Intensification of preexisting auroral arc at substorm expansion phase onset: Wave‐like disruption during the first tens of seconds

Abstract

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.

Features of auroral breakup obtained using data of ground-based television observations: case study

“…Brightness of this spot continuously increased till 20:32:21 UT for 15 s interval. Auroral arc with waved structure was observed only after 20:32:21 UT, which is consistent with the previous results by Mende et al (2007) and Liang et al (2008). The process was followed later by the breakup arc flash and by the appearance of new bright rayed formations, which propagated poleward.…”

Magnetic conjugacy of northern and southern auroral beads

“…However, the abovementioned interhemispheric similarities strongly suggest that there must be a common driver in the magnetotail equatorial region that was controlling the major temporal evolution of the auroral beads. Most of the previous studies of auroral beads [e.g., Donovan et al, 2006;Liang et al, 2008] inferred that the magnetospheric driver of the auroral beads may be a ballooning type instability [Cheng, 2004]. Of course, some different processes at a lower altitude along the field line (i.e., M-I coupling region) may also contribute to the structuring of the bead structures: for example, ionospheric feedback instability in the so-called ionospheric Alfven resonator [Lysak and Song, 2002] and instabilities in the auroral acceleration region [Chaston and Seki, 2010].…”

“…The scale of the beads and their separation are typically on the order of the gyroradius of 1-10 keV protons in the source magnetosphere. Thus, as suggested by Liang et al [2008] and Sakaguchi et al [2009], the plasma instability creating the beads in the primary stage of their development would not be a pure MHD scale process but include kinetic effect.…”

“…Recently, attention has been drawn to the temporal evolution of the initial brightening arc during the few minutes before the onset, because it could illustrate the process that triggers the expansion phase onset. Optical observations from the ground [Donovan et al, 2006;Liang et al, 2008;Sakaguchi et al, 2009] and from spacecraft [Elphinstone et al, 1995;Henderson, 2009] have shown that in the initial brightening arc, there exists a characteristic small-scale auroral structure, consisting of azimuthally arrayed wave-like forms (often referred to as "auroral beads") with a wavelength of 30-100 km and an azimuthal propagation speed of 5-8 km s À1 . As time progresses, the auroral beads evolve into wave-like undulations with a larger scale of 50-200 km, and finally lead to auroral breakup.…”

Fine structures and dynamics in auroral initial brightening at substorm onsets

“…Donovan et al (2006b) reported a pseudo breakup occurred on a pre-existing auroral arc with a brightening that consisted of eastward propagating beads with a wavelength of ∼100 km and an azimuthal propagation speed of ∼5 km/s. Liang et al (2008) found intensification of aurora occurring within ∼10 s over an arc segment of ∼01:00 MLT and featuring wave-like formations with longitudinal wavelengths between 50 and 200 km. Similar structures were seen by Rae et al (2009) at scales along the arc of ∼70 km (see also Freidrich et al, 2001).…”