Dynamical coupling of neutrals and ions in the high-latitude F region: Simultaneous FPI and HF radar observations at Syowa Station, Antarctica

Simultaneous measurements of neutral winds and plasma drifts have been carried out to clarify the local neutral-ion coupling processes in the auroral F region. High-resolution maps of thermospheric winds and plasma drifts were made using data obtained by the Fabry-Perot Doppler Imaging System (FPDIS) and by the Syowa-South HF radar, respectively, both installed at Syowa Station (66.4° in magnetic latitude), Antarctica during the solar minimum period in 1996. We made case studies for two nights: one on 13 May 1996 during a rather quiet period including the growth phase of a substorm, and the other on 24-25 April 1996 during a moderately active period. From 2000 to 2200 UT on 13 May, the velocities of neutral winds and plasma drifts showed similar time variations: a negative excursion followed by a positive excursion. However, the variations of plasma drifts preceded those of neutral winds with a time lag of ∼30 min, and plasma drift velocities were about 7-10 times greater than neutral wind velocities. Therefore, it is suggested that the variations of neutral winds were driven by the ion drag force. To interpret results of the 13 May case, we simulated the relationship between neutral winds and ion drifts by including the effect of auroral heating, such as Joule and particle heating. The results suggest that strong neutral winds were driven by the ion drag and the pressure gradient force induced by auroral heating. In contrast, in the case of 24-25 April, no similarity was found between neutral winds and plasma drifts. Although it is difficult to specify sources for the discrepancy, some possible origins are proposed. For example, if the pressure gradient force is much greater than the ion drag force as in the case of disturbed period, neutral wind variations become independent of plasma drift variations. The discrepancy is also caused by the existence of vertical winds associated with active aurora.