Quantitative modelling of fine scale auroral emissions and electric fields at sub-second resolution

Abstract

The structuring and dynamics of auroral emissions at fine spatial and temporal scales, down to tens of metres and fractions of a second, is an oft-observed but poorly understood phenomenon. One particular theory for such fine scale structure involves magnetic reconnection in the auroral acceleration region initiating Alfven waves which structure and energise the precipitating particles. Such theories are testable using measurements of the ionospheric electric fields associated with the observed fine scale structure. This thesis presents attempts to measure ionospheric electric fields, using a novel model that tracks emission from a long-lived ion produced at times of auroral particle precipitation, so that the theories that seek to explain the fine scale structuring of the aurora can be evaluated. However, such modelling requires knowledge of the energy spectrum of the precipitation, and the resulting emissions, at the spatial and temporal resolutions of the observed fine scale aurora.

This thesis presents such new work concerning the modelling of auroral electron energy spectra and fine scale auroral emissions at sub-kilometre and sub-second resolutions using ground based observations, and the subsequent application of a novel method to estimate ionospheric electric fields using a long-lived ion produced during times of auroral precipitation. Two novel methods are presented. The first method utilises a fusion of multi-monochromatic auroral observations at optical and near-infrared (NIR) wavelengths together with simultaneous radar observations. The second uses only multimonochromatic observations, to be used when complementary radar observations are unavailable. Each technique is applied to an observed auroral event to determine the energy spectra of the precipitating electrons and resulting 3-D distribution of auroral emissions. Modelled images of the emissions verify the accuracy of the recovered spectra.The recovered spectra are used as input to a novel model which solves the continuity equation of a long-lived ion produced at times of auroral precipitation. This model uses a parameterised ion velocity, and optimises the velocity parameters by comparing observed and modelled images of emission from this ion. A simple velocity parameterisation, a uniform flow perpendicular to the magnetic field, yields plasma velocities of 0.4-2.4 km s-1, with the plasma velocities being enhanced at times when the auroral brightness is high. Comparison of the recovered velocities to radar observations of ionospheric plasma velocities shows agreement in direction, but the recovered velocities are larger, more so when the aurora is brighter. Electric fields, inferred from the modelled plasma velocities, of up to 120 mV m-1 are found at the time when the auroral brightness was intensified. A more complex flow parameterisation is presented and tested, but does not succeed for the event analysed.

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