Very low frequency phase advances during solar X-ray flares

Abstract

Very Low Frequency (VLF) waves propagate in the Earth-ionosphere waveguide bounded by the base of the D-region and by the ground. VLF waves are very sensitive to the electron density profile in the D region, and they can propagate over long distances keeping a high stability for the observed phase and amplitude parameters. For these reasons VLF waves provide a useful diagnostic of the low ionosphere, in terms of its parameters β (conductivity sharpness) and H (reflection height). This study reports on Sudden Phase Anomalies (SPA) of VLF waves received at Atibaia (São Paulo, Brazil) and at Inubo (Japan), from several long distance (over 2800 kilometers) paths during solar flares. The time coverage of the data sample includes periods of high solar activity (January-March 1991 and October-December 1991) as well as epochs of reduced solar activity (January 1994-July 1997). This report allows us to investigate the detect ability of X-ray producing ionization excess in the low ionosphere, and how this situation is modified along the solar cycle. The results are also discussed in terms of solar flare properties like intensity and spectrum. In particular during solar minimum, we were able to identify a lower X-ray flux level needed to account for a SPA (~5.10−7 and 4.10−8 Wm−2 in the 1-8 Ǻ and 0.5-4 Ǻ energy ranges respectively). We also confirm the importance of the X-ray spectral characteristics for the properties of the observed SPAs.