AN OVERVIEW OF TYPE-3 RADAR AURORAL RESEARCH - BASIC OBSERVATIONAL PROPERTIES AND NEW INTERPRETATION PROPOSITIONS

Type-3 radio auroral backscatter is recognized by its narrow Doppler s pectrum centered at sub-ion acoustic velocities and its spectrum stabi lity in both space and time. Originally-this scatter was attributed to electrostatic ion cyclotron waves in the upper E-region, a propositio n which has been disproved by experiment. Today, We know that these ec hoes are due-to plasma waves generated inside the electrojet where ion s are unmagnetized. Also, whenever type-3 and type-1 (Farley-Buneman) 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, th ere are results showing that type-3 echoes originate in the vicinity o f optical auroral arcs. In this paper we use past and new data to eval uate 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 den sity gradients in reducing the phase velocity of Farley-Bunemann waves to sub-ion acoustic speeds. This is examined in the framework of the linear theory and also of a recent non-linear theory of spectrum broad ening due to turbulence induced by mode-doupling. Further, pie reexami ne some published data In an attempt to evaluate the hypothesis that t he required sharp density gradients can exist at the edges of discrete auroral arcs. We conclude that the existing evidence in favour of the plasma gradient hypothesis is necessary but not sufficient. Also, wit h this mechanism it would be difficult to explain the preferential typ e-3 velocities which are independent of radar frequency, and the stabi lityl of the Doppler spectrum. At this stage of research, there is a n eed for well-designed, high-resolution, multi-instrument experiments i n order to obtain an integrated observational test of the current inte rpretation propositions.