Unexpected Southern Hemisphere ionospheric response to geomagnetic storm of 15 August 2015

When studying the ionospheric response to a geomagnetic storm of 15 August 2015, an unexpected phenomenon was observed at higher middle latitudes of the Southern Hemisphere. This phenomenon was a localized TEC enhancement (LTE) in the form of two separated plumes, which peaked southward of South Africa. The plumes were first observed at 5 UT near the southwestern coast of Australia. The southern plume was associated with local time slightly after noontime (1-2 hours after local noon). The plumes moved with the Sun. They peaked near 13 UT southward of South Africa. The southern plume kept constant geomagnetic latitude (63-64oS); it persisted for about 10 hours, whereas the northern plume persisted by about two hours more. Both plumes disappeared over the South Atlantic Ocean. No similar LTE event was observed during the prolonged solar activity minimum period of 2006-2009. In 2012-2016 we detected altogether 26 LTEs and all of them were associated with the southward excursion of Bz. The negative Bz excursion seems to be necessary but not sufficient condition for the LTE occurrence.