GEOTAIL LOW-ENERGY PARTICLE AND MAGNETIC-FIELD OBSERVATIONS OF A PLASMOID AT X(GSM)=-142 R(E)

Data from the GEOTAIL Low Energy Particle (LEP) instrument and magneto meter (MGF) for a plasmoid event on October 8, 1993 when the spacecraf t was located at X(GMS) similar to -142 R(E) were analyzed. The event started 16 minutes after a substorm onset with a tailward flow of elec trons whose characteristic energy was several keV with an apparent ene rgy dispersion. This was followed by the arrival of a beam of high ene rgy ions, which also had a similar energy dispersion, and an enhanceme nt of the magnetic field intensity. There was appreciably high flux of cold ions in the lobe region, and it was further enhanced at the fron t of the plasmoid. These cold ions were energized at the boundary of t he plasmoid where a sharp decrease in the total magnetic field intensi ty 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. Afte r passing the main part of the plasmoid, the spacecraft entered a regi on characterized by high energy tailward flowing ions and coexisting c old ions. The level of low frequency magnetic field fluctuations was r elatively high, and there were some time variations in both high-energ y and cold ion fluxes. Synthesizing these features, we conclude that o ur observations are consistent with the creation of a plasmoid and sur rounding energetic particle layers following a substorm as predicted b y the near-Earth neutral line model of a magnetospheric substorm.