Four Solar Cycle Spectrum Variation of the Sun-as-a-Star

The chromospheric activity of the Sun is governed by the magnetic field anchored in the photosphere. The solar cycle 23 was a smaller cycle compared to the recent cycles, in terms of sunspot number and total disk integrated magnetic field. Comparison of the chromospheric lines in past cycles may provide insight of the effect of magnetic field on solar atmosphere. In this paper, we study the dependence of chromospheric activity on magnetic field of the Sun-as-a-star in four solar cycles during 1977-2018. The study is conducted by merging the data obtained by Dr. W. Livingston and the observations by Integrated Sunlight Spectrometer (ISS) and Vector Spectromagnetograph (VSM) of Synoptic Optical Long-term Investigations of the Sun (SOLIS) instrument. The chromospheric activity is measured as the line depth and equivalent width (EW) of spectral lines in Hα, He I 10830 nm, Ca II 854.2 nm, Ca II H and K, and Na D I 589.6 nm obtained with the ISS. The full disk mean total magnetic flux (FDMTMF) observed with the VSM is used as the measure of magnetic activity of the Sun. The equivalent width of Ca II K and He I 10830 nm measured by Livingston along with the Magnetic Plage Strength Index (MPSI) value and a Mount Wilson Sunspot Index (MWSI) obtained with 150-Foot Solar Tower in Mt. Wilson Observatory are used to further study the relationship between the magnetic field and chromospheric activity.