Current structures associated with dipolarization fronts

Abstract

[1] Recently, observational results on currents around the dipolarization fronts (DFs) of earthward flow bursts have attracted much research attention. These currents are found to have close association with substorm intensifications. This paper devotes to further study of the current system ahead and within the DFs with high-resolution magnetic field measurements from Cluster constellation in 2003. The separation of four spacecraft is much smaller than the scales of spatial structures ahead and within the DF layer so that the currents can be reliably obtained. Based on features of the magnetic field variations prior to the fronts, we categorized the DFs into two types: DFs with magnetic dips immediate ahead of the fronts (type I) and DFs without magnetic dips (type II). For type I DFs, it is found that dawnward currents along the DFs exist in the dip region; duskward currents exist within the fronts. Furthermore, the dawnward currents in the dip region are found to be mainly parallel to the local magnetic field with a spatial scale of~1000 km, whereas the duskward currents within the fronts have both significant parallel and perpendicular components. On the other hand, for type II DFs, only significant duskward and mainly perpendicular currents show up within the fronts; no dawnward currents exist ahead of DFs. The dawnward and mainly parallel current in the type I DFs is important in the current coupling process between magnetosphere and ionosphere and may lead to local current disruptions for substorm initiations.

A physical explanation for the magnetic decrease ahead of dipolarization fronts

“…4 that was not shown in the current system described in Fig. 4b by Yao et al (2013), we suggest this pressure increase may be caused by the compression of the plasma flow behind the DF. This dawnward current is naturally expected from Fig.…”

“…This B z signature is seen for all distance ranges shown in the figure. Yao et al (2013) showed that such a dip in the magnetic field ahead of DFs is a common, and found that field-aligned currents usually exist in this region. We now compare DFs based on the magnitude of the change in B z prior to the DF arrival.…”

“…Figure 4b is a cartoon illustrating the banana current loop between the magnetic dip region and the dipolarization front layer. This cartoon presents similar current system as the current system associated with DF in Yao et al (2013), however, they represent a very different physical process. In Yao et al (2013), the dawnward and duskward currents are the current system of DF, while here the dawnward and duskward currents are additional structures asso- ciated with the magnetic dip.…”

“…This cartoon presents similar current system as the current system associated with DF in Yao et al (2013), however, they represent a very different physical process. In Yao et al (2013), the dawnward and duskward currents are the current system of DF, while here the dawnward and duskward currents are additional structures asso- ciated with the magnetic dip. In addition, the difference of magnetic field B z is almost the same at t 0 -10 s and t 0 + 10 s, which strongly suggests that these three current form circuits, which are independent from the background magnetotail current.…”

Current reduction in a pseudo‐breakup event: THEMIS observations

“…The current disruption started at the leading edge of the flow, as a sharp dipolarization front (DF) with a magnetic dip ahead of the front layer. The magnetic dip current, which is of Region 2 sense, has been studied recently [Liu et al, 2013a[Liu et al, , 2013bYao et al, 2013b;Sun et al, 2013]. Those studies presented the FAC features ahead of the DF, while this paper has studied the cross-tail current density evolution ahead of earthward flow.…”