Statistical analysis of the plasmoid evolution with Geotail observations

Abstract

Plasmoids in the Earth's magnetotail were studied statistically, using low energy particle (LEP) and magnetic field (MGF) data from the Geotail spacecraft. Their evolution along the tail axis from X GSM′ = −16 to −210 R E was investigated with 824 plasmoid events. Their dependence on Y GSM′ was studied as well to derive the three‐dimensional structure of the plasmoids. (The coordinates are aberrated to remove the average effects of the orbital velocity of the Earth about the Sun.) We defined a plasmoid as a structure with rotating magnetic fields and enhanced total pressure. In the near tail ( X GSM′ ≥ −50 R E ), there was a tendency for the plasmoids to be observed in the premidnight sector around the tail axis (| Y GSM′ − 3| ≤ 10 R E ), while they were observed widely (| Y GSM′ ≤ 20 R E ) in the middle tail (−50 > X GSM′ ≥ −100 R E ) and in the distant tail (−100 R E > X GSM′ ). The plasmoids expanded in the ± Y GSM′ direction with typical velocities of ±130 ± 100 km/s in the near tail. This strongly supports the view that plasmoids are initially formed at the near‐Earth neutral line which has a limited extent in the Y GSM′ direction. The plasmoids accelerated in the downtail direction from 400 ± 200 km/s to 700 ± 300 km/s from the near to the middle tail. Then, it is suggested that they decelerated to 600 ± 200 km/s as they traveled to the distant tail. The ion temperature inside plasmoids was 4.5 ± 2 keV in the near and middle tail, and then rapidly decreased to 2 ± 1 keV from the middle to the distant tail region. The ion temperature in the distant tail was 2 times higher than the values deduced previously. Typical plasmoid dimensions were estimated to be 10 R E (length) × 40 R E (width) × 10 R E (height) in the middle and distant tail. The energy carried by each plasmoid was ∼2 × 10 14 J in the middle tail, half of which was lost from the middle to the distant tail. Inside plasmoids, the thermal energy flux exceeded the bulk energy flux and Poynting flux. The energy released tailward in the course of a substorm was estimated to be roughly 10 15 J.