AURORAL RADAR FREQUENCY BROADENING MEASUREMENTS AT LARGE ASPECT ANGLES AND COMPARISON WITH THEORY

The observed characteristics of radio auroral backscatter at large asp ect angles have never been adequately explained by theories of plasma turbulence that otherwise seem to be satisfactory. Recently, the first nonlinear theory on the subject has appeared in the open literature. In this theory, under the action of a driving electric field, the maxi mum aspect angle depends on the combined contribution of frequency bro adening due to mode coupling and anisotropic anomalous collisions caus ed by the interaction of electrons with the fluctuating electric field s in the waves. In this paper an attempt is made to test the theoretic al predictions by comparing them to observations of mean frequency bro adening during typical auroral electrojet conditions. The analysis was based on a large number of Doppler spectra compiled from several VHF radar backscatter experiments with magnetic aspect angles in the range from about 1 to 10 deg. The theory fails to explain echo occurrence a t directions perpendicular to the E x B flow, but otherwise it can acc ount for most of the observations. The evidence shows that the effect on large aspect angle echoes of frequency broadening alone is insuffic ient and that the key role in fitting the data is played by the parall el anomalous collision frequency nu(e) parallel-to. The agreement bet ween theory and experiment, however, is possible only for large enhanc ements of nu(e) parallel-to relative to the normal electron neutral c ollision frequency nu(e) parallel-to, with the increase ranging betwee n 1 and 2 orders of magnitude. This imposes difficulties because the l arge nu(e) parallel-to/nu(e) ratios imply a pronounced ion acoustic p lasma turbulence along B inside the electrojet layer. To our knowledge , there is no observational evidence to support this theoretical impli cation.