Abstract
Abstract. Recent studies have shown that the ambient plasma in the near-Earth magnetotail can be compressed by the arrival of a dipolarization front (DF). In this paper we study the variations in the characteristics of currents flowing in this compressed region ahead of the DF, particularly the changes in the cross-tail current, using observations from the THEMIS satellites. Since we do not know whether the changes in the cross-tail current lead to a field-aligned current formation or just form a current loop in the magnetosphere, we thus use redistribution to represent these changes of local current density. We found that (1) the redistribution of the cross-tail current is a common feature preceding DFs; (2) the redistribution of cross-tail current is caused by plasma pressure gradient ahead of the DF and (3) the resultant net current redistributed by a DF is an order of magnitude smaller than the typical total current associated with a moderate substorm current wedge (SCW). Moreover, our results also suggest that the redistributed current ahead of the DF is closed by currents on the DF itself, forming a closed current loop around peaks in plasma pressure, what is traditionally referred to as a banana current.
Highlights
Bursty bulk flows (BBFs) are intervals of fast Earthward plasma and magnetic flux transport in the plasma sheet, and are usually considered the most important carriers of mass and energy towards the near-Earth region (Angelopoulos et al, 1992, 1994)
Based on the observations that the |Bx| decrease is in concert with the pressure increase over several minutes, while the BZ decrease only occurs within ∼ 6 s of the dipolarization front (DF)’s arrival in the central plasma sheet region, we suggest that a small-scale duskward current on the DF itself is only able to significantly affect the limited region passing the spacecraft over the few seconds immediately prior to the DF’s arrival, while the longer duration of the |Bx| decrease over the blue shaded region must occur mainly as a result of a redistribution in the larger-scale cross-tail current ahead of the DF
Using a database of 698 DF events observed by THEMIS in plasma sheet, we have shown that in the ∼ 1.5 min ahead of the arrival of a dipolarization front, there is an increase in the thermal plasma pressure and a corresponding decrease in the absolute value of the X-component of the magnetic field, |Bx|
Summary
Bursty bulk flows (BBFs) are intervals of fast Earthward plasma and magnetic flux transport in the plasma sheet, and are usually considered the most important carriers of mass and energy towards the near-Earth region (Angelopoulos et al, 1992, 1994). DFB emphasizes magnetic perturbation signatures, whereas FB describe mass and magnetic flux transport Both the plasma velocity and the north–south component of the magnetotail’s magnetic field inside the BBF are significantly larger than in the surrounding region. Individual FBs within the BBF are associated with field-aligned currents (FACs) that flow into the ionosphere on their dawnward edge and out of the ionosphere on their duskward edge, which forms a wedgelet (Birn and Hesse, 1996; Forsyth et al, 2008; Sergeev et al, 1996) Both simulations (Birn and Hesse, 2013; Birn et al, 1999) and observations (Yao et al, 2012, 2014a) have shown that the braking of BBFs in the nearEarth magnetotail may lead to current disruption and the formation of the FACs necessary to support an SCW. Our study presents the compression effect ahead of DF, the consequent current system and the magnetic perturbations
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