D-region electron density evaluated from VLF amplitude time delay during X-ray solar flares

Abstract

VLF amplitude enhancements of the NAA/24.0 kHz signal (Maine, USA), registered by the Belgrade AbsPAL facility during solar flares, have been related to solar X-ray fluxes measured by the GOES-12 satellite. The observed regular appearance of the VLF amplitude/phase peak behind the peak of the corresponding X-ray flux has been characterized by the time delay assigned to each of the 97 out of 114 flare-induced distinct amplitude/phase enhancement events analysed. The measurements reported pertain to the period May–August of the years 2004 and 2005 and have been used to model the electron density enhancements of the D-region during solar flares. On the grounds of the continuity equation for the electron component and the amplitude time delay, the effective recombination coefficient has been estimated to range from 10 - 13 m 3 / s to 10 - 11 m 3 / s , depending on the X-ray maximum flux. By integrating the continuity equation, the time variation of the electron density during solar flare occurrence has been restored. Enhancements up to two orders of magnitude, for X-class flares, with respect to undisturbed conditions were predicted. The results obtained have been found to be in good agreement with measurements by different techniques, as well as with the independent estimates of β and H ′ . Errors introduced by the applied approach have been critically examined to place the uncertainty of the results arrived at between 10% and 20%.