With the deployment of the all‐sky imager array of the THEMIS mission, we were able to construct a preliminary database of auroral substorm expansion phase onsets, from which we have established a number of common features characterizing the first tens of seconds of the substorm auroral intensification. We find that the intensification occurs within ∼10 sec over an arc segment extending approximately 1 h MLT and featuring wave‐like formations distributed in longitude. The longitudinal wave number ranges between 100 and 300 such that the wavelength is comparable to the ion gyroradius in the central plasma sheet. The scale the intensification is about 10–30 sec. This study casts important observational constraints on substorm onset theories.

[1] We propose a theoretical model to investigate the effects of the curvature/gradient (c/g) drift and the finite azimuthal extent of the dipolarization region on the electron injection process associated with the substorm dipolarization. We study the azimuthal structure of high-energy (>30 keV) electron precipitation flux and compare the result with riometer observations. We are able to reproduce three basic archetypes of riometer responses to substorms, namely, the spike, dispersionless injection, and dispersed injection events catalogued in previous observations. The electron injection near the duskward edge of the dipolarization region is most subject to azimuthal c/g drift loss, appearing in riometer observations as the ''spike'' feature. The ''dispersionless injection'' response is seen inside the dipolarization region but some distance away from its western border: or, alternatively, when the substorm has a rapid westward expansion, so that the gain and loss of electrons from the duskside and dawnside of a dipolarizing flux tube roughly balance. The ''dispersed injection'' feature is seen east of the dipolarization region. Our theory successfully explains the statistical differences in terms of magnetic local time location and peak intensity between spikes and injection events. Through the substorm event on 23 May 1998 we demonstrate that our theoretical predictions of riometer responses are very consistent with the observations. We highlight the potential of riometers in resolving the azimuthal extent and evolution of the dipolarization region, which provides a new ground-based technique of remote sensing the substorm process.

[1] From a survey of the cross-tail current disruption (CD) events in the near-Earth plasma sheet collected from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we identify a highly repeatable class of event occurring at the current sheet boundary in a few minutes before the local CD onset. Salient features of this class of event include (1) a precipitous drop of the ion temperature, (2) concurrent growth of a neutral sheet-pointing electric field, and (3) ULF wave activations at Pi1/Pi2 bands. We interpret the ion temperature drop as a manifestation of the extremely thinning of the local current sheet prior to its disruption. This thinning process is inferred as nonadiabatic by nature. Particularly, when the current sheet thickness is down to ion kinetic scales, the ions are demagnetized, and a quasi-electrostatic neutral sheet-pointing electric field emerges owing to the charge separation. The ULF fluctuations of electric/magnetic field have a two-band structure. The lower-frequency band with a period of 50 to 80 s is interpreted as an Alfvenic mode coupled from other preonset wave modes excited at the equatorial plasma sheet such as the ballooning. The higher-frequency wave at 10-to 20-s periods is attributed to some instability mode, directly leading to the disruption of the thin current sheet (TCS). We suggest that an extremely thinned non-Harris TCS and the emergence of quasi-electrostatic field constitute the conducive conditions for a local CD to occur. A companion paper by covers some theoretical aspects of this study.Citation: Liang, J., W. W. Liu, and E. F. Donovan (2009), Ion temperature drop and quasi-electrostatic electric field at the current sheet boundary minutes prior to the local current disruption,

We present observation of plasma waves in the current sheet across multiple THEMIS satellites in a substorm event on March 5, 2008. It is shown that the arrival of a kinetic ballooning perturbation interacts with the local current sheet to generate a quasi‐electrostatic wave a few minutes before local onset, consistent with the prediction that the current sheet thins after the passage of a rarefaction wave. The propagation speed of current disruption front is found to be ∼100 km/s, about a tenth of the fast mode speed. The observed pattern of interaction was constant across radial distances between 10 and 20 RE in the event reported. It is further proposed that the presence of the quasi‐electrostatic field may change the local stability condition and induce local current disruption.

[1] The importance of monitoring particle injection into the inner magnetosphere from the plasma sheet is exceeded perhaps only by its difficulty. The recent progress in using ground-based riometer data [e.g., Spanswick et al., 2007] to detect by proxy particle injection has raised much hope that this important aspect of substorms can be more consistently monitored. In this paper we develop a theoretical model for explaining the observed dispersionless injection events reported by Spanswick et al. The substorm event on 3 October 1998 is analyzed to give the empirical context for riometer responses during a typical substorm. Our simulation shows that riometer electrons produce a clean and strong signal that can be uniquely and easily related to magnetic field dipolarization that normally gives rise to injection.