Abstract
A synoptic survey of Sudden Phase Anomalies (SPAs) observed in the phase variation measurements of 16 kHz VLF transmissions from Rugby (England) made at Visakhapatnam (India) has been carried out. These Sudden Ionospheric Disturbances caused during solar flares are examined in relation to flare time enhancements of X-ray radiation fluxes. It is found that nearly 81% of SPAs recorded have accompanying X-ray enhancements and in 80% of the cases Hα flares and microwave bursts also occur concurrently. Using SPA magnitudes and flare time X-ray flux values, the threshold level of X-ray flux to induce an SPA has been estimated as 1.5×10-3 ergs/cm2/s. In majority of the events, the change in reflection height during the flare is observed to be less than 4 kms. Other SPA characteristics like onset times, growth and relaxation times etc. have also been studied. These results are consistent with those obtained for other propagation paths.
