ON TYPE-3 AURORAL VHF COHERENT RADAR BACKSCATTER

Type 3 echoes of radar aurora relate to coherent backscatter signals with narrow power spectra peaking below ion acoustic Doppler speeds in the 100- to 300-m/s range and preferentially close to 200 m/s. The first observations, all made at 50 MHz, suggested that these echoes are due to electrostatic ion cyclotron (EIC) plasma waves generated at altitudes well above the electrojet where ions become magnetized. Most recent evidence, however, based on multifrequency studies and altitude interferometry results in conjunction with theoretical difficulties, placed the EIC interpretation in jeopardy; thus today the type 3 echo question remains unresolved. In the present paper, after a brief review of past studies and an update of the problem, we present new evidence of type 3 auroral scatter obtained at 140 MHz from cross-beam radar measurements made at optimum magnetic aspect angles. The observations do not favor the EIC mechanism, and they show that type 3 waves resemble in several ways the type 1 waves generated by the two-stream instability, e.g., they propagate in the same direction inside the same azimuthal sector at small angles to the electrojet E x B flow. This suggests a direct role for the ambient electric field in generating type 3 plasma waves as well. Further, type 3 and type 1 echoes can occur simultaneously over extended ionospheric areas and thus possibly originate at different altitudes. Prompted by the present results, we consider the option that type 3 echoes are simply due to type 1 waves originating in narrow sporadic E(s) layers located at lower electrojet altitudes, and we investigate the effect of both an enhanced mean ionic mass (because of the predominance of heavy metallic ions) and a large destabilizing electron density gradient component on lowering the phase velocity well below typical ion acoustic speed values. Despite limitations and some controversy, approximate numerical estimates show this option might be feasible and therefore ought to be studied further.