MgII index datasets discussed in this paper. The top three blue bars are the Solar Backscatter Ultraviolet (SBUV/2) instruments on NOAA 16, 17, and 18. The purple bar shows the time range covered by SORCE SOLSTICE. The bottom three bars (light blue, peach, and green) show the observational time coverage for GOME, SCIAMACHY, and GOME-2 respectively. An arrowhead on a colored bar indicates that the dataset continues to the present day, or in the case of GOME, it indicates that the dataset begins before the earliest date on the plot.