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Ann. Geophys., 14, 1462-1472, 1996
www.ann-geophys.net/14/1462/1996/
© European Geosciences Union 1996
Enhanced incoherent scatter plasma lines
H. Nilsson1, S. Kirkwood1, J. Lilensten2, and M. Galand2
1Swedish Institute of Space Physics, Box 812, S-981 28 Kiruna, Sweden
2Centre d'Etudes des Phenomènes Aléatoires et Géophysiques - ENSIEG, St Martin d'Heres Cedex, France
Abstract. Detailed model calculations of auroral
secondary and photoelectron distributions for varying conditions have been used
to calculate the theoretical enhancement of incoherent scatter plasma lines.
These calculations are compared with EISCAT UHF radar measurements of enhanced
plasma lines from both the E and F regions, and published EISCAT VHF radar
measurements. The agreement between the calculated and observed plasma line
enhancements is good. The enhancement from the superthermal distribution can
explain even the very strong enhancements observed in the auroral E region
during aurora, as previously shown by Kirkwood et al. The model
calculations are used to predict the range of conditions when enhanced plasma
lines will be seen with the existing high-latitude incoherent scatter radars,
including the new EISCAT Svalbard radar. It is found that the detailed
structure, i.e. the gradients in the suprathermal distribution, are most
important for the plasma line enhancement. The level of superthermal flux
affects the enhancement only in the region of low phase energy where the number
of thermal electrons is comparable to the number of suprathermal electrons and
in the region of high phase energy where the suprathermal fluxes fall to such
low levels that their effect becomes small compared to the collision term. To
facilitate the use of the predictions for the different radars, the expected
signal- to-noise ratios (SNRs) for typical plasma line enhancements have been
calculated. It is found that the high-frequency radars (Søndre Strømfjord,
EISCAT UHF) should observe the highest SNR, but only for rather high plasma
frequencies. The VHF radars (EISCAT VHF and Svalbard) will detect enhanced
plasma lines over a wider range of frequencies, but with lower SNR.
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