Mathematics – Probability
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995angeo..13....2k&link_type=abstract
Annales Geophysicae (ISSN 0992-7689), vol. 13, no. 1, p. 2-9
Mathematics
Probability
5
Auroral Echoes, Auroral Spectroscopy, Doppler Effect, Doppler Radar, Radar Echoes, Backscattering, Earth Ionosphere, Histograms, Spectral Signatures, Spectrum Analysis
Scientific paper
Type-3 echoes observed at auroral latitudes by coherent radars have two distinguishing spectral characteristics: they are relatively narrow and have small Doppler shifts. It has never been entirely clear whether this is characteristic of a new mechanism of plasma instability or whether these echoes are just a subset of normal Type-1 (Farley-Bunemann) spectra with unusually small Doppler shifts. In this paper, a large number of spectra gathered during two 50-MHz radar observing campaigns in 1981 and 1983 have been examined in order to see whether there is any statistical basis for postulating a separate class of echo and, if so, to determine which properties these echoes have. From the data presented, it appears that Type-3 echoes are, indeed, a separate class. In agreement with previous observations, the data show that these echoes do not appear to be quantified in Doppler velocity but are spread over a range of velocities. There is a strong dependence on aspect angle, with Type-3 echoes appearing preferentially on radar paths where the aspect angles are significantly different from `perfect' aspect. For these high aspect angle paths, there appears to be no marked dependence of the probability of occurrence of Type-3 echoes on the orientation of the radar beam with respect to the ambient electric field direction. Finally, there is some suggestion that Type-3 peaks are often accompanied by harmonically related spectral peaks.
Koehler James A.
Mu Jun
Sofko George J.
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