2021/09/14 by Laura A. Hayes, Oscar O’Hara, Oscar S. D. O'Hara +3
Earth and Planetary Sciences · Physics and Astronomy · #Amplitude #Astronomy #Astrophysics #Coronal mass ejection #Earthquake Detection and Analysis #Flare #Flux (metallurgy) #Geophysics #Geostationary Operational Environmental Satellite #Ionosphere #Ionosphere and magnetosphere dynamics #Optics #Physics #Plasma #Satellite #Solar and Space Plasma Dynamics #Solar cycle #Solar cycle 23 #Solar cycle 24 #Solar flare #Solar wind #Space weather #astro-ph.SR #physics.space-ph
paper · pdf · doi:10.1007/s11207-021-01898-y
19 pages, 8 figures, accepted in Solar Physics
arxiv created 2021/09/14 · openalex publication_date 2021/11/01 · arxiv updated 2021/11/17 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
Solar flares significantly impact the conditions of the Earth's ionosphere. In particular, the sudden increase in X-ray flux during a flare penetrates down to the lowest-lying D-region and dominates ionization at these altitudes (60-100 km). Measurements of very low frequency (VLF: 3-30kHz) radio waves that reflect at D-region altitudes provide a unique remote-sensing probe to investigate the D-region response to solar flare emissions. Here, using a combination of VLF amplitude measurements at 24kHz together with X-ray observations from the Geostationary Operational Environment Satellite (GOES) X-ray sensor, we present a large-scale statistical study of 334 solar flare events and their impacts on the D-region over the past solar cycle. Focusing on both GOES broadband X-ray channels, we investigate how the flare peak fluxes and position on the solar disk dictate an ionospheric response and extend this to investigate the characteristic time delay between incident X-ray flux and the D-region response. We show that the VLF amplitude linearly correlates with both the 1-8 A and 0.5-4 A channels, with correlation coefficients of 0.80 and 0.79, respectively. Unlike higher altitude ionospheric regions for which the location of the flare on the solar disk affects the ionospheric response, we find that the D-region response to solar flares does not depend on the flare location. By comparing the time delays between the peak X-ray fluxes in both GOES channels and VLF amplitudes, we find that there is an important difference between the D-region response and the X-ray spectral band. We also demonstrate for several flare events that show a negative time delay, the peak VLF amplitude matches with the impulsive 25-50 keV hard X-ray fluxes measured by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI).