| Summary: | PhD (Space Physics), North-West University, Potchefstroom Campus Black auroras show a significant reduction in optical brightness, i.e. reduced flux of particle precipitation, compared to the surrounding diffuse aurora. This phenomenon also exhibits lower mean energy than the surrounding brighter aurora it is embedded in. This shift in particle precipitation energy to a lower mean value is confirmed by using synchronised dual wavelength optical and EISCAT incoherent scatter radar observations that ran in parallel. A newly observed type of aurora not yet reported in the literature, the anti-black aurora, is introduced. Anti-black auroras sometimes appear as brighter patches paired with the black aurora, always moving together. The underlying mechanisms that cause black auroras are not yet fully understood, although several theories have been proposed: a coupled ionospheric magnetospheric generation mechanism, and a magnetospheric generation mechanism. No theories exist as to the formation of the anti-black aurora. A possible origin of the mechanism is investigated by mapping the observed motion of the black aurora as well as the anti-black aurora into the magnetosphere, to determine the probable energy of the source electron population, assuming gradient-curvature drift. This inferred particle energy in the equatorial plane in the magnetosphere is found to be higher than the source particle energy in the ionosphere, which leads to the hypothesis of an anomalous electric field in the auroral acceleration region in the upper ionosphere causing the loss of precipitating particle energy along the magnetic field line. Doctoral
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