Multi-spectral analysis of fine scale aurora

Sammanfattning: The Aurora Borealis is the visible manifestation of the complex plasma interaction between the solar wind and the Earth's magnetosphere and ionosphere. Ground based and in situ measurements demonstrate a prevalence of dynamic fine structure within auroral displays, with spatial scales down to tens of metres and time variations occurring on a fraction of a second.The fine-scale morphology is related to structuring of auroral currents and electric fields and detailed spatial, spectral and temporal observations of the aurora are crucial in understanding the electrodynamic processes taking place in the ionosphere and in its coupling to the magnetosphere.In this thesis, the low-light optical instrument ASK (Auroral Structure and Kinetics) is used to image small-scale structures in the aurora at very high spatial and temporal resolution. ASK is a multi-spectral instrument, imaging the aurora in three selected emissions simultaneously. This provides information on the energy of the precipitating electrons. The SIF (Spectrographic Imaging Facility) instrument has been used in conjunction with ASK, to give a more complete picture of the spectral characteristics of the aurora, and to determine the degree of contaminating emissions present in the same spectral interval as the emission lines observed by ASK.Data from ASK and SIF are used to study the relation between the morphology and dynamics of small-scale structures in the aurora and the energy of the precipitating electrons. By comparing electron density profiles provided by EISCAT (European Incoherent SCATter) radar measurements with modeling results, information on the characteristic energy and the energy flux of the precipitating electrons can be obtained. One of the ASK channels is imaging a metastable O+ emission, which has a lifetime of about 5 s. By tracing the afterglow in this channel optically a direct measure of the E x B drift is obtained from which the local ionospheric electric field can be calculated. ASK data has also been used to analyse the properties of a distorted auroral arc, in which auroral structuring was found to take place simultaneously at different spatial scales. The smallest features, 'ruffs', are undulations found to develop on the edge of an auroral curl, fold or shear. Detailed optical studies of black aurora, including both the type which is associated with plasma shear motions and no or weak shear motions were conducted from ASK data, to investigate the spectral properties and fine scale morphology of the black structures and to shed light on the processes behind this phenomenon.