A case study of HF radar spectra and 630.0 nm auroral emission in the pre-midnight sector

• Published in: Annales Geophysicae
• Main Authors: M. Lester, S. E. Milan, V. Besser, R. Smith
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2001
• Subjects: Science; Q; Physics; QC1-999; Geophysics. Cosmic physics; QC801-809; Longyearbyen
• Online Access: https://doi.org/10.5194/angeo-19-327-2001; https://doaj.org/article/2b5db6ce652c4791b243ef6082d0c986

A comparison of HF radar backscatter observed by the CUTLASS Finland radar, meridian scanning photometer data from Longyearbyen, magnetic field variations from IMAGE stations, and particle precipitation measured by the DMSP F12 spacecraft is presented. The interval under discussion occurred in the pre-midnight local time sector, during a period of weakly northward interplanetary magnetic field. A region of HF backscatter, typically 8 degrees wide, occurred in the field of view of the CUTLASS Finland radar. A well defined gradient in the spectral width parameter was present, with mainly low (< 200 m s - 1 ) spectral widths in the lower latitude part of the scatter and predominantly large (> 200 ms - 1 ) spectral widths in the higher latitude part. The relationship between the spectral width and the red line (630.0 nm) emission measured by the meridian scanning photometer is considered. The poleward border of the red line emission, which has, in the past, been proposed as being representative of the polar cap boundary, was co-located to within 1° of magnetic latitude with the gradient in spectral width for part of the interval. Statistically, large spectral widths occurred poleward of the red line emission, while small spectral widths occurred within or equatorward of the red line emission. Near simultaneous DMSP particle observations in the 20 eV to 20 keV range indicate that the poleward border of the red line emission and the gradient in spectral width occurred at the same latitude as the transition from auroral oval to polar rain particle energies. We conclude that the large spectral widths were not caused by particle precipitation associated with the auroral oval. There were two periods of special interest when the relationship between the red line and the spectral width broke down. The first of these happened during enhanced red line and green line (557.7 nm) emission, with a drop out of the radar scatter and an enhanced, narrow westward electrojet. We conclude that this event was a magnetospheric ...