TC‐1 observations of flux pileup and dipolarization‐associated expansion in the near‐Earth magnetotail during substorms

Fifty‐three substorms measured by Double Star/TC‐1 in the near‐Earth magnetotail from July to October, 2004 are studied. The main features of these events are: (a) Magnetic flux pileup characterized by continuous enhancement of B z is observed, which starts almost simultaneously with aurora breakup within 1–3 minutes, indicating that substorm onset is in close relation to flux pileup. (b) Sudden plasma sheet expansion with sharp increases in ion temperature and density is seen in all events, which occurs typically ∼11 minutes after the beginning of pileup. The plasma sheet expansion is shown to be in close relation with the primary substorm dipolarization and, hence, can be referred to as ‘dipolarization‐associated expansion’. (c) Evidence indicates that the substorm current wedge first forms earthward of TC‐1 position and, hence, inward of the flow braking region, and then propagates tailward with an expansion in the Z‐direction. Possible implications of these observations are briefly discussed.