[1] 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.

Abstract. Ionospheric E × B plasma drift velocities derived from the Super Dual Auroral Radar Network (SuperDARN) Doppler data exhibit systematically smaller (by 20-30%) magnitudes than those measured by the Defence Meteorological Satellites Program (DMSP) satellites. A part of the disagreement was previously attributed to the change in the E/B ratio due to the altitude difference between the satellite orbit and the location of the effective scatter volume for the radar signals. Another important factor arises from the freespace propagation assumption used in converting the measured Doppler frequency shift into the line-of-sight velocity. In this work, we have applied numerical ray-tracing to identify the location of the effective scattering volume of the ionosphere and to estimate the ionospheric refractive index. The simulations show that the major contribution to the radar echoes should be provided by the Pedersen and/or escaping rays that are scattered in the vicinity of the F-layer maximum. This conclusion is supported by a statistical analysis of the experimental elevation angle data, which have a signature consistent with scattering from the F-region peak. A detailed analysis of the simulations has allowed us to propose a simple velocity correction procedure, which we have successfully tested against the SuperDARN/DMSP comparison data set.

[1] Gillies et al. (2009) proposed the use of interferometric measurements of the angle of arrival as a proxy for the scattering region refractive index n s needed to estimate the line-of-sight Doppler velocity of the ionospheric plasma from HF [Super Dual Auroral Radar Network (SuperDARN)] radar observations. This study continues this work by comparing measurements of line-of-sight velocities by SuperDARN with tristatic velocity measurements by the EISCAT incoherent scatter radar from 1995 to 1999. From a statistical viewpoint, velocities measured by SuperDARN were lower than velocities measured by EISCAT. This can, at least partially, be explained by the neglect in the SuperDARN analysis of the lower-than-unity refractive index of the scattering structures. The elevation angle measured by SuperDARN was used as a proxy estimate of n s and this improved the comparison, but the velocities measured by SuperDARN were still lower. Other estimates of n s using electron densities N e based on both EISCAT measurements and International Reference Ionosphere model values did not increase the SuperDARN velocities enough to attain the EISCAT values. It is proposed that dense structures that were of comparable size to the SuperDARN scattering volume partially help resolve the low-velocity issue. These dense, localized structures would provide the N e gradients required for generation of the coherent irregularities from which the SuperDARN radar waves scatter, whereas EISCAT incoherent radar measurements provide only the background N e and not the density of the small-scale structures. The low-velocity SuperDARN results suggest that small-scale dense structures with refractive indices well below unity must exist within the SuperDARN scattering volume and may contribute greatly to the scattering process.

The combined fields of view of the two Resolute Bay Incoherent Scatter Radars (RISR‐Canada and RISR‐North) significantly overlap the field of view of the Super Dual Auroral Radar Network (SuperDARN) radar located in Rankin Inlet. These radars measure ionospheric flow velocities in the polar cap region. Velocity data from the first multiple‐day combined operations of the two RISR radars and Rankin Inlet have been compared. Direct comparisons between line‐of‐sight measurements by both types of radars have been performed. These comparisons included data from 40 days of radar operations and used velocity data from 35 SuperDARN range gates (spanning 1,575 km). Overall, 5.2 × 105 comparison sets were analyzed. In particular during the daytime, signatures of groundscatter in the SuperDARN data often existed at most of the ranges considered in this comparison. This groundscatter could be partially removed from the comparison by only considering SuperDARN data points that had ionospheric velocity measurements in surrounding range cells. It was found that after removing groundscatter contamination at medium SuperDARN ranges (range gates 18–45), velocities measured by the two radar systems agreed when the high‐frequency results were adjusted to account for the refractive index effect. In regions dominated by groundscatter and E region scatter, lower SuperDARN velocities were measured and the overall comparison with the F region RISR velocities was poor.

[1] Measurements of ionospheric drift velocities using HF coherent scatter radars, such as SuperDARN, are generally underestimated because the refractive index in the scattering volume has not been taken into account. Refractive index values evaluated from electron density measurements, international reference ionosphere predictions, or elevation angle measurements have been applied to SuperDARN velocities in past studies. However, the SuperDARN velocities so obtained were, on average, statistically lower than velocities measured by other instruments. One possible explanation for this underestimation is that HF coherent scatter preferentially occurs in regions of the ionosphere where the scattering cross section is largest, and such regions are characterized by small-scale structures which have higher-than-average electron densities. This was not accounted for in past studies because the refractive index estimates used were from large scale and therefore smoothed estimates of electron density. In this paper, a new method of estimating the actual electron density (or plasma frequency) at the location of SuperDARN scatter (instead of the larger-scale background electron density) is presented. This method takes advantage of the frequency shifts which occur in normal SuperDARN operations. If it is assumed that, on average, the actual ionospheric drift velocity and plasma frequency are roughly constant before and after a shift in frequency, any change in measured velocity as SuperDARN changes frequency is due to a change in refractive index. An analysis of the change in the measured velocity resulting from each shift in frequency gives an experimentally based estimate of the electron density in the scattering volume. A statistical analysis of essentially all frequency shifts by SuperDARN and the estimated electron densities in the scattering volume has been performed. The resulting electron densities are appreciably higher than previous methods to estimate electron density predict. Application of this new method to velocity comparisons between SuperDARN and other instruments results in agreement between the HF radar and non-HF radar velocities for the first time. This new method allows for the first direct measurements of electron densities in the exact locations where the cross section for SuperDARN scatter maximizes.