Near-Earth plasma sheet azimuthal pressure gradient and associated auroral development soon before substorm onset

Abstract

[1] The azimuthal plasma pressure gradient in the near-Earth plasma sheet makes crucial contributions to field-aligned current (FAC) formation. Numerical simulations and statistical observations have shown that a plasma pressure peak tends to build up in the premidnight region of the near-Earth plasma sheet during the substorm growth phase owing to enhanced magnetic drift. This leads to azimuthal pressure gradients in this region. The temporal variation of the azimuthal pressure gradient may provide an indication for the FAC variations associated with the substorm growth phase and may set up a plasma sheet precondition for the substorm onset being triggered near this region. We take advantage of two of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft separated azimuthally near the orbit apogee and investigate the azimuthal plasma pressure gradient before substorm onset in the R ∼10-12 R E region. Equatorial plasma pressure is estimated by removing the curvature force effect. Five events with the spacecraft footprints mapped very close to the aurora onset region were selected. These events show substantial duskward pressure gradient enhancement 1-5 min before onset. The onset arc, which results from enhanced energetic electron precipitation, was found to intensify simultaneously with the pressure gradient enhancement before onset breakup occurs. Since the energy and energy flux of precipitating electrons reflect the upward FAC strength, these results indicate that the duskward azimuthal pressure gradient enhancement is associated with enhanced upward FAC during the late growth phase and leads to the intensification of the onset auroral arc soon before it breaks up. It is possible that this pressure gradient enhancement may lead to ballooning mode instability and thus substorm onset along the intensifying arc.Citation: Xing, X

A Case Study of Near‐Earth Magnetotail Conditions at Substorm and Pseudosubstorm Onsets

“…Xing et al (2011) reported similar results that the azimuthal pressure gradient increases between substorm initial brightening and poleward expansion. It slightly decreased at ∼0337 UT, just before the first pseudosubstorm subsided.…”

Substorm simulation: Insight into the mechanisms of initial brightening

“…For example, Saito et al [2010] presented global auroral images taken by the Polar satellite during substorm expansion, to show that the auroral evolution is spatially and temporally separable into two periods of intense auroral activities, which were separated by 4.5 min. [Lui, 1996], dawnward inertial currents [Shiokawa et al, 1997], pressure gradients [Shiokawa et al, 1998;Birn et al, 1999;Yao et al, 2012], flow vortices near the equatorial plane [Birn et al, 2004;Keiling et al, 2009], and low-entropy magnetic flux tubes (or plasma bubbles) [Sergeev et al, 1996;Chen and Wolf, 1999;Nakamura et al, 2001;Birn et al, 2004;Xing et al, 2011].…”

Plasma pressure generated auroral current system: A case study

“…In this case study good agreement was found between the magnitude of the FAC sheet current pair as obtained by FAST and Polar in a simultaneous coordinated measurement. The azimuthal plasma pressure gradient of 0.01 nPa/R E used in our calculations is characteristic of quiet times [Xing et al, 2011] and it was sufficient to power the auroral current system as seen by the two satellites. As the pressure gradient current generation is proportional to 1/B it is highly dependent on the configuration of the magnetic field.…”

“…This was a remarkably successful statistical study considering the averaging over different geophysical conditions and the use of the magnetic field model to map low altitude satellite data to model magnetospheric particle densities and temperatures. With the advent of multi satellite missions such as CLUSTER and THEMIS, it is now possible to make direct measurements of the plasma pressure and hence its gradient [Xing et al, 2011]. These measurements show that during quiescent times the night side azimuthal pressure gradients are of the order of 0.01 nPa/Re and it can increase by a large factor before and during substorm onset.…”

“…This was a typical pressure gradient measured during quiescent times prior to substorm onsets [Xing et al, 2011]. We calculated the pressure gradient current flowing through the surface separating the up and the down going FAC.…”