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

Abstract

The onset of an auroral substorm is generally thought to occur on a quiet, homogeneous auroral arc. We present a statistical study of independently selected substorm onset arcs and find that over 90% of the arcs studied have resolvable characteristic spatial scales in the form of auroral beads. We find that the vast majority (~88%) of auroral beads have small amplitudes relative to the background, making them invisible without quantitative analysis. This confirms that auroral beads are highly likely to be ubiquitous to all onset arcs, rather than a special case phenomena as previously thought. Moreover, as these auroral beads grow exponentially through onset, we conclude that a magnetospheric plasma instability is fundamental to substorm onset itself.

Using ultra-low frequency waves and their characteristics to diagnose key physics of substorm onset

“…This supports the hypothesis that Pi1-2 waves are the magnetic counterparts of auroral beads. Detailed analysis of these auroral beads (Rae et al 2010;Kalmoni et al 2015Kalmoni et al , 2017 and auroral and plasma sheet dynamics at substorm onset Murphy et al 2015) has provided strong evidence that auroral beads are the ionospheric manifestation of a plasma instability driven by substorm onset, whether it be in the near-Earth magnetotail (e.g. Lui 1991) or closer to the ionosphere (e.g.…”

“…Since both Pi1-2 ULF waves (e.g. Murphy et al 2011b;Rae et al 2011) and auroral beads (Kalmoni et al 2017) are ubiquitous to substorm onset, the Pi1-2 waves appear to be exclusively related to the substorm onset instability at the inner edge of the plasma sheet (e.g. Samson et al 1992).…”

“…Structuring of the brightening auroral arc subsequently develops into latitudinally confined periodic azimuthal undulations, termed "auroral beads" after Henderson (1994), and subsequently re-discovered in the THEMIS era (e.g. Rae et al 2009a, b, 2010Murphy et al 2015Kalmoni et al 2015Kalmoni et al , 2017. Following the development of these auroral beads, auroral breakup occurs.…”

“…These auroral beads, which mark the onset of the substorm in the ionosphere (e.g. Rae et al 2010;Murphy et al 2015;Kalmoni et al 2015Kalmoni et al , 2017, have been shown to grow exponentially in brightness at the same time as the onset of Pi1-2 ULF waves (~ 0.05 s −1…”

“…Further studies of the onset of the ULF waves shows that the exponential growth in ULF wave amplitudes occurs immediately prior to the onset of an auroral substorm (Rae et al 2011). Finally, it has been shown that both the ULF wave amplitudes and auroral brightness grow exponentially at the location of ionospheric substorm onset (Rae et al 2012;Kalmoni et al 2015Kalmoni et al , 2017. Although the relationship between the enhancement of ULF waves and auroral brightness is no longer in doubt (see review by Rae and Watt 2016), the relative timing and localisation of Pi1, Pi1-2 and Pi2 waves across the auroral zone at substorm onset have not yet been investigated.…”