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

Wild, J. A. and Milan, S. E. and Owen, C. J. and Bosqued, J. M. and Lester, M. and Wright, D. M. and Frey, H. U. and Carlson, C. W. and Fazakerley, A. N. and Rème, H. (2004) The location of the open-closed magnetic field line boundary in the dawn sector auroral ionosphere. Annales Geophysicae, 22 (10). pp. 3625-3639. ISSN 0992-7689

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Abstract

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 (140–180 nm) nor the Doppler spectral width boundary were trustworthy indicators of the polar cap boundary location, while narrow band UV emissions in the range 130–140 nm appear to be much more reliable.