Convective bursty flows in the near-Earth magnetotail inside 13 R-E

Abstract

With the 4-s resolution data of the magnetometer and the ion plasma analyzer on Double Star TC-1 from June to November of each year during the period of 2004-2006, we statistically analyzed the occurrence rate of both convective and field-aligned bursty flows (FABFs). In the region -13 R-E < X < -9 R-E, vertical bar Y vertical bar < 10 R-E, vertical bar Z vertical bar < 4 R-E, there are in total 80 convective bursty flow events and 430 FABF events with a criterion that the duration time of both V-perpendicular to x > 180 km/s and V-parallel to x > 180 km/s exceeds 30 s, where V-perpendicular to x and V-parallel to x are irrespectively the X component of the velocity vector perpendicular and parallel to the local magnetic field. The statistical results show that the occurrence rate of the convective bursty flow (CBF) reached a minimum around -11.5 R-E; the occurrence rate of the FABF has a sudden decrease around -11.5 R-E, but has no clear increase inside -11.5 R-E. The spatial distribution of the all types of bursty flows shows that there had no deflection flows in the azimuthal direction related to the absence of the convective bursty flows inside 13 R-E explored by TC-1. The magnetic field analysis indicates that there exists a minimum in Bz around 12 R-E. Here, we suggest that the CBFs inside -11.5 R-E may not relate to the flows outside -12 R-E. The CBFs outside 12 R-E are likely to be the classical bursty bulk flow, originating 20-30 R-E downtail and braking outside 12 R-E. The minimum Bz may have close relation with the flow braking. The CBFs inside 12 R-E may be produced by another mechanism. The minimum Bz may be prone to the excitation of instabilities and, consequently, initiation of the substorm expansion and/or acceleration of ions resulting in CBFs inside.