MMS Observation on the Cross‐Tail Current Sheet Roll‐up at the Dipolarization Front

We perform a case study on the evolution of the current sheet in different regions around the dipolarization front (DF), including magnetic‐dip preceding the DF, front at the DF, and magnetic pileup region (MPR) behind the DF based on magnetospheric multiscale (MMS) observation on July 31, 2017. In this event, MMS1 stays inside the current sheet during the whole bursty bulk flow (BBF) interval. Our analysis reveals that the cross‐tail current sheet at the DF is rolled up, signified by the depression (−Vz/−Bz) at the dip and elevation (+Vz/+Bz) at the front. The minimum variance analysis on the magnetic field method is applied to obtain the normal direction of the current sheet. The result confirms the roll‐up, that is, downward at the depressed current sheet and upward at the elevated current sheet. The current sheet roll‐up at the DF is asymmetric, with steeper elevation than depression. The elevation angle of the elevated current sheet is evaluated to be ∼30°. Strong duskward and predominantly perpendicular J spike (∼90 nA/m2) concentrate at the interface between the dip and the front. The strength of the current of the J‐spike is about nine/three times the current at the dip/front. The front is characterized by positive E·J. In the dip/MPR, no such preference is seen. Ion/Electron pitch angle distributions exhibit significant and different evolutions in the roll‐up current sheet from dip to front, including their energy‐dependence and distributions. Finally, the roll‐up current sheet could decelerate BBF and change the flow structure. The potential significance of the roll‐up current sheet on BBF evolution is emphasized. Key Points The cross‐tail current sheet at the dipolarization front (DF) is rolled‐up, with a depression at the dip and elevation at the front Strong dawnward J‐spike is formed in the roll‐up current sheet at the interface between dip and front Current sheet roll‐up at the DF has a potential significance on the evolution of the bursty bulk flow