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Abstract The lowest region of the ionosphere, theDregion, plays an important role in magnetosphere‐ionosphere coupling but is challenging to directly observe. The group velocity of the extremely low frequency (ELF; 3–300 Hz) portion of lightning induced electromagnetic radiation can be used to diagnose theDregion electron density profile. Day‐night conditions can be assessed using ELF receivers and lightning detection networks. Analytical formulations and the Long Wave Propagation Capability software package show that ELF group velocity has particular sensitivity to the sharpness of the exponential electron density profile. Applying the technique to sudden ionospheric disturbances shows that the group velocity increases in response to incidence of solar X‐ray flux . A small number of ELF receivers can provide a large‐scale diagnostic of theDregion. ELF remote sensing using lightning is complementary to very low frequency remote sensing and can be used to assess the Earth‐ionosphere propagation channel for very low frequency transmitters.
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This content will become publicly available on December 1, 2025
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.
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- PAR ID:
- 10610157
- Publisher / Repository:
- Journal of Geophysical Research
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 129
- Issue:
- 12
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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