Dayside and nightside reconnection rates inferred from IMAGE FUV and Super Dual Auroral Radar Network data

The spectrographic imager at 121.8 nm ( SI12) of the far ultraviolet ( FUV) experiment onboard the IMAGE spacecraft produces global images of the Doppler- shifted Lyman alpha emission of the proton aurora. This emission is solely due to proton precipitation and is not contaminated by dayglow, allowing us to monitor the auroral oval on the dayside as well as on the nightside. Remote sensing of the polar aurora can be advantageously supplemented by use of ground- based data from the Super Dual Auroral Radar Network ( SuperDARN) that monitors the ionospheric convective flow pattern in the polar region. In the present study, the SI12 images are used to determine the location of the open/ closed field line boundary and to monitor its movement. The SuperDARN data are then used to compute the ionospheric electric field at the location of the open/ closed boundary. The total electric field is then computed along the boundary accounting for its movement via Faraday's law so that the dayside and nightside reconnection voltages can be derived. This procedure is applied to several substorm intervals observed simultaneously with IMAGE FUV and SuperDARN. The dayside reconnection voltage feeds the magnetosphere with open flux, which is later closed by nightside reconnection. The calculated dayside reconnection rate is consistent with the solar wind properties measured by the Geotail, Wind, and ACE satellites. We identify the presence of nightside reconnection due to pseudobreakups taking place during the growth phase. In several cases, we establish that the nightside reconnection rate is maximum at the time of the substorm expansion phase onset or shortly after, reaching similar to 120 kV, and then slowly returns to undisturbed values of similar to 30 kV. The flux closure rate can also start intensifying prior to expansion phase onset, producing pseudobreakups.