Neutral thermospheric dynamics observed with two scanning Doppler imagers: 3. Horizontal wind gradients

This is the third and final article in a series of papers reporting on observations of the 630.0 nm thermospheric airglow emission by two spatially separated scanning Doppler imagers (SDI's) in Alaska. In this article, bistatic winds derived from the combined measurements of both instruments in a region of field-of-view overlap were used to derive local-scale maps of horizontal neutral wind gradients. Averaged over the bistatic 'field-of-view', these gradient estimates were compared with the monostatic gradient estimates routinely produced by the two SDI's. The key findings to emerge from this study include: 1) the bistatic gradient estimate agreed very well with monostatic estimates for the majority of the time which, given the very different methods involved in each technique, gives us great confidence in our ability to measure F-region neutral wind gradients; 2) the strongest gradient was that which describes the magnetic meridional shear of the zonal wind, which is driven by momentum deposition from convecting ions; 3) vortical flow was more often observed than divergent flow, and both types of flow showed systematic variations with magnetic local time; 4) viscous heating due to non-negligible gradients was on the order of 10(-11) Wm(-3) which, while small compared to typical F-region Joule heating rates, may be comparable to particle heating, and in a time-integrated sense may be an appreciable source of heating.