Remote Sensing of the D-region Ionosphere Response to Solar Flares using Lightning as a Signal of Opportunity

The D-region of the ionosphere (60-100 km altitude) exists at the boundary between Earth’s neutral gas atmosphere and the plasma environment of near-earth space. Very low frequency (VLF, 3-30 kHz) radio waves propagate to great distances with low attenuation in the Earth-Ionosphere waveguide (EIWG) formed by the D-region ionosphere and the surface of the Earth. Propagation characteristics depend on waveguide characteristics (e.g., electron density profile of the D-region) and thus measuring propagating waves allows for ground-based remote sensing of the ionosphere. VLF waves propagating through the EIWG experience modal interference. As a part of the analysis, Stokes parameters will be used as a heuristic for modal content. Lightning is an abundant source of VLF radio waves (radio atmospherics, or sferics) with diverse frequency content and source geometry, which makes it ideal for remote sensing. In this work, we present comparisons of more established VLF remote sensing techniques evaluating the amplitude and phase of VLF transmitters to observations of lightning sferics and discuss how those changes might be interpreted as changes in the D-region ionosphere.