Abstract:
Sudden phase anomalies (SPA’s) observed in the phase of GBR 16 kHz VLF signals during the years 1977 to 1983 have been analyzed in the light of their associated solar X-ray fluxes in the 0.5-4 Å bands. An attempt has been made to investigate the solar zenith angle (x) dependence of the integrated solar X-ray flux for producing SPA’s. It is deduced from the observations for x < 81° that the phase deviation increases linearly as a whole with increasing solar X-ray fluxes in these two bands. The threshold X-ray flux needed to produce a detectable SPA effect has been estimated to be 1.6 x 10-⁴ erg cm - ² s-¹ and 1.8 x 10-³ erg cm-² s-¹ in the 0.5 – 4 Å and 1-8 Å bands, respectively. For both bands the average cross section for fall all atmospheric constituents at a height of 70 km in almost equal to the absorption cross section for the 3 Å X-ray emissions.
