The effect on SuperDARN HF propagation predictions of polar cap plasma density gradients measured with the RISR radars

The global array of High Frequency (HF) Super Dual Auroral Radar Network (SuperDARN) radars continuously monitors ionospheric convection from the mid-latitude to the polar cap in both hemispheres. These radars rely on coherent backscatter from field-aligned, decameter-scale irregularities. In order to scatter from field-aligned irregularities in the high-latitude region, where the field lines are nearly vertical, significant refraction of the HF radar waves is required. This refraction may also occur in the lateral direction when horizontal electron density gradients are present at the appropriate orientation to the wave propagation direction. Density structures, such as polar cap patches are ideal locations for receiving coherent backscatter, but also have horizontal gradients which will cause uncertainty in propagation paths and resulting echo locations. Under the most extreme conditions, HF waves from SuperDARN may be laterally deviated 100s of km (the equivalent of several SuperDARN range bins) and have a bearing change of several degrees (the equivalent of several SuperDARN beams). In this study, electron density measurements from the Resolute Bay Incoherent Scatter Radars (RISRs) are used to determine the occurrence probability and magnitude of density gradients in the polar cap. Ray tracing simulations are then performed to estimate more typical location and bearing offsets that would be expected on HF waves from the SuperDARN radars under most conditions.