[1] Doppler-shift measurements of the thermospheric 630.0 nm emission recorded by two spatially separated imaging Fabry-Perot spectrometers in Alaska have been combined to infer F region horizontal wind vectors at approximately 75 locations across their overlapping fields-of-view. These "bistatic" horizontal wind estimates rely only on an assumption regarding the local vertical wind (and assume a common observing volume), and thus represent a more direct measurement of the wind than do the monostatic (single-station) vector wind fields routinely inferred by these instruments. Here we present comparisons between both the independently derived monostatic wind fields from each instrument and the bistatic wind estimates inferred in their common observing volumes. Data are presented from observations on three nights during 2010. Two principal findings have emerged from this study. First, the monostatic technique was found to be capable of estimating the actual large-scale wind field reliably under a large range of geophysical conditions, and is well suited to applications requiring only a large-scale, 'big picture' approximation of the wind flow. Secondly, the bistatic (or tristatic) technique is essential for applications requiring detailed knowledge of the small-scale behavior of the wind, as for example is required when searching for gravity waves.

[1] Results are presented from two nights of bistatic Doppler measurements of neutral thermospheric winds using Fabry-Perot spectrometers at Mawson and Davis stations in Antarctica. A scanning Doppler imager (SDI) at Mawson and a narrow-field Fabry-Perot spectrometer (FPS) at Davis have been used to estimate the vertical wind at three locations along the great circle joining the two stations, in addition to the vertical wind routinely observed above each station. These data were obtained from observations of the 630.0 nm airglow line of atomic oxygen, at a nominal altitude of 240 km. Low-resolution all-sky images produced by the Mawson SDI have been used to relate disturbances in the measured vertical wind field to auroral activity and divergence in the horizontal wind field. Correlated vertical wind responses were observed on a range of horizontal scales from ∼150 to 480 km. In general, the behavior of the vertical wind was in agreement with earlier studies, with strong upward winds observed poleward of the optical aurora and sustained, though weak, downward winds observed early in the night. The relation between vertical wind and horizontal divergence was seen to follow the general trend predicted by Burnside et al. (1981), whereby upward vertical winds were associated with positive divergence and vice versa; however, a scale height approximately 3-4 times greater than that modeled by NRLMSISE-00 was required to best fit the data using this relation.

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Abstract.A new all-sky imaging Fabry-Perot spectrometer has been installed at Mawson station (67 • 36 S, 62 • 52 E), Antarctica. This instrument is capable of recording independent spectra from many tens of locations across the sky simultaneously. Useful operation began in March 2007, with spectra recorded on a total of 186 nights. Initial analysis has focused on the large-scale daily and average behavior of winds and temperatures derived from observations of the 630.0 nm airglow line of atomic oxygen, originating from a broad layer centered around 240 km altitude, in the ionospheric F-region.The 1993 Horizontal Wind Model (HWM93), NRLMSISE-00 atmospheric model, and the Coupled Thermosphere/Ionosphere Plasmasphere (CTIP) model were used for comparison. During the geomagnetically quiet period studied, observed winds and temperatures were generally well modelled, although temperatures were consistently higher than NRLMSISE-00 predicted, by up to 100 K. CTIP temperatures better matched our data, particularly later in the night, but predicted zonal winds which were offset from those observed by 70-180 ms −1 westward. During periods of increased activity both winds and temperatures showed much greater variability over time-scales of less than an hour. For the active night presented here, a period of 45 min saw wind speeds decrease by around 180 ms −1 , and temperatures increase by approximately 100 K. Activeperiod winds were poorly modelled by HWM93 and CTIP, although observed median temperatures were in better agreement with NRLMSISE-00 during such periods.Average behavior was found to be generally consistent with previous studies of thermospheric winds above Mawson. The collected data set was representative of quiet geomagnetic and solar conditions. Geographic eastward windsCorrespondence to: C. Anderson (c.anderson@latrobe.edu.au) in the afternoon/evening generally continued until around local midnight, when winds turned equatorward. Geographic meridional and zonal winds in the afternoon were approximately 50 ms −1 weaker than expected from HWM93, as was the transition to equatorward flow around midnight. There was also a negligible geographic zonal component to the post-midnight wind where HWM93 predicted strong westward flow.

[1] 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 À11 Wm À3 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.