Auroral morphology of the midday oval

Auroral displays in the noon sector were examined by using hundreds of Defense Meteorological Satellite Program auroral images, taken over the southern polar region in austral winters, in order to determine the morphology. The auroral displays in the midday part of the auroral oval can be grouped into five characteristic types, depending on the geomagnetic activity and the Bz component of the interplanetary magnetic field. An important characteristic is the clear disconnection in appearance between the noonside and the nightside discrete auroras. Also, there is the lack of correlation between the concurrent nightside substorm activity and midday discrete auroral activity. Thus, the occurrence of bright, discrete auroras in the midday oval may be caused by the local injection of the magnetosheath plasma into the dayside boundary layer. The observed discontinuity of discrete auroras between the dayside and the nightside ovals is consistent with the existence of two separated major injection regions along the auroral oval: the dayside cusp and the nightside plasma sheet.

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