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

Abstract

Enhancements of D-region electron densities caused by solar flares are determined from observations of VLF subionospheric amplitude changes and these enhancements are then related to the magnitudes of the X-ray fluxes measured by the GOES satellites. The electron densities are characterised by the two traditional parameters, H′ and β (being measures of the ionospheric height and the rate of increase of electron density with height, respectively), which are found by VLF radio modelling of the observed amplitudes using the NOSC Earth-ionosphere waveguide programs (LWPC and Modefinder) mainly on two paths, one short and one long. The short path measurements were made near Cambridge, UK, on the 18.3 kHz signals from the French transmitter 617 km to the south while the long path measurements were made near Dunedin, NZ, on the 24.8 kHz signals from NLK in Seattle, USA, 12.3 Mm across the Pacific Ocean. The observations include flares up to a magnitude of about M5 (5×10 −5 W m −2 at 0.1– 0.8 nm ) which gave VLF amplitude enhancements up to about 8 dB ; these corresponded, under near solar maximum conditions (1992), to a reduction in H′ from about 71 km down to about 63 km and an increase in β from 0.43 km −1 up to about 0.49 km −1 . The increased values of β during a flare are caused by the solar X-rays dominating all sources of ionisation during the flare in contrast with the normal unperturbed daytime values of β which are significantly lower than for a single solar UV or X-ray source due to the extra electrons from the normal galactic cosmic ray ionisation in the lowest parts of the D-region. This steady, normal (unperturbed) cosmic ray influence on β, and hence unperturbed VLF attenuation, is more marked at times of reduced solar Lyman- α flux in the D-region such as at solar minimum, high latitudes or early or late in the day, thus explaining the normal (unperturbed) higher VLF attenuation rates previously reported in these conditions.