Effects of a Solar Flare on Global Propagation of Extremely Low Frequency Waves

Abstract

Solar flares have profound impacts on the lower ionosphere and long-distance radio propagation. Extremely low frequency (ELF: 3–3,000 Hz) waves are challenging to observe and experience unique interactions with the lower ionosphere. The primary natural sources of ELF waves are thunderstorm lightnings across the globe. Using a newly developed azimuth determination technique and improved observation hardware we show that ELF attenuation in the Earth-Ionosphere spherical cavity decreases and propagation velocity increases under the influence of an M-class solar flare. Using a two-parameter model of the lower ionosphere, the observations are shown to be consistent with increased electron density and sharper gradients in the D-region resulting from X-ray radiation. The sharper electron density gradient is primarily responsible for the propagation velocity increase, suggesting a unique capability that ELF observations can bring to global remote sensing of the lower ionosphere under space weather perturbations.