Improvement of HF coherent radar line-of-sight velocities by estimating the refractive index in the scattering volume using radar frequency shifting

Abstract

[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.

A statistical analysis of SuperDARN scattering volume electron densities and velocity corrections using a radar frequency shifting technique

“…As presented in Section 1, the velocities assumed by HF radars have been underestimated by a factor equal to the refractive index at the point of scatter (). Using this relationship and the dependence of refractive index n s on radar wave frequency f and plasma frequency f p : Gillies et al [2011] developed the following formula to calculate f p at the point of scatter given two velocities ( v 1 and v 2 ) at two frequencies ( f 1 and f 2 ): Since f p (in units of Hz) is related to N e (in units of m −3 ) by: an examination of velocities at two different frequencies will provide a measurement of the scattering volume electron density.…”

“…Unfortunately, there were not enough frequency shifting data available to make strong conclusions. Two main factors limited the amount of data that was used for the Gillies et al [2011] study: 1) due to the superposed epoch nature of the study, there was a requirement that velocity data existed in a given range gate for several scans before and after a shift, and 2) because the operating mode required that the frequency be constant for a number of scans prior to and after the shift in frequency, it was only possible to look at common mode data where a single frequency shift occurred in the sequence. Modes in which a radar could change frequencies after every scan were not employed.…”

“…The main goal of the Gillies et al [2011] study was to use a superposed epoch analysis to demonstrate that, on average, a shift in the operating frequency of a SuperDARN radar corresponds to a shift in the LOS plasma drift velocity measured by the radar. This velocity shift can then be used to infer the plasma frequency and electron density in the region where the scattering occurred.…”

“…Data were only used if the virtual height was above 200 km (note that, based on results presented by Chisham et al [2008, Figure 4], virtual heights that were calculated to be above 900 km were re‐calculated assuming ‐hop scatter). These criteria resulted in 15 × 10 6 frequency shifts to analyze, which is ∼40 times the number used by Gillies et al [2011]. Each of these frequency shifts has a value associated with v 1 , v 2 , f 1 , and f 2 .…”

“…This value of N e is attributed to the entire range gate, but in reality it may represent a subset of the plasma contained therein. This present study greatly expands the data set used by Gillies et al [2011] and will determine values of n s based on various parameters, such as solar activity, magnetic latitude, local time, and time of year. These values of n s may then be used as correction factors to improve the velocities measured by SuperDARN.…”