On determining the noon polar cap boundary from SuperDARN HF radar backscatter characteristics

Previous work has shown that ionospheric HF radar backscatter in the noon s ector can be used to locate the footprint of the magnetospheric cusp partic le precipitation. This has enabled the radar data to be used as a proxy for the location of the polar cap boundary, and hence measure the flow of plas ma across it to derive the reconnection electric held in the ionosphere. Th is work used only single radar data sets with a field of view limited to si milar to2 h of local time. In this case study using four of the SuperDARN r adars, we examine the boundary determined over 6 h of magnetic local time a round the noon sector and its relationship to the convection pattern. The v ariation with longitude of the latitude of the radar scatter with cusp char acteristics shows a bay-like feature. It is shown that this feature is shap ed by the variation with longitude of the poleward flow component of the io nospheric plasma and may be understood in terms of cusp ion time-of-flight effects. Using this interpretation, we derive the time-of-flight of the cus p ions and find that it is consistent with approximately 1 keV ions injecte d from a subsolar reconnection site. A method for deriving a more accurate estimate of the location of the open-closed field line boundary from HF rad ar data is described.