THEMIS observations of the near‐Earth plasma sheet during a substorm

Abstract

We present observations of a substorm on 13 March 2008 by Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the near‐Earth tail during the plasma sheet expansion evidenced by increase in the plasma density and temperature. The main features of the event are as follows: (1) Cross‐tail current reduction or current disruption (CD) was observed in the near‐Earth tail at X ∼ −8.0 RE and Y ∼ 2.0 RE, marked by a sharp drop of ∣Bx∣ and accompanied by sharp increases in the plasma density and temperature, manifesting a rapid expansion (recovery) of the local plasma sheet. During the course of the plasma sheet expansion, the propagation speed of the dipolarization is ∼48 km/s in tailward direction and ∼35 km/s in azimuthal direction. (2) In the inner edge of the plasma sheet, slow flux pileup is observed. The magnetic flux pileup is characterized by continuous enhancement of Bz and Bt and a reduction of the plasma density, pressure Pth, and β in the flow‐braking region. The tailward moving plasma sheet expansion (CD) is also passing across the flow‐braking region. In short, the dipolarization in the inner edge of the plasma sheet can be attributed to CD, while the tailward progression of dipolarization in the flow‐braking region can be attributed to magnetic flux pileup. (3) A sharp decrease in the magnitude of Bx at P1 (−13.1 RE, 2.5 RE, −0.56 RE) prior to the dipolarization at P5 is difficult to explain as part of the outward evolution of the CD. The rapid change in the magnetic field topology, and signatures of earthward flows and dipolarization observed at P1 prior to Pi2 onset may be caused by inward motion of flux from magnetic reconnection in the midtail (20–30 RE). Tail reconnection prior to substorm expansion can result in a sudden change of the near‐Earth configuration, which may result in instabilities related to the onset of CD.