Characterizing Unexpectedly Localized Slowing of the Thermospheric Cross-Polar Jet of Neutral Wind Over Alaska in the Midnight Sector

We have measured auroral zone thermospheric neutral winds in the midnight local time sector, using ground-based optical Doppler spectroscopy of the 630.0 nm emission from atomic oxygen, originating at around 240 km altitude over Alaska. One of the most prominent features seen in winds at these latitudes is the cross-polar jet emerging from the polar cap at local times around magnetic midnight. The standard view is that wind flows anti-sunward in the midnight sector and spills equatorward over magnetic latitudes extending well below those of the auroral zone. The purpose of this paper is to show that this view is too simplistic. From our observatory at Poker Flat, Alaska (similar to 65.12oN), the anti-sunward flow is frequently seen to stall over surprisingly short horizontal distances (100-200 km), without spilling further equatorward. This behavior is most prevalent during a low solar activity at mid-winter when the combination of pressure gradient established by solar heating and the ion drag is not enough to allow the jet to push through the background atmosphere on the nightside. At higher latitudes, by contrast, the flow is relatively uniformly anti-sunward around magnetic midnight even during quiet conditions. During periods of high solar and magnetic activity, the expected spilling of the midnight sector cross-polar jet to lower latitudes often is indeed observed over Alaska. Our observation of abrupt stalling during quiet solar and geomagnetic conditions is a very significant difference from the model predictions, with potentially important ramifications- which is the motivation for the present study.