[1] Using Time History of Events and Macroscale Interactions during Substorms observations from four tail seasons, we study the three-dimensional structure of the dipolarization front current sheet (DFCS), which demarcates the magnetic boundary of a dipolarizing flux bundle (DFB, the strong magnetic field region led by a dipolarization front) in Earth's magnetotail. An equatorial cross section of the DFCS is convex; a meridional cross section is consistent with a dipolarized field line. The equatorial flow pattern in the ambient plasma ahead of the DFCS exhibits diversions of opposite sense on its evening and morning sides. The magnetic field perturbations are consistent with local field-aligned current generation of region-2 sense ahead of the front and region-1 sense at the front. The median thickness of the DFCS increases from 800 to 2000 km with increasing distance from the neutral sheet, indicating bundle compression near the neutral sheet. On a meridional cross section, DFCS's linear current density (1.2-1.8 nA/m) peaks~AE0.55 l from the neutral sheet (where l is the ambient cross-tail current sheet half-thickness, l~1.5 R E in our database). This peak, reminiscent of active-time cross-tail current sheet bifurcation noted in past studies, suggests that the intense but thin DFCS (10 to 20 nA/m 2 ) may be produced by redistribution (diversion) of the extended but weaker cross-tail current (~1 nA/m 2 ). Near the neutral sheet, the average DFCS current over the dipolarization front (DF) thickness is perpendicular to both the magnetic field interior to the DFB and the average field direction over the DF thickness. Away from the neutral sheet, the average current becomes progressively parallel to the internal field and the average field direction. The average current directions are indicative of region-1-sense field-aligned current on the DF. As few as approximately three DFBs can carry sufficient total current that, if redirected into the auroral ionosphere, can account for the substorm current wedge's peak current for a sizable substorm (~1 MA). A collapsing DFB could thus be an elemental substorm current wedge, or "wedgelet," that can divert a sizable portion of the cross-tail current into the auroral ionosphere.Citation: Liu, J., V. Angelopoulos, A. Runov, and X.-Z. Zhou (2013), On the current sheets surrounding dipolarizing flux bundles in the magnetotail: The case for wedgelets,

Recent observations have suggested that spatially localized flows of high‐temperature, low‐density plasma carrying a dipolarized magnetic field (dipolarizing flux bundles, DFBs) play a key role in hot plasma transport toward the inner magnetosphere. What controls plasma heating in DFBs and how do thermodynamic parameters (such as density, temperature, pressure, and specific entropy) and spectral properties of the DFB population depend on ambient plasma sheet properties and geocentric distance R remains unknown. By statistical analysis of 271 DFB events detected by the Time History of Events and Macroscale Interactions during Substorms mission during the 2008–2009 tail seasons, we find that on average, plasma inside DFBs is a factor of 0.6 less dense and a factor of 1.5 to 2 hotter than ambient tail plasma. The radial profiles of average thermodynamic parameters inside and outside DFBs are similar; when fitted by the κ‐function, their energy spectra have similar κ‐exponents, but a factor of 2 larger peak energies inside DFBs. Our analysis suggests that average DFB plasma properties are closely linked to those of the ambient plasma sheet population. Estimations show that on average, adiabatic heating of the ambient plasma in the increased magnetic field is the major factor in DFB plasma heating.

A dipolarizing flux bundle (DFB) is a small magnetotail flux tube (typically <~3 R E in X GSM and Y GSM ) with a significantly more dipolar magnetic field than its background. Dipolarizing flux bundles typically propagate earthward at a high speed from the near-Earth reconnection region. Knowledge of a DFB's flux transport properties leads to better understanding of near-Earth (X = À6 to À30 R E ) magnetotail flux transport and thus conversion of magnetic energy to kinetic and thermal plasma energy following magnetic reconnection. We explore DFB properties with a statistical study using data from the Time History of Events and Macroscale Interactions during Substorms mission. To establish the importance of DFB flux transport, we compare it with transport by bursty bulk flows (BBFs) that typically envelop DFBs. Because DFBs coexist with flow bursts inside BBFs, they contribute >65% of BBF flux transport, even though they last onlỹ 30% as long as BBFs. The rate of DFB flux transport increases with proximity to Earth and to the premidnight sector, as well as with geomagnetic activity and distance from the neutral sheet. Under the latter two conditions, the total flux transport by a typical DFB also increases. Dipolarizing flux bundles appear more often during increased geomagnetic activity. Since BBFs have been previously shown to be the major flux transporters in the tail, we conclude that DFBs are the dominant drivers of this transport. The occurrence rate of DFBs as a function of location and geomagnetic activity informs us about processes that shape global convection and energy conversion.

