Improvement of SuperDARN velocity measurements by estimating the index of refraction in the scattering region using interferometry

In past calculations of convective velocities from Super Dual Auroral Radar Network (SuperDARN) HF radar observations, the refractive index in the scattering region has not been taken into account, and therefore the inferred ionospheric velocities may be underestimated. In light of the significant contribution by SuperDARN to ionospheric and magnetospheric research, it is important to refine the velocity determination. The refractive index in the ionosphere at SuperDARN observation F region altitudes has typical values between 0.8 and close to unity. In the scattering region, where conditions are more extreme, the index of refraction may be much lower. A simple application of Snell's law in spherical coordinates (Bouguer's law) suggests that a proxy for the index of refraction at the scattering location can be determined by measuring the elevation angle of the returned ionospheric radar signal. Using this approximation for refractive index, the Doppler velocity calculation can be refined for each SuperDARN ionospheric echo, using the elevation angles obtained from the SuperDARN interferometer data. A velocity comparison of DMSP and SuperDARN observations has revealed that the SuperDARN speeds were systematically lower than the DMSP speeds. A linear regression analysis of the velocity comparisons found a best fit slope of 0.74. When the elevation angle data were used to estimate refractive index, the best fit slope rose 12% to 0.83. As most SuperDARN radars employ an interferometer antenna array for elevation angle measurements, the improvement in velocity estimates can be done routinely using the method outlined in this paper.