University of Oulu

Belcher, S. R. G., Clilverd, M. A., Rodger, C. J., Cook, S., Thomson, N. R., Brundell, J. B., & Raita, T. (2021). Solar flare X-ray impacts on long subionospheric VLF paths. Space Weather, 19, e2021SW002820.

Solar flare X-ray impacts on long subionospheric VLF paths

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Author: Belcher, Samuel R. G.1; Clilverd, Mark A.2; Rodger, Craig J.1;
Organizations: 1Department of Physics, University of Otago, Dunedin, New Zealand
2British Antarctic Survey (UKRI-NERC), Cambridge, UK
3Sodankylä Geophysical Observatory, University of Oulu, Sodankylä, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 1.8 MB)
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Language: English
Published: American Geophysical Union, 2021
Publish Date: 2022-01-03


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.

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Series: Space weather
ISSN: 1542-7390
ISSN-E: 1542-7390
ISSN-L: 1542-7390
Volume: 19
Issue: 11
Article number: e2021SW002820
DOI: 10.1029/2021SW002820
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: The authors would like to thank AntarcticaNZ for providing support for the VLF observations made at Arrival Heights through event K060, NOAA for providing the GOES X-ray flux data, and the Canadian Natural Research Council for the solar UV, F10.7 cm flux. The authors would like to thank Paul Muir of Otago University for his assistance with Figure 1. MAC would like to acknowledge support from the UK Research and Innovation (UKRI-NERC) through National Capability Space Weather Observatory funding (NC-SS SWO).
Copyright information: © 2021 The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.