Abstract
As of January 1981, it has passed a decade since we began phase measurements of frequency standard VLF signals. So we report statistical properties of their sudden phase anomalies (SPA's). According to statistics of SPA events for eleven years from 1971, SPA's have the following characters; (1) SPA's are observed only in the day-side hemisphere, build up rapidly (1 to 10min.) and recover slowly (30min. to 3hr.). (2) Time variations of SPA'S correlate well with those of the flux changes of solar X-ray bursts. (3) Changes in the reflection height are from 0.2 to 20km. But height changes for moderate SPA's are no more than 6km. (4) As the correlation between the flux densities of solar X-ray bursts and the height changes at the time of SPA events is very good, we can estimate the flux density of a solar X-ray burst at the ground using SPA's data, which cannot be observed at ground level because of the absorption by the atmosphere.
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