Analysis of Low-Frequency Radio Array Mapping of D-Region Electron Densities and HF Absorption

We present results from an array of low-frequency (500 Hz - 500 kHz) radio receivers, passively recording VLF (3–30 kHz) emissions from natural lightning and navy VLF transmitters. A series of signal processing, machine learning, and tomographic techniques amalgamates this dataset to reconstruct the 3D D-region (60–90 km) ionospheric electron density profiles over the Gulf of Mexico and Southeastern USA. We present profiles over a selected 13-day period in August 2017 during which time there was a solar eclipse and multiple major solar flares. The VLF electron density profiles are also used to infer HF absorption as a function of location and time, a technique described in this paper. We use this to validate our D-region inferences by comparing against a ground-based ionosonde in-strument.