Extreme Ultraviolet Flashes Of Solar Flares Observed Via Sudden Frequency Deviations

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.

This report describes a radio investigation of traveling ionospheric disturbances carried out near Boulder, Colorado, over a 1-year period from June 1967 to June 1968. The three-dimensional motions of F2 layer disturbances were measured by the high frequency Doppler technique with spaced transmitters and at several probing frequencies. Horizontal motions were determined by cross-correlating three signals on frequencies near 5 MHz, whose reflection points were approximately at the corners of a horizontal equilateral triangle with 40-km sides. Vertical motions were determined from cross-correlation of signals on frequencies of 3.3, 4.0, and 5.1 MHz, whose reflection points were aligned vertically.