Pressure gradient evolution in the near-Earth magnetotail at the arrival of BBFs

Abstract

Using in situ observations from THEMIS A, D and E during the 2008–2011 tail season, we present a statistical study of the evolution of pressure gradients in the near-Earth tail during bursty bulk flow (BBF) convection. We identified 138 substorm BBFs and 2,197 non-substorm BBFs for this study. We found that both the pressure and the B Z component of the magnetic field were enhanced at the arrival of BBFs at the spacecraft locations. We suggest that the increase of B Z during non-substorm BBFs is associated with flux pile-up. However, the much stronger enhancement of B Z during substorm BBFs implies the occurrence of magnetic field dipolarization which is caused by both the flux pile-up process and near-Earth current disruption. Furthermore, a bow-wave-like high pressure appears to be formed at the arrival of substorm BBFs, which is responsible for the formation of region-1-sense FACs. The azimuthal pressure gradient associated with the arrival of substorm BBFs lasts for about 5 min. The enhanced pressure gradient associated with the bow wave is caused by the braking and diversion of the Earthward flow in the inner plasma sheet. The results from this statistical study suggest that the braking and azimuthal diversion of BBFs may commonly create azimuthal pressure gradients, which are related to the formation of the FAC of the substorm current wedge.