Abstract
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 (140-180nm) 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.
Highlights
The concept of the “open” polar cap is a natural consequence of the open description of the magnetosphere proposed by Dungey (1961)
By employing space- and ground-based observations of the dawn-sector auroral zone on 8 December 2001, we have compared the locations of the open/closed field line boundary, the poleward boundary of the auroral oval when viewed over two UV wavelength bands, and a Doppler spectral width boundary apparent in HF coherent scatter radar echoes
During the interval 15:00–16:00 UT, the nightside UV aurora observed by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) Far UltraViolet (FUV) Wideband Imaging Camera (WIC) sensor was dominated by the fading remnants of a pre-existing substorm auroral bulge which extended to ∼75◦ Mlat and approximately spanned 00:00–03:00 MLT, while in the 04:00– 06:00 MLT sector, the aurora was generally not observed poleward of ∼71◦ Mlat
Summary
For a variety of reasons, the majority of the techniques developed to date are not globally applicable. Sometimes the backscattered power of the low spectral width echoes (corresponding to closed magnetic field lines) is insufficient to be detected by the radar system and only the high-width echoes are observed (Milan and Lester, 2001) In such cases, it is the equatorward edge of the high-width echoes that indicates the spectral width boundary (SWB) and the OCB. Previous comparisons between in-situ particle measurements and space-based observations of the UV aurora have revealed a discrepancy of a few degrees of latitude between the OCB and the poleward boundary of UV auroral emission (e.g. Kauristie at al., 1999; Baker et al, 2000), with the largest discrepancy in the 05:00 MLT sector (Carbary et al, 2003) We note that these studies have generally utilised wideband UV auroral imagery, typically corresponding to the Lyman-Birge-Hopfield (LBH) region of the UV spectrum (∼140–180 nm). We will investigate the correspondence between estimates of the OCB location in the dawn sector using (a) insitu particle measurements, (b) observations of UV emission made over multiple wavelength-bands, and (c) ground-based coherent-scatter radar measurements of the ionosphere during a case study based upon data from 8 December 2001
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