Photometric quantities for solar irradiance modeling

We analyze photometric quantities for the modeling of the total solar irradiance, S . These quantities are derived from full‐disk solar images taken at the San Fernando Observatory. We introduce a new quantity, the photometric sum , Σ, which is the sum over an entire image of each pixel's contribution to the irradiance in that image. Σ combines both bright and dark features; and because the sum is over the entire image, it will include low contrast features that cannot be identified directly. Specifically, we examine Σ r , Σ b , and Σ K , the photometric sums over broadband red, broadband blue, and 1‐nm bandpass Ca II K images, respectively. Σ r and Σ b measure the effects of solar features on the variability in S at two different continuum wavelengths. Σ K measures the variability in spectral lines due to solar features. We find that Σ r and Σ b have no long‐term trend. Σ K , however, varies in phase with the solar cycle. We carry out several multiple linear regressions on the value of S from cycle 22; the best fit uses Σ r and Σ K and reproduces the observed composite S with a multiple regression coefficient R = 0.96. We conclude that the long‐term change in S over the solar cycle can be accounted for by the variability in the spectral lines as measured by Σ K , assuming no change in the quiet Sun; the contribution of the continuum to the variations in S is only on active region timescales.