Abstract
Sudden fmin enhancements (SFmE's) and sudden cosmic noise absorptions (SCNA's) associated with increments of X-ray fluxes during solar flares are studied on the basis of X-ray flux data measured by SOLRAD 9 and 10 satellites. Some statistical analyses on SFmE's observed at five observatories in Japan, corresponding to increased X-ray fluxes in the 1-8Å band are made for 50 solar flare events during the period January 1972 to December 1973, and value of fmin is expressed as functions of cos χ (χ; solar zenith angle) and 1-8Å band X-ray flux. Similar study is also made for SCNA's observed by 30MHz riometer at Hiraiso for 15 great solar flare events during the same period, together with 27.6MHz riometer data reported by SCHWENTEK (1973) and 18MHz data published by DESHPANDE and MITRA (1972b). It is found that fmin value (MHz) and SCNA value (L, dB) of a radio wave with frequency f (MHz) are related to X-ray flux (F0, erg cm-2sec-1) in the 1-8Å band and to cos χ, by following approximate expressions,
fmin (MHz)=10F01/4cos1/2χ,
L (dB)=4.37×103f-2F01/2cos χ,
respectively. Blackout seems to occur for F0 values causing fmin's greater than about 5MHz. It is shown that these expressions can be derived from a brief theoretical calculation of radio wave absorption in the lower ionosphere. Also it is suggested that threshold X-ray fluxes in the 1-8Å band which may produce a minimum SFmE (2MHz), blackout and minimum SCNA (0.27-0.36dB for 30MHz noise) are 1.6×10-3, 6.2×10-2 and (3-8)×10-3erg cm-2sec-1, respectively, for cos χ=1.
