Solar Flare X‐Ray Impacts on Long Subionospheric VLF Paths. Issue 11 (11th November 2021)

• Record Type: Journal Article
• Authors: Belcher, Samuel R. G.; Clilverd, Mark A.; Rodger, Craig J.; Cook, Sophie; Thomson, Neil R.; Brundell, James B.; Raita, Tero
• Is Part Of: Space weather. Volume 19:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-11
• Subjects: VLF propagation -- solar flares -- regression analysis -- X‐ray flares -- nowcasting -- ICAO; Space environment -- Periodicals; 551.509992
• DOI: 10.1029/2021SW002820
• Languages: English
• ISSNs: 1542-7390

Abstract

Abstract: Solar flares increase the electron number concentration in the daytime ionosphere, potentially affecting radiowave propagation over several frequency ranges. In this study, we use ionospheric observations to determine both peak magnitudes and time variations of solar flare X‐rays without using the direct measurement from the flare. Ground‐based observations of VLF transmitter phase perturbations are compared against measured X‐ray flux levels during solar flares. Flare fluxes derived here from VLF phases on a west‐east subionospheric path are compared with those from a previously analyzed north‐south path. Using a wider selection of solar flares, including M‐class flares for the first time, the best‐fit equations and root mean square (RMS) errors are computed with improved standard deviation (SD) uncertainty estimates for the peak fluxes. Good agreement is found between peak long X‐ray wavelength fluxes (XL, 0.1–0.8 nm) derived for M‐class and X‐class flares and those measured by the GOES satellites. Linear regression analysis on the two paths shows the uncertainties increase in inverse proportion to the path length. Investigations were made with a limited set of "operational" parameters that could be used to derive XL fluxes. No increases in RMS or SD uncertainty levels were introduced by the removal of satellite‐based regression parameters such as the XL flux level measured before the flare onset. As such, these techniques support the idea of nowcasting M‐class and X‐class flares from entirely ground‐based measurements. Plain Language Summary: In this study, previous analysis of solar flare impacts on the propagation of radiowaves beneath the Earth's ionosphere is extended. These space weather effects can cause disruptions to aviation navigation and communications systems, affecting flight routing and causing passenger delays. Perturbations of signals coming from man‐made communication transmitters in the very low frequency range are used to measure solar flare X‐ray flux levels over a wider range of event sizes than done previously. We have shown that the accuracy of the determined flare size is dependent on the distance between the transmitter and the measuring receiver, with longer paths being better. Using only ground‐based measurements to estimate the flare size is a reasonable proxy for satellite X‐ray data, suggesting a technique for an independent solar flare monitoring capability. Key Points: Ground‐based subionospheric VLF phase measurements of M‐class and X‐class flare impacts are analyzed on N‐S and W‐E propagation paths Good agreement is found between the peak XL flux derived using VLF phase for M‐class and X‐class flares and those measured by GOES Regression analysis on the two paths shows the flux uncertainties increase in inverse proportion to the transmitter to receiver path length