The Solar Ultraviolet Spectrum Estimated Using theMgii Index and CaiiK Disk Activity

• Published in: Solar physics 2011-04, Vol.269 (2), p.253-267
• Main Authors: Morrill, J S, Floyd, L, McMullin, D
• Format: Article
• Language: English
• ISSN: 0038-0938; 1573-093X
• DOI: 10.1007/s11207-011-9708-7

As part of a program to estimate the solar spectrum back to the early twentieth century, we have generated fits to UV spectral irradiance measurements from 1-410nm. The longer wavelength spectra (150-410nm) were fit as a function of two solar activity proxies, the Mgii core-to-wing ratio, or Mgii index, and the total Caii K disk activity derived from ground based observations. Irradiance spectra at shorter wavelengths (1-150nm) where used to generate fits to the Mgii 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 derived from 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 150nm. To generate fits between solar irradiance and solar proxies, this study uses the above irradiance data, the NOAA composite Mgii index, and daily CaiiK disk activity determined from images measured by Big Bear Solar Observatory (BBSO). In addition to the fitting coefficients between irradiance and solar proxies, other results from this study include an estimated relationship between the fraction of the disk with enhanced CaiiK activity and the Mgii index, an upper bound of the average solar UV spectral irradiance during periods where the solar disk contains only regions of the quiet Sun, as was believed to be present during the Maunder Minimum, as well as results indicating that slightly more than 60% of the total solar irradiance (TSI) variability occurs between 150 and 400nm.