The boundary between quasi-dipolar (closed) geomagnetic field lines and those connected to the interplanetary magnetic field (open) is a key diagnostic for the magnetospheric system. This study presents an HF radar technique for determining the location of this boundary in the nightside ionosphere and calibrates it with the boundary determined from simultaneous particle precipitation data. The latitudes of spectral width boundaries (SWBs) identified from 5 years (1997--2001) of data from the Halley HF radar of the Super Dual Auroral Radar Network (SuperDARN), in the range 1800--0200 magnetic local time (MLT), have been compared with nightside particle precipitation boundaries (PPBs) identified from measurements from the Defense Meteorological Satellite Program (DMSP) spacecraft. Latitudinal differences were measured between the PPBs and the nearest SWB within a ¿10 min UT window and within a ¿1 hr MLT window. These differences were then organised into 2-hour MLT sectors. The statistical distributions of the latitudinal differences show that the nightside SWB correlates best with the b6 PPB, which is a good marker for the open-closed field line boundary (OCB) in the nightside ionosphere. When considering SWBs determined using different spectral width threshold values, those determined using the lower threshold values (<150 m/s) are offset ~1--2¿ equatorward of the b6 boundary suggesting that there is a gradual latitudinal change in spectral width between the b5 and b6 boundaries rather than a step change at the b6 boundary. The results enable the OCB to be determined with confidence in the 1800--0200 MLT sector using SuperDARN radar data.