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| Titel |
A case study of HF radar spectra and 630.0 nm auroral emission in the pre-midnight sector |
| VerfasserIn |
Mark Lester, S. E. Milan, V. Besser, R. Smith |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 19, no. 3 ; Nr. 19, no. 3, S.327-339 |
| Datensatznummer |
250014222
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| Publikation (Nr.) |
 copernicus.org/angeo-19-327-2001.pdf |
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| Zusammenfassung |
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
substorm occurring at much higher than usual latitudes. The second period of
special interest happened when equatorward moving bands of large spectral width
occurred within the region of scatter. Up to 4 of these bands were present
during an interval of 100 minutes. Associated with these narrow bands of large
spectral width were narrow channels of enhanced westward ion velocities. We
conclude that these equatorward moving bands of large spectral width may be
related to reconnection processes in the tail. The observations demonstrate that
the tail continues to be active even under low solar wind energy input
conditions. Furthermore, we conclude that the gradient in the spectral width may
be used as a proxy for the polar cap boundary, but only with extreme caution.
Key words. Ionosphere
(ionosphere-magnetosphere inter-actions; polar ionosphere) – Magnetospheric
physics (storms and substorms) |
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