Characteristics of the near-Earth magnetotail variations at the time of substorm onset

From the result of our statistical analysis with GEOTAIL and THEMIS data, we confirmed that tailward plasma flows are created in association with the plasma flows that propagate from X ~ -12 Re toward the earth at the time of substorm onset. To understand the physical mechanism of the formation of such tailward flows, we have performed a case study of substorm event occurred at 10:46 UT on June 22, 1997 when the GEOTAIL was located at (X, Y) ~ (-9, 6) Re and encountered with the flow bursts. The event started with earthward flows for about 2 min with the northward magnetic field enhancement, followed by slow tailward flows. Such an earthward and tailward flow sequence repeated three times. By calculating the electric current from electron and ion moment data, we found the variation in the X-component of JxB force that can be interpreted to have close relationship with the enhancement of earthward flows. Namely, the flows are not simply generated at a distant location and reach the GEOTAIL, but they are still under acceleration at X ~ -10 Re. After the passage of the flow front, the (JxB)x term takes negative values. The time derivative of Vx does not necessarily correspond to (JxB)x/mn. This inconsistency might be due to the contribution of the terms such as -grad P or (Vgrad) V. It is also necessary to check the possibility of the decoupling between electrons and ions as well as the effect of anomalous resistivity.