Developing a nowcasting capability for X‐Class solar flares using VLF radiowave propagation changes

A technique for analysing very low frequency (VLF) radiowave signals is investigated in order to achieve rapid, real‐time detection of large solar flares, through the monitoring of changes in VLF radio signal propagation conditions. The reliability of the use of VLF phase and amplitude perturbations to determine the X‐ray fluxes involved during 10 large solar flare events (>X1) is examined. Linear regression analysis of signals from the NPM transmitter in Hawaii, received at Arrival Heights, Scott Base, Antarctica over the years 2011‐2015 shows that VLF phase perturbations during large solar flares have a 1.5‐3 times lower mean square error when modelling the long wavelength X‐ray fluxes than the equivalent short wavelength fluxes. The use of VLF amplitude observations to determine long or short wavelength X‐ray flux levels have a 4‐10 times higher mean square error than when using VLF phase. Normalised linear regression analysis identifies VLF phase as the most important parameter in the regression, followed by solar zenith angle at the mid‐point of the propagation path, then the initial solar X‐ray flux level (from 5 min before the impact of the solar flare), with F10.7 cm flux from the day beforehand providing the least important contribution. Transmitter phase measurements are more difficult to undertake than amplitude. However, networks of VLF receivers already exist which include the high quality phase capability required for such a nowcasting product. Such narrowband VLF data can be a redundant source of flare monitoring if satellite data is not available.