Near-Earth plasma sheet boundary dynamics during substorm dipolarization

Abstract

We report on the large-scale evolution of dipolarization in the near-Earth plasma sheet during an intense (AL ~ −1000 nT) substorm on August 10, 2016, when multiple spacecraft at radial distances between 4 and 15 R E were present in the night-side magnetosphere. This global dipolarization consisted of multiple short-timescale (a couple of minutes) B z disturbances detected by spacecraft distributed over 9 MLT, consistent with the large-scale substorm current wedge observed by ground-based magnetometers. The four spacecraft of the Magnetospheric Multiscale were located in the southern hemisphere plasma sheet and observed fast flow disturbances associated with this dipolarization. The high-time-resolution measurements from MMS enable us to detect the rapid motion of the field structures and flow disturbances separately. A distinct pattern of the flow and field disturbance near the plasma boundaries was found. We suggest that a vortex motion created around the localized flows resulted in another fieldaligned current system at the off-equatorial side of the BBF-associated R1/R2 systems, as was predicted by the MHD simulation of a localized reconnection jet. The observations by GOES and Geotail, which were located in the opposite hemisphere and local time, support this view. We demonstrate that the processes of both Earthward flow braking and of accumulated magnetic flux evolving tailward also control the dynamics in the boundary region of the near-Earth plasma sheet.

Lower Hybrid Drift Waves and Electromagnetic Electron Space‐Phase Holes Associated With Dipolarization Fronts and Field‐Aligned Currents Observed by the Magnetospheric Multiscale Mission During a Substorm

“…In the present study, we have shown two DF‐like signatures associated with fast plasma flows and field‐aligned current signatures detected during the expansion phase of a large substorm (see also Nakamura et al, ). The first event corresponds to a fast dawnward flow and antiparallel current, which could be associated with a vortex structure generated by the wake of a previous fast earthward plasma flow (Sergeev et al, ).…”

“…The first event corresponds to a fast dawnward flow and antiparallel current, which could be associated with a vortex structure generated by the wake of a previous fast earthward plasma flow (Sergeev et al, ). The second event is more classic and corresponds to a fast earthward and duskward plasma flow and a parallel current consistent with a localized substorm current wedge‐like signature (“wedgelet”) expected to be associated with such a fast earthward flow (Cao et al, ; Liu et al, ; Nakamura et al, ; Palin et al, ). This second event is preceded by a southward/dawnward plasma flow consistent with a brief expansion of the plasma sheet generated by the arrival of the fast earthward/duskward flow.…”

“…Based on a multimission study including GOES, THEMIS, Van Allen Probes, Geotail, and MMS satellites and ground‐based measurements, Nakamura et al () carried out a large‐scale study of this substorm event. In particular, they showed that four field‐aligned current (FAC) signatures were detected accompanied by dipolarization fronts (increase of B z or of the elevation angle) at 1001:22, 1001:43, 1002:41, and 1003:01 UT.…”

“…Considering that the front crossing lasts about 1 s for the first event (3 s for second) due to the fast dawnward motion in the y direction, a rough estimate of the front thickness as well as the scale of the density gradient is about 1,500 km or 2d i for the first event (with v ⊥ y , e ∼−1,500 km/s and n ∼ 0.12 cm −3 ) and also ∼1,500 km for the second event (with v ⊥ x , e ∼ v ⊥ z , e ∼−500 km/s). Based on a timing analysis, Nakamura et al () gave more precise estimates of the current sheet thickness associated with these DFs and found 1,310 km and 710 km, respectively. Therefore, the spatial scale of these fronts crossed out of equator is about the ion inertial length, similar to those calculated closer to the magnetic equator by previous missions (e.g., Deng et al, ; Khotyaintsev et al, ; Runov et al, ; Zhou et al, ).…”

“…The AKR already detected before the local onset around 1001 UT is probably associated with the first substorm and produced by the corresponding auroral electron precipitations (e.g., Ergun et al, 1998;Pritchett & Strangeway, 1985;Strangeway et al, 1998); it indicates that the magnetosphere is very perturbed and has never reached a relatively quiet steady state during the whole period. At lower frequencies, strong electric and magnetic field fluctuations are detected up to the electron Based on a multimission study including GOES, THEMIS, Van Allen Probes, Geotail, and MMS satellites and ground-based measurements, Nakamura et al (2017) carried out a large-scale study of this substorm event.…”