A diagnosis of the plasma waves responsible for the explosive energy release of substorm onset

During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy. This rapid reconfiguration of Earth's nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. Progress in understanding of how substorms are initiated is hindered by a lack of quantitative analysis of the single consistent feature of onset; the rapid brightening and structuring of the most equatorward arc in the ionosphere. Here, we exploit state-of-the-art auroral measurements to construct an observational dispersion relation of waves during substorm onset. Further, we use kinetic theory of high-beta plasma to demonstrate that the shear Alfven wave dispersion relation bears remarkable similarity to the auroral dispersion relation. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail.

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

Substorm onset is marked in the ionosphere by the sudden brightening of an existing auroral arc or the creation of a new auroral arc. Also present is the formation of auroral beads, proposed to play a key role in the detonation of the substorm, as well as the development of the large-scale substorm current wedge (SCW), invoked to carry the current diversion. Both these phenomena, auroral beads and the SCW, have been intimately related to ultra-low frequency (ULF) waves of specific frequencies as observed by ground-based magnetometers. We present a case study of the absolute and relative timing of Pi1 and Pi2 ULF wave bands with regard to a small substorm expansion phase onset. We find that there is both a location and frequency dependence for the onset of ULF waves. A clear epicentre is observed in specific wave frequencies concurrent with the brightening of the substorm onset arc and the presence of "auroral beads". At higher and lower wave frequencies, different epicentre patterns are revealed, which we conclude demonstrate different characteristics of the onset process; at higher frequencies, this epicentre may demonstrate phase mixing, and at intermediate and lower frequencies these epicentres are characteristic of auroral beads and cold plasma approximation of the "Tamao travel time" from near-earth neutral line reconnection and formation of the SCW.

Auroral streamer and its role in driving wave-like pre-onset aurora

The time scales of reconnection outflow, substorm expansion, and development of instabilities in the terrestrial magnetosphere are comparable, i.e., from several to tens of minutes, and their existence is related. In this paper, we investigate the physical relations among those phenomena with measurements during a substorm event on January 29, 2008. We present conjugate measurements from ground-based high-temporal resolution all-sky imagers and in situ THEMIS measurements. An auroral streamer (north-south aligned thin auroral layer) was formed and propagated equatorward, which usually implies an earthward propagating plasma flow in the magnetotail. At the most equatorward part of the auroral streamer, a wave-like auroral band was formed aligning in the east-west direction. The wave-like auroral structure is usually explained as a consequence of instability development. Using AM03 model, we trace the auroral structure to magnetotail and estimate a wavelength of ~0.5 R _E. The scale is comparable to the drift mode wavelength determined by the in situ measurements from THEMIS-A, whose footpoint is on the wave-like auroral arc. We also present similar wave-like aurora observations from Cassini ultraviolet imaging spectrograph at Saturn and from Hubble space telescope at Jupiter, suggesting that the wave-like aurora structure is likely a result of fundamental plasma dynamics in the solar system planetary magnetospheres.

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

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.