is, the energy flux lost by ring current ions flowing through the magnetopause, is defined as the energy flux normal to the magnetopause and is calculated based on measurements made adjacent to the earthward side of the low-latitude boundary layer. Our statistics show that the outflowing energy flux is about 105–108 keV/(cm2s) during both the main phase and the recovery phase. It is higher on the afternoonside than on the morningside. It is better correlated with the square root of the dynamic pressure of the solar wind than the electric field of the solar wind, which is a proxy for the strength of the convection electric field. The contribution of the outflow to the rapid decay of the ring current is estimated to be at least 23% and could be much higher than 23% for the 23 September 2001 storm, based on an underestimated leakage area which is determined from magnetic field measurements. We suggest that the drift governing the ion outflow mainly is the B drift which has a radial component that arises from a day-night gradient of the magnetic field in the magnetosphere caused by the solar wind compression. We conclude that the ion outflow contributes significantly to the rapid decay of the ring current, even in the case of a sudden northward turning of the interplanetary magnetic field which causes a sudden decrease in the convection electric field.