Optical characterization of the growth and spatial structure of a substorm onset arc

Abstract

[1] We present a detailed case study of the characteristics of auroral forms that constitute the first ionospheric signatures of substorm expansion phase onset. Analysis of the optical frequency and along-arc (azimuthal) wave number spectra provides the strongest constraint to date on the potential mechanisms and instabilities in the near-Earth magnetosphere that accompany auroral onset and which precede poleward arc expansion and auroral breakup. We evaluate the frequency and growth rates of the auroral forms as a function of azimuthal wave number to determine whether these wave characteristics are consistent with current models of the substorm onset mechanism. We find that the frequency, spatial scales, and growth rates of the auroral forms are most consistent with the cross-field current instability or a ballooning instability, most likely triggered close to the inner edge of the ion plasma sheet. This result is supportive of a near-Earth plasma sheet initiation of the substorm expansion phase. We also present evidence that the frequency and phase characteristics of the auroral undulations may be generated via resonant processes operating along the geomagnetic field. Our observations provide the most powerful constraint to date on the ionospheric manifestation of the physical processes operating during the first few minutes around auroral substorm onset.

Statistical azimuthal structuring of the substorm onset arc: Implications for the onset mechanism

“…Auroral beads can statistically be observed~2 min before auroral onset at low spatial scales and are the earliest signature of waves which exists in the near-Earth magnetosphere on closed field lines. In the minutes around auroral onset, the waves start to grow exponentially over a wide range of spatial scales; this is characteristic of the linear stage of the instability [Rae et al, 2010;Motoba et al, 2012;Murphy et al, 2014;Kalmoni et al, 2015]. During the transition from preonset to postonset times, the consistent median statistical characteristic spatial scale of the instability is indicative that a single instability is active at these times.…”

“…Statistically, the instability simultaneously excites exponentially growing waves over a wide range of spatial scales. Rae et al [2010] and Kalmoni et al [2015] demonstrate that, in individual events, specific wavenumbers grow faster than others.…”

“…The authors demonstrated that, for all of these arcs, the auroral brightness grew exponentially with time for a common range of spatial scales, suggesting these are evidence of wave processes in the magnetosphere. This exponentially growing auroral beads signature is indicative of plasma instability [e.g., Rae et al, 2010]. The statistically dominant magnetospheric spatial scales of k = 2.5-3.75 × 10 À6 m À1 and growth rates of~0.05 s À1 obtained by Kalmoni et al [2015] are consistent with both the shear-flow ballooning instability [Voronkov et al, 1997] and the cross-field current instability [Lui, 2004] at the inner edge of the plasma sheet, with the majority of events mapping to 9-12 R E in the magnetotail.…”

“…Structures known as auroral rays or beads have been observed to form azimuthally along the substorm onset arc in the minutes before auroral substorm onset [Davis, 1962;Henderson, 1994]. In recent years there have been many studies of auroral beads from data acquired by high resolution (temporal and spatial) ground-based all-sky imagers (ASIs) [e.g., Friedrich et al, 2001;Kepko et al, 2009;Sakaguchi et al, 2009;Rae et al, 2009Rae et al, , 2010Motoba et al, 2012;Murphy et al, 2014;Kalmoni et al, 2015] in which beads are reported to have spatial scales, or wavelengths, in the range 30-150 km in the ionosphere. Sakaguchi et al [2009] reported two case studies of substorm onset arcs which exhibited beading with spatial scales of 30-60 km in the ionosphere.…”

Investigations of the auroral luminosity distribution and the dynamics of discrete auroral forms in a historical retrospective

“…The poleward discrete arc is spatially distinct from the dynamic auroral activity, which originated in the epicentre of magnetic disturbances. A subsequent analysis of the frequencies of the magnetic oscillations, their spatial scales, and the growth rates of auroral forms resulted in the finding, that the onset phase of the substorm is most likely triggered close to the inner edge of the ion plasma sheet (Rae et al, 2010).…”