A Statistical Study on the Properties of Dips Ahead of Dipolarization Fronts Observed by MMS

摘要

Magnetic reconnection in the magnetotail accelerates plasma in short-duration bursts of fast flows, referred to as bursty bulk flows toward the Earth. These bursty bulk flows are typically accompanied by a sharp increase in the northward magnetic field component, the so-called dipolarization front (DF). This rapid increase in northward magnetic field is often preceded by a decrease, the so called DF-dip, where the northward magnetic field component sometimes even changes sign and turns southward. Here we present a statistical study of the DF-dip of 43 events, using observations from the Magnetospheric Multiscale (MMS) mission during summer 2017, when MMS's apogee was located in the magnetotail around 25 R_E. The 43 events are subdivided into two categories according to their DF-dip: 20 events where the DF-dip stays northward (positive DF-dip category) and 23 events where the DF-dip turns fully southward (negative DF-dip category). We find that (1) the magnetic field depression ahead of the DF of the positive DF-dip events correlates mainly with perpendicular currents and diamagnetic currents flowing ahead of the DF and (2) the magnetic field depression ahead of the DF of the negative DF-dip events correlates mainly with field-aligned currents and that this type of events might be (a) earthward moving flux rope-like structures caused by multiple X-line reconnection and/or (b) are a result of localized, single X-line reconnection under a guide field.