Abstract
NASA/National Solar Observatory Spectromagnetograph (SPM) data are compared with spacecraft measurements of total solar irradiance (TSI) variations for 8 yr beginning with the declining phase of solar cycle 22 and extending into the maximum of cycle 23. Previously reported conclusions based on a similar comparison for a shorter time period appear to be robust: three factors (sunspots, strong unipolar regions, and strong mixed-polarity regions) describe most of the variation in the SPM record, but only the first two are associated with TSI. Additionally, the residuals of a linear multiple regression of TSI against SPM observations over the entire 8 yr period show an unexplained, increasing, linear time variation with a rate of about 0.05 W m-2 yr-1. Separate regressions for the periods before and after 1996 January 1 show no unexplained trends but differ substantially in regression parameters. This behavior may reflect a solar source of TSI variations beyond sunspots or uncompensated nonsolar effects in one or both of the TSI and SPM data sets.
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