Temporal variations of solar EUV, UV, and 10,830‐Å radiations

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Abstract
The temporal characteristics of the full‐disk chromospheric EUV fluxes agree well with those of the ground‐based measurements of the chromospheric He I absorption line at 10,830 Å and differ systematically from those of the coronal EUV and 10.7‐cm flux. The ratio of the flux increase during the rise of solar cycle 21 to that during solar rotation variations is uniformly high for the chromospheric EUV and corroborating 10,830‐Å fluxes, highest for the transition region and “cool” coronal EUV fluxes (T < 2×106 °K), and lowest for the “hot” coronal EUV and 10.7‐cm flux. The rise and decay rates of episodes of major activity progress from those for the hot coronal EUV lines and the 10.7‐cm flux to slower values for the chromospheric H Lyman alpha line, 10,830‐Å line, and photospheric 2050‐Å UV flux. We suggest that active region remnants contribute significantly to the solar cycle increase and during the decay of episodes of major activity. The ratio of power in 13‐day periodicity to that for 27 days is high (1/3) for the photospheric UV flux, medium (1/6) for the chromospheric EUV and 10,830‐Å fluxes, and small to negligible for the hot coronal EUV fluxes. These ratios are used to estimate the dependence of active region emission on the solar central meridian distance for chromospheric and coronal EUV fluxes.