Ion precipitation in the dawn sector during geomagnetic storms

Abstract

Three southward turnings of the IMF during the magnetic storm of 29–31 October 2003 led to ring current intensifications characterized by Dst minima. Defense Meteorological Satellite Program (DMSP) satellites detected low‐energy ions precipitating equatorward of auroral electrons in the dawn/morning local time sector. These ion fluxes weakened and/or vanished subsequent to Dst recoveries. A survey of the DMSP database reveals that near‐dawn ion precipitation is a main‐phase characteristic of all large magnetic storms. DMSP and Combined Release and Radiation Effects Satellite (CRRES) satellites also detected similar ion precipitation during the main phase of the March 1991 magnetic storm. To reconcile these observations with elementary concepts of ion drifts, the data suggest two source populations. The lowest‐energy ions were initially energized earthward of the plasma sheet electron boundary in the evening sector then corotated eastward. Higher‐energy ions originated in the plasma sheet and drifted close to the Earth under the combined influences of time‐varying convective electric fields and azimuthal gradients in the Earth's magnetic field. We developed a modified Volland Stern model to study test‐particle trajectories in asymmetric magnetic fields and variable electric fields compatible with CRRES measurements. Ions with magnetic moments μ ≥ μc ≈ 14 eV/nT drift duskward. Ions with μ < μc are injected to low L shells during a spike in the convective electric field. As the electric field diminishes and shielding increases, these ions drift toward dawn much closer to Earth than any test electron trajectories. Precipitation in the dawn sector results from a combination of pitch‐angle scattering by intense ambient waves and the dictates of ion drift paths.