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

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. Plain Language Summary Solar flares are large emissions of energy from the Sun that hit the Earth with X‐rays and ultraviolet emissions. A solar flare has significant effects on the upper atmosphere of the Earth, known as the ionosphere. The solar flare X‐ray radiation leads to an increase in free electron density in the lower ionosphere by ionizing more of the neutral atmosphere. This increase in electron density can in turn have profound effects on long range radio wave propagation. In this study we focus on observations of Extremely Low Frequency (ELF: 3–3,000 Hz) waves and how they are affected by a solar flare. ELF waves are challenging to observe and generate. The primary source of ELF radiation on Earth is thunderstorm lighting. We use a new analysis technique and improved observation hardware to show that ELF waves propagate more effectively and faster under the influence of a solar flare. The unique response of ELF waves to solar flares can be leveraged in global monitoring of the lower ionosphere which is important for a wide range of communication technologies. Key Points Applying new data analysis, we show solar flare effects on the extremely low frequency emissions from global lightning An M‐class solar flare causes decreased attenuation and increased group velocity in ELF band The measured propagation parameters agree with theoretical modeling and suggest global remote sensing capabilities using ELF observations