The variability of the sun's radiation in the UV spectrum is far greater than in its aggregate output. The UV light measured by SUSIM originates in the upper photosphere, chromosphere, and corona of the sun. Generally, the longer the wavelength, the lower in the solar atmosphere it originates. Strong absorption lines (such as Ca II) are exceptions to this rule, originating at much higher altitudes than the surrounding spectrum. Various ground based indices of solar activity such as sunspot number and fluxes at various radio frequencies have been shown to correlate with aspects of the UV spectrum. Measurements of the solar UV spectrum through time help our understanding of the processes at the various altitudes in the solar atmosphere and their connection to other solar indices and events.