High-resolution spectrographic studies of strange aurora over Svalbard.

Abstract

The Arctic archipelago of Svalbard experiences unique auroral conditions, due to its ability to observe aurora on the dayside of Earth during the polar night. Here, many auroral forms deviate from the traditional bright green and red emissions of electron precipitation. These strange aurora include dayside proton aurora, pseudo-continuum emissions, and green fragments.

High-resolution spectrography is an extremely powerful analytical tool in auroral studies. Spectra can be used to determine the intensity, emitting species, temperature, energy partition, and many more physical characteristics by examination of the rotational spectrum, or by well-resolving profiles such as the proton blueshift velocity profile. The High-Throughput Imaging Echelle Spectrograph (HiTIES), located on Svalbard, is capable of reaching sub-Ångström resolution, and is the only such instrument in Svalbard. This thesis presents the results of using high-resolution spectrograph data to provide evidence for new and unanswered topics in aurora science.

While low latitude and nightside proton aurora is strongly linked to electromagnetic ion-cyclotron (EMIC) wave activity, we present the first observational study directly comparing Pc1 pulsations and proton aurora on the dayside. Using statistical methods to analyse HiTIES data and magnetometer measurements of Pc1 pulsations, either no - or only a very weak - link is found between EMIC waves and the acceleration and precipitation of auroral protons. This is contrary to initial expectations, and indicates that the mechanisms driving dayside proton aurora are more separate from the nightside regime than first assumed.

The study of pseudo-continuum aurora is a recent development in the field. We present the discovery of pseudo-continuum in the dayside region, poleward of the auroral oval, naming our new strange aurora GHOST. In this thesis I present my contributions to the discovery and characterisation of GHOST using high-resolution spectrography. The use of this method has allowed me to present the first spectra of the pseudo-continuum at high-resolution, with the ability to discern the rotational spectrum. Using HiTIES we also identify patchy proton events concurrent with the GHOST sightings, which we identify as reconnection signatures, locating most of our events in the cusp.

By spectral fitting of the pseudo-continuum, I made the discovery that the spectrum was consistent with N2 at extreme temperatures. We present the first discussion of N2 as a possible emitting mechanism for GHOST and for atmospheric pseudo-continuum in general. The magnetospheric origin of the drivers for this excited hot N2 are also investigated for the first time, and we postulate that strong By dominance in the Interplanetary Magnetic Field (IMF) combined with negative Bz, induces both active reconnection and strong shear flow in the polar cap. GHOST is also brightest and most often found in the cusp, where it is additionally heated by soft precipitation, and excited by sunlight. This combination could produce an ionosphere primed with the high-energy N2, which is then enhanced by precipitation, resulting in the pseudo-continuum characteristic of GHOST.

Weird aurora such as GHOST, STEVE, and fragments, are also some of the core topics of interest of the ISSI working group Auroral Research Coordination: Towards Internationalised Citizen Science (ARCTICS), of which I am a core member. As an ARCTICS member I have been involved in a variety of citizen science projects, using citizen science observations to understand some of these particularly strange and rare features. Within the ARCTICS group I have also been a core contributor to the creation of the Aurora Field Guide and Handbook for Citizen Science, an open-access pair of documents laying out the basic principles of collaboration in aurora science for both scientists and the public.

Finally, we present preliminary findings on quantifying the effects of proton precipitation on the Arctic mesopause region using HiTIES data. This work outlines possible mechanisms by which the OH* airglow spectrum, visible in HiTIES, could be disturbed by energetic proton precipitation, and how that impact might appear in optical measurements, and presents initial results.

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Identifiers

ORCID for Daniel Whiter: orcid.org/0000-0001-7130-232X

ORCID for Robert Fear: orcid.org/0000-0003-0589-7147

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