Near-Earth Tailward Flow and the Dipolarization Process---TC-1 Exploration

On Oct. 12, 2004, the Geotail spacecraft was located in the solar wind near the dayside magnetopause and observed sustaining southward IMF from 01:30UT to 04:30UT. During the interval, a weak storm is driven with minimum Sym-H index of -33 nT at 04:12UT. In the main phase of the storm, AE index had been kept at a high level with a maximum value of about 400nT. The TC-1 satellite located at (-10.6, 3.2, -0.1) Re observed evident characteristics of substorm expansion phase and the process of magnetic field dipolarization at the near-Earth magnetotail during 02:00---03:00UT. Prior to the dipolarization, tailward flow (vx<-100km/s) lasting over 3min with the characteristics of high hot ion temperature, low density and flowing along the magnetic field occurred. The features of the tailward flow indicated that it was originated from the ionosphere. During the interval of tailward flow, both the Bx component and the intensity of the magnetic field had been enhanced which suggested that the tailward flow had certain effects on the magnetic field configuration. According to the TC-1 observations, near-Earth tailward flow may be in close relation with the process of the magnetic field dipolarization.