Extreme Ultraviolet Flashes Of Solar Flares

High time and intensity resolution satellite measurements of X-ray and extreme ultraviolet (EUV) radiation during solar flares are studied to determine the wavelength dependence of the flare radiation responsible for sudden frequency deviations (SFD). SFD's measure the flare-induced effects in the E and F1 regions of the ionosphere and are in effect like a broadband (1-1030 Å) detector for impulsive flare enhancements. He II 303.8 Å, O V 629.7 Å, H Ly [upsilon] 972.5 Å, C III 977.0 Å, and H Ly [alpha] 1215.7 Å were found to have essentially the same time dependence as the total ionizing radiation producing SFD's, except that they decay faster than the net 1-1030 Å radiation. Flare enhancements of Fe XV 284.1 Å, Fe XVI 335.3 Å, Si XII 499.3 Å. Mg X 625.3 Å, and Ne VIII 770.4 Å, which are normally coronal lines, appear to have a much slower time dependence than the radiation responsible for SFD's. X-rays in the 0.5-3 Å range are slightly slower than the radiation responsible for SFD's during the decay stage; 1-8 Å X-ray flares are slower, especially during the decay stages; and 8-20 Å flare radiation enhancements are slower throughout the entire SFD.

Solar X-ray, extreme ultraviolet, H[alpha] and radio emission were studies to determine what solar radiation is best suited for an automatic flare alarm system aboard a satellite for the detection of the start of a solar flare. Although hard X-rays ([lambda] “1 Å), centimeter-wavelength solar radio bursts, and flashes at certain EUV wavelengths usually have faster rise times and peak earlier than soft X-rays in the 2-16 Å range, the data available to date show that on the average the start time of the 2-16 Å X-rays occurs earlier than the start times for these other types of data. The early start times and large percentage increase of 2-16 Å X-rays make this radiation the best suited for the automatic detection of solar flares for the present state of the art of solar radiation measurements.

ATS-satellite observations of sudden increases in total electron content (SITECs) produced by extreme-ultraviolet (EUV) and X-ray bursts of solar flares are presented quantitatively and interpreted qualitatively. Large SITECs for the white-light flare of July 4, 1974, and for the large flares on July 5, September 10, and September 19, 1974, are illustrated. The time rate of change of total electron content dN (subscript)T/dt was directly compared with SFD (Sudden Frequency Deviations) measurements. The ATS-6 dN(subscript)T/dt measurements are essentially transionospheric SFD measurements. The main difference with respect to ground-based SFD measurements is that, in addition to the 100 to 200 km altitude range where ionospheric electron loss rates are high, the ATS-6 measurements also observe the low loss-rate F2 region. Because of this low ionization loss rate, dN(subscript)T/dt includes more of the slow radiation effects and proceeds to a negative decay phase much later than the ground-based SFDs.