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As a measure of the degree of coupling between the solarwind-magnetosphere-ionosphere systems, the rate at which the size of thepolar cap (the region corresponding to ionospheric termini of open magneticflux tubes) varies is of prime importance. However, a reliable technique bywhich the extent of the polar cap might be routinely monitored has yet to bedeveloped. Current techniques provide particularly ambiguous indications ofthe polar cap boundary in the dawn sector. We present a case study of space-and ground-based observations of the dawn-sector auroral zone and attempt todetermine the location of the polar cap boundary using multi-wavelengthobservations of the ultraviolet aurora (made by the IMAGE FUV imager),precipitating particle measurements (recorded by the FAST, DMSP, andCluster 1 and 3 satellites), and SuperDARN HF radar observations of the ionosphericDoppler spectral width boundary. We conclude that in the dawn sector, duringthe interval presented, neither the poleward edge of the wideband auroral UVemission (140-180nm) nor the Doppler spectral width boundary weretrustworthy indicators of the polar cap boundary location, while narrow bandUV emissions in the range 130-140nm appear to be much more reliable

The location of the open-closed magnetic field line boundary in the dawn sector auroral ionosphere