Sudden frequency deviations induced by solar flares

Abstract

The sudden frequency deviation (SFD), an effect induced on high-frequency radio waves by solar flares, is studied in detail. The method of identifying and analyzing SFD's and related theories on absorption, recombination, and the shape of SFD's are described. More than 5200 hours of continuous and simultaneous recordings of the instantaneous frequencies of four long-distance stable-frequency transmission paths were used. The percentage of solar flares accompanied by SFD's is a function of the size (or class) of the solar flare and the solar zenith angle (or local time), but does not show dependence on the season of the year or the heliographic location of the solar flares. The maximum amplitude of a SFD usually precedes that of an optical solar flare for large flares and lags for subflares. The several peaks in a SFD are interpreted as due to successive energy outbursts from the sun during a solar flare. The period between successive bursts varies from 1/2 to 4 minutes, the average interval being 2 minutes. From the recombination time constant of the enhanced ionization, and the change of absorption of the received signal, the enhanced ionization is estimated to occur between 120 and 200 km in the ionosphere. SFD's are found to be well correlated with centimeter-wave impulsive bursts of high intensity, meter-wave fast-drift bursts, and high-energy X-ray bursts, all of which usually occur during the first phase of solar flares. With respect to duration, recovery time constant, time of maximum, and height of enhanced ionization, SFD's are distinguished from other flare-associated ionospheric effects on radio waves, such as SID's, SCNA's, SEA's, and SPA's, all of which usually occur during the second phase of solar flares.