Effect of the theta aurora on the polar thermosphere

The theta aurora is characterized by a transpolar auroral arc, connecting the dayside to the nightside auroral oval. It is observed during geomagnetically quiet times when the polar cap becomes contracted. A high-resolution time-dependent model of the global thermosphere is used to estimate the effects of the transpolar arc on the polar thermosphere. A background ionospheric model is assumed, and a simple but physically reasonable representation of the ionospheric conditions within the transpolar arc is “superimposed” on the background ionosphere. The thermospheric model is then solved both with and without the imposed transpolar arc to determine the arc's effects on the neutral gas. In particular, percentage changes in thermospheric mass density and temperature are computed. These percentage differences are seen to vary systematically with the level of the ionospheric density enhancement within the arc. A strong dependence on solar activity level is also noted. While results are presented mainly for an altitude of 300 km, the typical variation of these results with altitude is also illustrated.