Observation of the plasma sheet during a contracted oval substorm in a prolonged quiet period

Abstract

This report presents an interesting observation of a substorm which occurred on a contracted oval, with simultaneous particle measurements by Imp 6 (r ∼ 11 RE) and ATS 5 (r ∼ 6.6 RE) satellites. At the substorm onset, immediate and substantial energization of both protons and electrons in the plasma sheet was observed by the Imp 6 satellite in the midnight sector. This change was accompanied by a strong earthward plasma flow at ∼500 km/s for ∼12 min. However, no significant mid-latitude positive bays were detected, and no substorm injection of plasma sheet particles was observed by the ATS 5 satellite, located then at ∼0130 MLT. This observation indicates that substorms occurring on a contracted oval are accompanied by two of the characteristic substorm changes in the plasma sheet, namely, the energization of the plasma sheet particles and the occurrence of strong plasma flows, but the substorm injection does not necessarily reach the synchronous distance in the midnight sector. Further, it suggests that contracted oval substorms and intense substorms represent the same physical phenomenon, differing only in magnitude. It is also shown that after a prolonged quiet period the average energies of protons and electrons in the near-earth plasma sheet (at ∼11 RE) are as low as 2 keV and 100 eV, respectively.