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.  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. . Developing a Nowcasting Capability for X‐Class Solar Flares Using VLF Radiowave Propagation Changes. Space Weather, 17. https://doi.org/10.1029/2019SW002297
—Michael A. Hapgood, Editor, Space Weather