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 atte ntion is given to the growth phase and the time immediately following expan sion onset. It is found that the growth phase is characterized by the enhan cement of the velocity shear near midnight and its extension to low latitud es 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 prio r to expansion onset and to decrease at expansion onset. Possible explanati ons for the establishment of the midnight velocity shear during growth phas e and its subsequent disappearance at expansion onset are presented. Also, arguments are presented regarding the decrease of ionospheric flow velocity at expansion onset.