GEOTAIL low energy particle and magnetic field observations of a plasmoid at XGSM = −142 RE

Abstract

Data from the GEOTAIL Low Energy Particle (LEP) instrument and magnetometer (MGF) for a plasmoid event on October 8, 1993 when the spacecraft was located at XGMS ∼−142 RE were analyzed. The event started 16 minutes after a substorm onset with a tailward flow of electrons whose characteristic energy was several keV with an apparent energy dispersion. This was followed by the arrival of a beam of high energy ions, which also had a similar energy dispersion, and an enhancement of the magnetic field intensity. There was appreciably high flux of cold ions in the lobe region, and it was further enhanced at the front of the plasmoid. These cold ions were energized at the boundary of the plasmoid where a sharp decrease in the total magnetic field intensity was observed. It was found that the main part of the plasmoid had a significant dusk‐to‐dawn magnetic field, indicating the plasmoid had a flux rope, or a distorted closed‐loop magnetic field structure. After passing the main part of the plasmoid, the spacecraft entered a region characterized by high energy tailward flowing ions and coexisting cold ions. The level of low frequency magnetic field fluctuations was relatively high, and there were some time variations in both high‐energy and cold ion fluxes.Synthesizing these features, we conclude that our observations are consistent with the creation of a plasmoid and surrounding energetic particle layers following a substorm as predicted by the near‐Earth neutral line model of a magnetospheric substorm.