EUV Flux Variation During End of Solar Cycle 20 and Beginning Cycle 21, Observed from AE-C Satellite

Minimum levels of solar EUV fluxes appear to have occurred about 14 months before July 1976, the month designated as the start of Cycle 21 (minimum of monthly mean value of sunspot number, R/sub Z/). The EUV minimum, observed by the EUVS experiment on AE-C, occurred around 20 April 1975 under conditions of a spotless disk and of a simultaneous minimum in F10/sub ./7. Another rise of quiet-disk EUV, observed a few months before July 1976, was found in the low-excitation lines such as 584 A He I and 1026 A H Ly-β. The transition-region lines of Fe X at 174.5 and 177.2 A showed a substantial gradual rise later, toward the end of 1976 and mainly in early 1977, when periods of spotless disk with simultaneously minimal F10/sub ./7<70 no longer existed. EUV fluxes for days of ''high'' activity (R/sub Z/>30 with F10/sub ./7>80) on different sides of the sunspot minimum show drastically different relations to F10/sub ./7