Dynamics of Particle Precipitation in the Auroral Ionosphere.

Wild, Paul (2011) Dynamics of Particle Precipitation in the Auroral Ionosphere. PhD thesis, Lancaster University.

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Abstract

Energetic electrons deposit significant amounts of energy into the ionosphere during precipitation events. Riometers provide a means of monitoring this precipitation by measuring the associated cosmic noise absorption. Individually however they cannot provide any details about the energies of the precipitating electrons. The first study in this thesis looks at estimating the characteristic energy of the precipitating electrons by the means of two imaging riometers with overlapping fields of view. Two methods of calculating the height of maximum cosmic noise absorption are developed, a method of height triangulation and tomographic reconstruction of the absorption events. These methods show a marked improvement when compared with a previously published method. A case study comparing the calculated height of maximum cosmic noise absorption with a deduced absorption profile from an EISCAT electron density profile shows good correlation. Using the height of maximum cosmic noise absorption estimates are made of the characteristic energy for three case studies; a morning absorption event, a substorm spike and an afternoon absorption event. The estimated energies for these events were 5keV, 17-20keV and 100+keV respectively. The second study concerns the statistics and mechanisms of daytime absorption events. Statistics of absorption during the course of a day show a deep minimum during the afternoon sector. However there are a number of discrete cases that do occur during this afternoon minimum. A statistical analysis of the time period, 12-16UT at Kilpisjarvi is undertook. They are found to be short lived, highly localised events. This is in agreement with previous studies. They tend to occur during periods of weak geomagnetic activity. A portion of these are found to be early onset substorms, and account for 7.4% of the events. To understand the mechanisms behind the rest of the events one year of data was analyised in greater detail. A portion of these events seem to agree with previous studies, that these events are reltavistic preciptiation events caused by ElectroMagnetic Ion Cyclotron (EMIC) wave scattering. However a greater number of the events seem to be due to the precipitation of lower energy electrons during dispersed electron injections of the radiation belts; a more localized and later occurring version of morning absorption caused by the eastward drift and scattering of lower energy (10-100keV) substorm injected electrons.