The Solar Ultraviolet Spectrum Estimated Using the Mg II K Index and Ca II K disk Activity

As part of a program to estimate the solar spectrum backward in time to the early twentieth century, we have generated fits to UV spectral irradiance measurements (150 - 410 nm) as a function of two solar activity proxies, the Mg II core-to-wing ratio, or Mg II index, and the total Ca II K disk activity derived from ground based observations. In addition, irradiance spectra at shorter wavelengths (1 - 150 nm) where used to generate fits to the Mg II core-to-wing ratio alone. Two sets of spectra were used in these fitting procedures. The fits at longer wavelengths (150 to 410 nm) were based on the high resolution spectra taken by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on the Upper Atmospheric Research Satellite (UARS). Spectra measured by the Solar EUV Experiment (SEE) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite were used for the fits at wavelengths from 1 to 150 nm. To generate fits between solar irradiance and solar proxies, this study uses the above irradiance data, the NOAA composite Mg II index, and daily Ca II K disk activity determined from images measured by Big Bear Solar Observatory (BBSO). Among the results of this study is an estimated relationship between the fraction of the disk with enhanced Ca II K activity and the Mg II index, an upper bound of the average solar UV spectral irradiance during periods of pure quiet sun as was believed to be present during the Maunder Minimum, and results indicating that more than 60 % of the Total Solar Irradiance (TSI) variability occurs between 150 and 400nm. In this presentation we will discuss the results of this study and the implications for estimating UV spectra for use in long-term climate models.