Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995angeo..13...10h&link_type=abstract
Annales Geophysicae (ISSN 0992-7689), vol. 13, no. 1, p. 10-24
Physics
2
Auroral Arcs, Auroral Echoes, Auroral Electrojets, Auroral Zones, Backscattering, E Region, Plasma Waves, Auroral Spectroscopy, Density Distribution, Doppler Radar, Electric Fields, Spectral Signatures
Scientific paper
Type-3 radio auroral backscatter is recognized by its narrow Doppler spectrum centered at sub-ion acoustic velocities and its spectrum stability in both space and time. Originally this scatter was attributed to electrostatic ion cyclotron waves in the upper E-region, a proposition which has been disproved by experiment. Today, we know that these echoes are due to plasma waves generated inside the electrojet where ions are unmagnetized. Also, whenever type-3 and type-1 (Farley-Bunemann) echoes are observed simultaneously they have the same Doppler polarity which suggests a direct role for the ambient electric field in type-3 generation under some critical conditions in the plasma. Moreover, there are results showing that type-3 echoes originate in the vicinity of optical auroral arcs. In this paper we use past and new data to evaluate and summarize first the basic observational properties of type-3 echoes and then take a critical view of newly proposed interpretation ideas. Mainly, we focus on the role of sharp, destabilizing plasma density gradients in reducing the phase velocity of Farley-Bunemann waves to sub-ion acoustic speeds. This examined in the framework of the linear theory and also of recent non-linear theory of spectrum broadening due to turbulence induced by mode-coupling. Further, we reexamine some published data in an attempt to evaluate the hypothesis that the required sharp density gradients can exist at the edges of discrete auroral arcs. We conclude that the existing evidence in favor of the plasma gradient hypothesis is necessary but not sufficient. Also, with mechanism it would be difficult to explain the preferential type-3 velocities which are independent of radar frequency, and the stability of the Doppler spectrum. At this stage of research, there is a need for well-designed, high-resolution, multi-instrument experiments in order to obtain an integrated observational test of the current interpretation propositions.
Haldoupis Christos
Hussey Glenn C.
Mu Jun
Sofko George J.
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