Global Auroral Observations from Polar UVI during THEMIS Events

In this poster, we compare auroral events seen in the ultraviolet emission imaged by Polar UVI with plasma and fields data detected by instruments onboard the THEMIS satellite. Global images offer a clear indication of onset times for isolated substorm events. An example of one such event took place on March 23, 2007. UVI recorded a substorm onset at 1110 UT near local midnight. The onset recorded by magnetometers on the ground was 1118 UT, 8 minutes later, only after the arrival of a westward surge at the ground station. This substorm, as in many substorms, was accompanied by pseudo-breakup events before the onset of the auroral expansion. Additionally, in the local morning sector, pulsations were seen during substorm recovery. The THEMIS probes at this time were located between 9 and 13 RE in the pre-midnight sector. THEMIS observed particle injections and dipolarization signatures propagating from one probe to another. Large ion flows and magnetic field fluctuations were seen during the substorm intensitifcation at 1118 UT as the disturbance propagated westward to the probe locations, thus illustrating the importance of global auroral images in placing the magnetospheric observations in context.