Observations of ionospheric plasma flows within theta auroras

[1] This paper reports results from a detailed analysis of theta auroras that occurred during the passage of a magnetic cloud on 8 November 2000. The interplanetary magnetic field (IMF) was generally northward for more than 12 hours, while the y-component of the IMF changed signs several times. Auroral images of the Northern Hemisphere acquired from the Ultraviolet Imager (UVI) on board the Polar satellite showed clear development of theta auroras during the prolonged northward IMF period. It is found that the theta aurora can be enhanced by compression of the magnetosphere associated solar wind pressure pulses. We also examine large-scale ionospheric plasma convection flow data from the SuperDARN radar network to study the ionospheric plasma flow pattern associated with the theta aurora. The radar data, when available, showed that two different plasma flows can coexist within the day-night part of the theta aurora. The nightside extension of the theta aurora is always on a region of antisunward convecting flows, whereas there is no consistent flow pattern found in the dayside portion of the theta aurora studied. In general, plasma flow outside the theta aurora cannot be determined because of lack of scattered radar signals, but from a few instances it suggested that the nightside theta aurora is flanked by antisunward flows. In contrast to the prevailing view that theta auroras reside in regions of closed magnetic flux that convect sunward, this study suggests one more aspect of the theta aurora and provides constraints for models and theories.