The variability of the sun's ultraviolet radiation

Solar UV intensity variations determined by direct irradiance measurements and by measurements of the plage/quiet sun contrast are reviewed. Typical values for the solar UV variability over a solar cycle are: less than 1% at wavelengths longer than 2100 A, 8% at 2080 A (continuum), 20% at 1900 A (continuum), 70% at H Ly alpha, 200% in certain emission lines in the 1200-1800 A range, and more than a factor of 4 in coronal lines at less than 1000 A.