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Ann. Geophys., 24, 3351-3364, 2006
www.ann-geophys.net/24/3351/2006/
© European Geosciences Union 2006
Naturally enhanced ion-acoustic lines at high altitudes
Y. Ogawa1,4, S. C. Buchert2, R. Fujii1, S. Nozawa1, and F. Forme3
1Solar Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan
2Swedish Institute of Space Physics, Uppsala, Sweden
3Centre d'Études des environnements Terrestre et Planétaires, Velizy, France
4now at: National Institute of Polar Research, Tokyo, Japan
Abstract. Naturally enhanced ion-acoustic lines (NEIALs) between 1200 and 1900 km
altitude are investigated. The NEIALs were found in the background gates of
data from the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) at
78° N looking field-aligned. Only strongly enhanced lines are detected at
such high altitudes. The estimated enhancement above incoherent scattering
integrated over the antenna beam and preintegration time of 10 s reaches
about 10 000. Both lines are always enhanced above 1000 km altitude, and the
downshifted line, corresponding to upward propagating ion-acoustic waves, is
always stronger than the upshifted line, for downgoing waves. The ratio of
the downshifted and upshifted peaks is often remarkably constant along a
profile. Using the line positions as indicators of the ion-acoustic speeds
and the bulk drift velocity, we find that the bulk drift does not exceed the
ion-acoustic (sound) speed, but extrapolation of the profiles suggests that
the sound barrier is reached around 2000 km in one event. The highest
ion-acoustic speed is seen near 600 km, above the density peak, indicating
that electrons are heated not only by ionizing precipitation but
significantly also by upgoing waves. Upflow continues to speed up above the
estimated temperature maximum. A certain qualitative similarity to the solar
corona seems to be the case.
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