A two-dimensional simulation of thermospheric vertical winds in the vicinity of an auroral arc

The observations made by Fabry-Perot interferometers (FPIs), radars, and satellites have indicated that large vertical motion in the polar region is occasionally generated in the thermosphere associated with auroral activities. However, the behavior of the vertical wind is often very complicated, and the cause of the vertical wind has not been explained by auroral heating or by ion-neutral drag alone. It has been pointed out that a background horizontal flow is likely to significantly alter the dynamics of the neutral atmosphere near an auroral arc. Recent observations have also suggested that strong downward motion is generated in the vicinity of an auroral arc. To study the thermospheric dynamics near a local heating region embedded in a large-scale horizontal flow, a two-dimensional numerical simulation of the thermospheric dynamics has been performed. It is found that interaction of local heating and strong horizontal flow could play an important role in generating vertical motion near an auroral arc. The simulation results indicate that for a horizontal wind speed larger than about 300 m/s, a steady wave-like structure of the neutral wind is formed within and downstream of the heated region. For a horizontal wind speed less than about 300 m/s, on the other hand, no significant vertical motion is generated outside the heated region. This process might account for at least some of the observed features of vertical motion within and outside an auroral arc.