Statistical characterization of the growth and spatial scales of the substorm onset arc

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 RE. 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.

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

“…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.…”

“…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.…”

“…Using ground-based and multipoint in situ data, Murphy et al [2014] presented a detailed analysis of two consecutive substorms and demonstrated that the development of auroral beads and disturbances in the inner magnetosphere preceded any evidence of tail reconnection further away from the Earth. Kalmoni et al [2015] presented a statistical study of auroral substorm onset arcs with clear, visually identified, auroral beads. 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.…”

“…We apply Fourier analysis to each time point of the differentiated keogram containing signatures of the beads only (Figure 1d) using the method outlined in Kalmoni et al [2015], resulting in a whitened power spectral density (PSD). We identify the temporal evolution of characteristic spatial scales from the dominant peaks of the PSD.…”

“…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.…”