The Unusual Response of the Magnetosphere to Solar Wind Conditions during the Galaxy 15 Substorm

On April 5, 2010, just after 09 UT, the NOAA Geosynchronous Operational Environmental Satellites (GOES) observed a large reconfiguration of the magnetospheric magnetic field in the midnight to dawn local time sector. Specifically, near midnight, the GOES-11 Hp (north-south) magnetic field increased by approximately 100 nT in 15 minutes. This is one of the largest dipolarizations of Earth's field ever observed by GOES, and along with energetic particle injections, these changes in the space environment occurred less than an hour before a serious anomaly on the Galaxy 15 satellite. At the time of the dipolarization, the auroral electrojet index (AL) reached extreme levels of about -1500 nT and within a half hour reached values of less than -2000 nT. Solar wind conditions were moderate, with Bz southward at about -10 nT for about 30 minutes prior to the event; however, the solar wind pressure was high at about 10 to 12 nPa. Using linear prediction filters derived from Es = -VBs and the Universal Coupling Function (that includes an IMF By effect) to predict AL, we find that the predicted AL is about half the observed value. This outcome suggests that the extreme AL resulted from a tail unloading event (substorm) that was not predictable from solar wind conditions using a linear prediction filter. In this presentation, we examine a number of similar events to better understand solar wind conditions that result in large electrojet disturbances such as those observed at the time of the Galaxy-15 anomaly.