A Quasi-stagnant Plasmoid Observed With Geotail on October 15, 1993

Nishida et al. (1986) proposed the category of quasi-stagnant plasmoids on the basis of ISEE-3 observations. Their quasi-stagnant plasmoids occurred during intervals of low geomagnetic activity, and the electron plasma moment data suggested that the plasmoids moved tailward very slowly (<300 km/s). This paper reports for the first time the low-energy (32 eV/e-43 keV/e) plasma ion signatures of a quasi-stagnant plasmoid, which was observed with Geotail at X=-170 RE on October 15, 1993. On this day Kp was generally quiet, but Geotail observed two prominent bipolar Bz perturbations identifiable as plasmoids. The first plasmoid moved tailward very slowly (~250 km/s), and it was associated with a Pi2 onset possibly localized at high latitudes and with a gradual enhancement in the power of Aurora] Kilometric Radiation (AKR) emissions. On the other hand, the second plasmoid moved tailward fairly rapidly (~500 km/s), and it was associated with a Pi2 onset observable at wide range of latitudes and with, a sharp enhancement in the AKR power. Thus the first plasmoid falls into the category of quasi-stagnant plasmoids but the second plasmoid would notj this is consistent with the previous observations by Nishida et al. that quasi-stagnant plasmoids are sometimes followed by substorm onset-related plasmoids. A unique feature of the plasma ions in the first plasmoid is that the earthward field-aligned beam was observed first and then the tailward field-aligned beam was observed, as the spacecraft entered into the plasmoid. This time sequence is rarely observed for plasmoids during geomagnetically active times. It is explained in the framework of the quasi-stagnant plasmoid model.