Very low frequency (VLF) radio receivers are a non-intrusive means of detecting geomagnetically induced currents (GICs) in power grids, an important manifestation of space weather. When transformers within power grids are stressed by GICs, they can generate harmonics of the 50 or 60 Hz AC currents flowing across those grids. Some of the power in these harmonics is emitted as radio waves at frequencies up to a few kHz, as shown in the figure above.
The existence of these power grid VLF emissions was first recognized some forty years ago by Hayashi et al. . However, there was then very limited interest in space weather and GICs, unlike today when these are topics of global concern. Thus Clilverd et al.  is an excellent and timely reminder that these VLF emissions have considerable potential as a diagnostic tool to monitor GICs in power grids without intruding on grid equipment. This could have major practical advantages in terms of safety and cost.
Citation: Clilverd, M. A., Rodger, C. J., Brundell, J. B., Dalzell, M., Martin, I., Mac Manus, D. H., et al. (2018). Long‐lasting geomagnetically induced currents and harmonic distortion observed in New Zealand during the 7–8 September 2017 disturbed period. Space Weather, 16. https://doi.org/10.1029/2018SW001822
—Michael A. Hapgood, Editor, Space Weather