Book excerpt: 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.