Substorm convection patterns observed by the Super Dual Auroral Radar Network

A study of convection patterns during substorms observed by the Super Dual Auroral Radar Network (SuperDARN) radar network is presented. Specific attention is given to the growth phase and the time immediately following expansion onset. It is found that the growth phase is characterized by the enhancement of the velocity shear near midnight and its extension to low latitudes and to local times across the midnight meridian. The velocity shear was observed to diminish at expansion onset. In addition, the ionospheric flow velocity magnitude was observed to be enhanced for some period of time prior to expansion onset and to decrease at expansion onset. Possible explanations for the establishment of the midnight velocity shear during growth phase and its subsequent disappearance at expansion onset are presented. Also, arguments are presented regarding the decrease of ionospheric flow velocity at expansion onset.