Abstract
We present the first multievent study of the spatial and temporal structuring of the aurora to provide statistical evidence of the near‐Earth plasma instability which causes the substorm onset arc. Using data from ground‐based auroral imagers, we study repeatable signatures of along‐arc auroral beads, which are thought to represent the ionospheric projection of magnetospheric instability in the near‐Earth plasma sheet. We show that the growth and spatial scales of these wave‐like fluctuations are similar across multiple events, indicating that each sudden auroral brightening has a common explanation. We find statistically that growth rates for auroral beads peak at low wave number with the most unstable spatial scales mapping to an azimuthal wavelength λ≈ 1700–2500 km in the equatorial magnetosphere at around 9–12 R E. We compare growth rates and spatial scales with a range of theoretical predictions of magnetotail instabilities, including the Cross‐Field Current Instability and the Shear Flow Ballooning Instability. We conclude that, although the Cross‐Field Current instability can generate similar magnitude of growth rates, the range of unstable wave numbers indicates that the Shear Flow Ballooning Instability is the most likely explanation for our observations.
Highlights
The causal sequence of kevents leading to energy release and auroral breakup during substorms remains unknown, primarily due to a lack of spatial and temporal resolution when investigating the physical processes occurring within the first 2 min of substorm onset in such a vast 3-D volume of space
This algorithm requires (a) that exponential growth must be continually present over a duration longer than 30 s, since this is the typical period of a bead fluctuation [Rae et al, 2010], (b) that it occurs before the aurora expand poleward out of the analysis field of view, and (c) that it must start within the window identified to contain substorm onset
The optical analysis technique presented in this paper provides a quantitative method to remote sense the physics of substorm onset from spatial analysis of substorm-related aurora
Summary
The causal sequence of kevents leading to energy release and auroral breakup during substorms remains unknown, primarily due to a lack of spatial and temporal resolution when investigating the physical processes occurring within the first 2 min of substorm onset in such a vast 3-D volume of space. Motoba et al [2012] observed magnetically conjugate auroral beads in ASI data from both Northern and Southern Hemispheres and suggested that the beads have a common driver originating in the magnetosphere In addition to these wave-like signatures in the aurora, simultaneous magnetic pulsations of ULF waves have been observed in the minutes surrounding substorm onset [Mann et al, 2008; Milling et al, 2008; Murphy et al, 2009a, 2009b; Rae et al, 2009a, 2009b; Walsh et al, 2010; Rae et al, 2011]. Rae et al [2010] provide optical analysis of substorm auroral arc azimuthal wave number spectra during a single event, which demonstrates that the beading of the substorm onset arc is characteristic of an instability in the near-Earth magnetosphere. This allows the statistical relationship between wave number and growth rate of auroral beads to be found, which we compare with theoretical predictions of instability characteristics
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