Estimating the Mg ii Index from 1961 Through 1981 Using Ca ii K Images from the Mt Wilson Observatory

Abstract

An empirical model of solar UV spectral irradiance has been developed that is based on observed spectral radiance measurements and full disk Ca ii K images. The Mg ii index is then calculated from the estimated spectra in a narrow wavelength range (180 A) near the Mg ii doublet at 2800 A. Our long term goal is to expand this wavelength range from 10 to 4000 A in continuing studies based on spectral data covering this wavelength range (e.g. Skylab, UARS/SUSIM, TIMED/SEE, etc.). Our previous modeling effort produced spectra in this 180 A range and the resulting Mg ii index values for the period from 1991 through 1995 and we have used observations during this time period to validate the model results. The current paper presents results from this model based on a 21-year portion of the recently digitized Ca ii K images from the Mt Wilson Observatory (MWO) film archive. Here we present details of the model, the required model modifications, and the resulting Mg ii index from 1961 through 1981. Since the NOAA Mg ii index did not begin until 1978, the present model results are compared to a Mg ii index estimated from the F10.7 radio flux over this 21-year period. The NOAA Mg ii index, which is derived from measured UV spectra, is also included for comparison from late 1978 through 1981.