Graph showing an example of very-low frequency signal phase response to solar X-ray emissions over a 24-hour period
An example of very-low frequency (VLF) signal phase response to solar X-ray emissions over a 24-hour period, thus giving periods of daylight and darkness on the signal propagation path (dusk is indicated by the first black line, and dawn by the second). The blue curves show the timelines of solar X-ray flux on 13 May 2013 (solid line is flux at longer wavelengths, “XL”, of 0.1-0.8 nm, and dashed line is shorter wavelengths, “XS”, of 0.05 to 0.4 nm). Solar flares appear in these curves as a sharp rise followed by slow decay. The orange curve shows the phase of a VLF signal received in Antarctica from a transmitter in Hawaii. The signal phase advances and then decreases when a large flare occurs around 02:00 UT and the signal path is in daylight. During the hours of darkness, there is no VLF response to flare, just a general phase decrease as the lower ionosphere naturally decays via recombination of electrons and ions. Another large flare occurs just after dawn, but the VLF response is dominated by a general advance in phase as the lower ionosphere reforms. But, later in the day, the VLF phase changes in response to a modest flare can be seen very clearly. Credit: George et al. [2019], Figure 3, top panel
Source: Space Weather

At very-low frequencies (VLF) radio waves can propagate around the world via the natural waveguide formed by the electrically conductive Earth below and the conductive ionosphere above. This effect has been exploited for long-distance, low-bandwidth, radio communications for over a century, and continues to be exploited today.

Solar flares narrow this waveguide on the dayside of the Earth by creating an ionospheric layer at lower altitudes, thus changing the phase and amplitude of the VLF signals. George et al. [2019] demonstrate how this narrowing may be used to monitor the occurrence and intensity of solar flares in near-real-time, complementing current flare monitoring by satellite-based X-ray sensors.

As the global requirements for operational monitoring of space weather increase, it is important to explore how complementary techniques can improve the resilience of that monitoring. This paper is a good example of the potential for such complementarity, especially between space-based and ground-based measurements, each of which has different strengths and weaknesses (and hence, working together, they improve resilience). The authors provide a proof-of-concept using flare impacts on a single VLF path and discuss how this might be expanded into a global network.

Citation: George, H. E., Rodger, C. J., Clilverd, M. A., Cresswell‐Moorcock, K., Brundell, J. B., & Thomson, N. R. [2019]. Developing a Nowcasting Capability for X‐Class Solar Flares Using VLF Radiowave Propagation Changes. Space Weather, 17.

—Michael A. Hapgood, Editor, Space Weather

Text © 2020. The authors. CC BY-NC-ND 3.0
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