The role of magnetospheric plasma instabilities in auroral and substorm dynamics

The auroral substorm is the manifestation of explosive energy release from the rapid and global reconfiguration of the magnetotail. The auroral substorm is marked by a sudden brightening and poleward expansion of the most equatorward auroral arc in the midnight sector of the ionosphere. The temporal sequence of magnetospheric processes which lead to the dynamic auroral substorm display remain disputed to this day. This thesis contains original research on the development and exploitation of novel data analysis techniques in order to analyse ground-based all sky imager data of the aurora, enabling the study of substorm processes in remarkable detail. Fourier analysis techniques are used to find the spatial scales of wave-like signatures (otherwise known as auroral beads/rays), which form along substorm onset arcs. Growth rates of ∼0.05 s⁻¹ are found from the exponential growth of the power spectral density of individual spatial scales. By analysing the dataset in this way, comparisons are made between observations and theoretical predictions of plasma instabilities at the near-Earth edge of the plasma-sheet which have been proposed to play a critical part in the substorm onset process. Auroral arc tracking techniques are developed to automate and increase the size of the database of events analysed. The vast majority of independently identified substorm onsets are preceded by azimuthal structuring along the onset arc with median wavelengths of ∼80 km. These beads grow and develop into a magnetospheric instability around 2 minutes prior to auroral substorm onset. Showing that beads are a common feature along the substorm onset arc provides unprecedented quantitative evidence that a near-Earth instability is a fundamental component of the substorm onset process. Finally, analysis techniques are extended to state-of-the-art high resolution multi-spectral auroral data to investigate the processes driving auroral beads. Beads can be resolved in the green-, blue- and red-line aurora with spatial scales as small as 30 km, which later develop into larger structures of ∼80 km. These observations are consistent with Alfvén wave accelerated auroral particle precipitation and therefore imply that the substorm onset arc and auroral beads are driven unstable by waves

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

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

The Triggering of the 2014 March 29 Filament Eruption

The X1 flare and associated filament eruption occurring in NOAA Active Region 12017 on SOL2014-03-29 has been a source of intense study. In this work, we analyze the results of a series of nonlinear force-free field extrapolations of the flare's pre- and post-flare periods. In combination with observational data provided by the IRIS, Hinode, and Solar Dynamics Observatory missions, we have confirmed the existence of two flux ropes present within the active region prior to flaring. Of these two flux ropes, we find that intriguingly only one erupts during the X1 flare. We propose that the reason for this is due to tether cutting reconnection allowing one of the flux ropes to rise to a torus unstable region prior to flaring, thus allowing it to erupt during the subsequent flare

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

An explanation of auroral intensification during the substorm expansion phase

A multiple auroral onset substorm on 28 March 2010 provides an opportunity to understand the physical mechanism in generating auroral intensifications during a substorm expansion phase. Conjugate observations of magnetic fields and plasma from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, of field-aligned currents (FACs) from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellites, and from ground-based magnetometers and aurora are all available. The comprehensive measurements allow us to further our understanding of the complicated causalities among dipolarization, FAC generation, particle acceleration, and auroral intensification. During the substorm expansion phase, the plasma sheet expanded and was perturbed leading to the generation of a slow mode wave, which modulated electron flux in the outer plasma sheet. During this current sheet expansion, field-aligned currents formed, and geomagnetic perturbations were simultaneously detected by ground-based instruments. However, a magnetic dipolarization did not occur until about 3 min later in the outer plasma sheet observed by THEMIS-A spacecraft (THA). We believe that this dipolarization led to an efficient Fermi acceleration to electrons and consequently the cause of a significant auroral intensification during the expansion phase as observed by the All-Sky Imagers (ASIs). This Fermi acceleration mechanism operating efficiently in the outer plasma sheet during the expansion phase could be a common explanation of the poleward auroral development after substorm onset. These results also show a good agreement between the upward FAC derived from AMPERE measurements and the auroral brightening observed by the ASI