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Previous work has shown that ionospheric HFradar backscatter in the noon sector can be used to locate the footprint of themagnetospheric cusp particle precipitation. This has enabled the radar data tobe used as a proxy for the location of the polar cap boundary, and hence measurethe flow of plasma across it to derive the reconnection electric field in theionosphere. This work used only single radar data sets with a field of viewlimited to ~2 h of local time. In this case study using four of the SuperDARNradars, we examine the boundary determined over 6 h of magnetic local timearound the noon sector and its relationship to the convection pattern. Thevariation with longitude of the latitude of the radar scatter with cuspcharacteristics shows a bay-like feature. It is shown that this feature isshaped by the variation with longitude of the poleward flow component of theionospheric plasma and may be understood in terms of cusp ion time-of-flighteffects. Using this interpretation, we derive the time-of-flight of the cuspions and find that it is consistent with approximately 1 keV ions injected froma subsolar reconnection site. A method for deriving a more accurate estimate ofthe location of the open-closed field line boundary from HF radar data isdescribed.  <b>Key words: </b>Ionosphere (ionosphere–magnetosphereinteractions; plasma convection) · Magnetospheric physics (magnetopause · cusp· and boundary layers

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