[1] As a dipolarizing flux bundle (DFB) moves earthward, it creates pressure and flow perturbations. These perturbations may play a significant role in controlling DFB motion and generating field-aligned currents (FACs) which render the DFB a "wedgelet", a traveling building block of the substorm current wedge. To investigate this hypothesis, we use DFB observations from the Time History of Events and Macroscale Interactions during Substorms mission to reconstruct the spatial profiles of the thermal and total (thermal plus magnetic) pressures and of the plasma flow near the DFB. The total pressure reaches maximum inside the dipolarization front (DF, the leading edge of the DFB). The resultant pressure gradient force pushes ambient plasma in the direction normal to the front and exerts a gradient force density of~0.15 nPa/R E against the DFB motion. The thermal pressure in the equatorial plane is strongest immediately ahead of the DFB's leading point; it decreases with distance from that peak: toward the ambient plasma, toward the DFB interior, and toward the DFB flanks. Combining our estimate of the flux tube volume distortion with the measured equatorial thermal pressure distribution, we obtain a region-1-sense FAC inside the DF layer and region-2-sense FAC in the~1 R E thick region immediately ahead of it. This system of FACs is indeed consistent with a wedgelet.

Dipolarization fronts (DFs), earthward-propagating structures in the Earth's magnetotail current sheet with sharp enhancements of the northward magnetic field B z , are typically preceded by minor decreases in B z . Other characteristic DF precursor signatures, including earthward flows and plasma density/pressure enhancements, have been explained in the context of ion acceleration and reflection at dipolarization fronts. In the same context here we simulate the spatial distribution of plasma pressure earthward of a convex DF. The resultant pressure distribution, which shows clear dawn-dusk asymmetries with greater enhancements at the DF duskside, agrees with statistical observations. The simulation further reveals that the reflected ions can carry a secondary current earthward of the advancing DF, which explains the characteristic signature of the B z dip immediately ahead of the DF.

[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.

Electromagnetic fluctuations associated with various plasma instabilities have been previously identified near dipolarization fronts in Earth's magnetotail. However, the potential effect of these fluctuations on particle distributions and energy conversion is poorly understood. The most important instabilities responsible for electron acceleration and scattering are the whistler instability and the electron firehose instability. Utilizing 10 years of Time History of Events and Macroscale Interactions during Substorms satellite observations, we explore the occurrence probability and intensity of these instabilities near dipolarization fronts, as function of electron anisotropy and plasma beta. Electron temperature anisotropies are well constrained by the marginal stability conditions of the whistler and oblique electron firehose instabilities. In fact, the observed enhancement of magnetic field fluctuations near the instability thresholds provides good evidence for the operation of these instabilities on electrons near fronts. Since the build-up of electron anisotropy is limited by wave-particle interactions, we conclude that such interactions are important enough to affect electron dynamics and energetics.Plain Language Summary Dipolarization fronts, transient phenomena frequently observed in the magnetotail, are closely related to substorms. Various types of instabilities are known to be present in the vicinity of dipolarization fronts. The effects of these instabilities on particle dynamics, however, are poorly understood. Our work, utilizing 10 years of Time History of Events and Macroscale Interactions during Substorms satellite observations, demonstrates that electron temperature anisotropies are well constrained by the marginal stability conditions of the whistler and oblique electron firehose instabilities. The occurrence probability of these instabilities and enhancement of magnetic and electric field fluctuations near the instability thresholds provide good evidence for the operation of these two wave types in the vicinity of dipolarization fronts. Our results reveal the importance of wave-particle interactions in the magnetotail near dipolarization fronts and potential effects of those interactions on global energy conversion.

[1] This paper presents THEMIS measurements of two substorm events to show how the substorm current wedge (SCW) is generated. In the late growth phase when an earthward flow burst in the near-Earth magnetotail brakes and is diverted azimuthally, pressure gradients in the X-and Y-directions are observed to increase in the pileup and diverting regions of the flow. The enhanced pressure gradient in the Y-direction is dawnward (duskward) on the dawnside (duskside) where a clockwise (counter-clockwise) vortex forms. This dawn-dusk pressure gradient drives downward (upward) field-aligned current (FAC) on the dawnside (duskside) of the flow, which, when combined with the FACs generated by the clockwise (counter-clockwise) vortex, forms the SCW. Substorm auroral onset occurs when the vortices appear, Near-Earth dipolarization onset is observed by the THEMIS spacecraft (probes) when a rapid jump in the Y-component of pressure gradient is detected. The total FACs from the vortex and the azimuthal pressure gradient are found to be comparable to the DP-1 current in a typical substorm. Citation: Yao, Z. H., et al. (2012), Mechanism of substorm current wedge formation: THEMIS observations, Geophys.