Annales Geophysicae www.ann-geophys.net 0992-7689 1432-0576 22 10 2004 10.5194/angeo-22-3625-2004 http://www.ann-geophys.net/22/3625/2004/ http://www.ann-geophys.net/22/3625/2004/angeo-22-3625-2004.html http://www.ann-geophys.net/22/3625/2004/angeo-22-3625-2004.pdf 3625 3639 2004-11-03 The location of the open-closed magnetic field line boundary in the dawn sector auroral ionosphere J. A. Wild S. E. Milan C. J. Owen J. M. Bosqued M. Lester D. M. Wright H. Frey C. W. Carlson A. N. Fazakerley H. Rème Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK CESR/CNRS, 9 Avenue du Colonel Roche BP 4346, 31028 Toulouse, Cedex 4, France Space Sciences Laboratory, University of California, Berkeley, USA As a measure of the degree of coupling between the solar wind-magnetosphere-ionosphere systems, the rate at which the size of the polar cap (the region corresponding to ionospheric termini of open magnetic flux tubes) varies is of prime importance. However, a reliable technique by which the extent of the polar cap might be routinely monitored has yet to be developed. Current techniques provide particularly ambiguous indications of the 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 to determine the location of the polar cap boundary using multi-wavelength observations of the ultraviolet aurora (made by the IMAGE FUV imager), precipitating particle measurements (recorded by the FAST, DMSP, and Cluster 1 and 3 satellites), and SuperDARN HF radar observations of the ionospheric Doppler spectral width boundary. We conclude that in the dawn sector, during the interval presented, neither the poleward edge of the wideband auroral UV emission (140-180nm) nor the Doppler spectral width boundary were trustworthy indicators of the polar cap boundary location, while narrow band UV emissions in the range 130-140nm appear to be much more reliable.