Physical mechanism to generate vertical motions in the polar lower thermosphere

Oyama, S.1, Maeda, S.2, Shinagawa, H.3 and Watkins, B. J.4

1 STEL Nagoya University, Nagoya, Japan
2 Kyoto Women’s University, Kyoto, Japan
3 NICT, Tokyo, Japan
4 GI UAF, Fairbanks, USA

An important aspect of the coupled thermosphere-ionosphere system at high latitudes is to know various temporal and spatial relationships in the dynamic interaction between the thermosphere and the ionosphere. While much is already known about the average characteristics of these systems, this subject has not yet been adequately investigated, in particular mesoscale phenomena. One of the most curious examples is the vertical wind in the lower thermosphere at high latitudes. Ground-based observations with Fabry-Perot Interferometers (FPI) have reported large amplitudes of the vertical wind in association with auroral activity. More recent rocket measurements at the Poker Flat Research Range in Alaska also showed considerably larger vertical thermospheric winds. Now there is no doubt in presence of the vertical wind larger than a few tens m/s. The current controversy in the field is then the physical mechanism to generate such large vertical motions. We conducted many experiments with the incoherent-scatter radars (ISR) in order to find the answer. Our recent experiments suggested that the thermospheric wind is accelerated obliquely upward/downward in association with Joule and/or auroral particle heating, and that the wind blows on the isothermal layer, which can be tilted by localized heating or climatological effects from the lower atmosphere. The paper will present the physical explanation using observational results.