Geometric determination of ionospheric total electron content from dual frequency radar sounding measurements

Abstract

Orbital radar sounding measurements are used to characterize the ionosphere by correcting for signal dispersion effects (de-focusing and delay) experienced as the radar wave propagates through it. For Mars, this is typically achieved either by 1) autofocusing surface echoes to maximize their strength or sharpness, or 2) comparing relative time delays to the surface at either cross-over points or with simulated radargrams. Here, a novel and robust geometric approach is proposed that is primarily applicable to Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) sounding measurements as, due to the design of the REASON and the nature of Europa’s ionosphere, typical Martian radiometric approaches may not be suitable. However, the methodology can be adapted to an arbitrary set of dual-frequency radar sounding measurements. The geometric approach leverages the dual-frequency nature of REASON and the time delay between the ionosphere-delayed 9 MHz High Frequency (HF) surface echoes and the simultaneously-recorded but differentially dispersed 60 MHz Very High Frequency (VHF) surface echoes. That time delay is then used to estimate the ionospheric total electron content (TEC). The robustness of the proposed methodology is demonstrated through testing with Martian sounding measurements. The geometric results reproduce the expected decrease in TEC in the Martian ionosphere as a function of increasing solar zenith angle (SZA), exhibit a consistent agreement between the two test regions, and quantitatively correlate with published radiometric TEC estimates. Testing provides key insights into the underlying assumptions and idiosyncrasies of the methodology and how it could best be applied with REASON datasets to characterize and search for anomalies (e.g. plumes) within the ionosphere of Europa.