The quantitative relationship between VLF phase deviations and 1–8 Å solar X-ray fluxes during solar flares

Abstract

An attempt is made to investigate the quantitative relationship between VLF phase deviations in SPA (sudden phase anomalies) events and associated solar X-ray fluxes in the 1–8 Å band during solar flares. The phase deviations (Δφ) of the 18.6 kHz VLF wave transmitted from NLK, USA are used in this analysis which were recorded at Nishinomiya, Japan during the period June 1974 to May 1975. The solar X-ray fluxes ( F 0) in the 1–8 Å band are estimated from f min variations using the empirical expression given by Sato (1975), because no observed data were available on the 1–8 Å X-ray fluxes during the period of the VLF observation. The result shows that the normalized phase variation, Δφ cos min , where min represents the minimum solar zenith angle on the VLF propagation path, increases with increasing log F 0. A theoretical explanation for this is presented assuming that enhanced ionizations produced in the lower ionosphere by a monochromatic solar X-ray emission are responsible for the VLF phase deviations. Also it is found that a threshold X-ray flux to produce a detectable SPA effect is approximately 1.5 × 10 −3 cm −2 sec −1 in the 1–8 Å band.