Dynamics of double-theta aurora: Polar UVI study of January 10-11, 1997

During the much studied magnetic cloud encounter with Earth on January 10-11, 1997, pairs of polar cap arcs twice became widely separated from the auroral oval, that is, an extremely rare double theta aurora configuration arose. These events exhibit many dynamic features hitherto rarely if ever reported in:the literature of theta aurora. The two independent bars proved capable of merging into a single bar, crossing the polar cap, then splitting again. Moreover, in both events, after completely crossing the polar cap, the transpolar arcs reversed direction, and crossed again. The dynamics of these double theta-aurora events appear to constrain any models of their origin. Interestingly, in the January 10 event, a double theta-aurora which had twice crossed the polar cap ended up in a configuration previously termed a double oval. Particle data from the DMSP F13 satellite demonstrates that an isolated plasma sheet fragment formed also in the southern hemisphere, at least for the January 11 event for which correlative data is available.