Analysis of NASA/NSO spectromagnetograph observations for comparison with solar irradiance variations

The NASA/NSO Spectromagnetograph (SPM) at the National Solar Observatory/Kitt Peak Vacuum Telescope (NSO/KPVT) has produced daily full-disk solar images of line-of-sight (LOS) magnetic flux, LOS velocity, continuum intensity, equivalent width, and central line depth in either Fe I 550.7 nm or Fe I 868.8 nm since 1992 April. Internally, these five images are strictly cospatial and cotemporal. This paper describes reduction techniques for removing spurious effects (principally from clouds and optical vignetting) from the images and presents summary statistics from the first 2 years of operation. Pairs of bivariate histograms as well as five-dimensional histograms are formed from the "cleaned" images. Factor analysis of various summary statistics from the histograms shows that mixed-polarity magnetic elements of weak to moderate strength; unipolar magnetic features, including active regions and strong enhanced network; and sunspots are the most important sources of variance in the SPM data. Multiple regressions of spacecraft measurements of bolometric solar irradiance on the dominant factors, however, yield strong correlations with only unipolar regions and sunspots; the weak and mixed-polarity features that account for most of the SPM variance are only weakly correlated with the spacecraft measurements.