Solar flare induced ionospheric D-region enhancements from VLF phase and amplitude observations

Ionospheric perturbations due to solar flares, measured at VLF in both phase and amplitude on long subionospheric paths, are used to determine the accompanying D-region electron density enhancements as a function of the flare X-ray fluxes measured by the GOES satellites. The electron densities are characterised by the two traditional parameters, H-1 and beta (being measures of the ionospheric height and the rate of increase of electron density with height, respectively), found by computational modelling of the observed phases and amplitudes using the NOSC Earth-ionosphere waveguide programs (LWPC and ModeFinder) over a wide range of VLF frequencies, 10.2-24.8 kHz, along a number of transequatorial paths across the Pacific Ocean to Dunedin, New Zealand. The transmitters monitored include Omega Japan, Omega Hawaii, NPM in Hawaii, and NLK near Seattle, USA, for which the paths range in length from 8.1 to 12.3 Mm. The observations include flares up to a magnitude of about X5 (5 x 10(-4) W m(-2) at 0.1-0.8 nm). These gave VLF phase delay reductions of up to about 52 mus and amplitude enhancements up to nearly 10 dB for the 12.3 Mm NLK to Dunedin path on 24.8 kHz which corresponded, under low to medium solar cycle conditions (1994-1998), to a reduction in H-1 from about 71 km down to about 58 km and an increase in beta from about 0.39 km(-1) up to a definite 'saturation' level of about 0.52 km(-1). These experimentally determined values of H-1 and beta were then used in LWPC to predict flare-induced VLF phase and amplitude perturbations over a wider range of frequencies than were actually available for observation. (C) 2003 Elsevier Ltd. All rights reserved.