Journal cover Journal topic
Annales Geophysicae An open-access journal of the European Geosciences Union
Ann. Geophys., 24, 3351-3364, 2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.
21 Dec 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.

Citation: Ogawa, Y., Buchert, S. C., Fujii, R., Nozawa, S., and Forme, F.: Naturally enhanced ion-acoustic lines at high altitudes, Ann. Geophys., 24, 3351-3364,, 2006.
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