Variability in Irradiance and Photometric Indices During the Last Two Solar Cycles

The Total Solar Irradiance (TSI) primarily varies on an 11-year time scale and is governed by features such as sunspots and associated decay products such as plage and faculae. These short-lived physical features can also modulate the solar irradiance at intermediate and short temporal scales. Here we investigate the periodic variations, at solar-surface-rotation time scales, of photometric indices derived from images obtained at the San Fernando Observatory (SFO), and we compare them to the properties of the contemporaneous TSI as measured by the Total Irradiance Monitor (TIM) onboard the SOlar Radiation and Climate Experiment (SORCE) spacecraft. Both the daily ground- and space-based data, which span from early 2003 to late 2018, present missing pixels. We use an autoregressive gap-filling method to construct continuous time series to be analyzed via Fourier and wavelet spectral techniques. Lomb–Scargle periodograms, which can handle time series with missing data, are used for comparison. Both the Fourier spectral power and the periodograms yield compatible results with statistically significant periodicities in the range 25 – 35 days. All of the time series have maximum power at 27 days. Significant secondary periods are found at 29 – 30 days and 34 – 35 days. Wavelet analyses of the full time series show that the photometric index resulting from the red-continuum photometric sum [ Σ r ] and the TSI exhibit common high power at surface-solar-rotation scales during the active part of the solar cycle. The phase relation at the surface-solar-rotation scales is not definite. During the solar minimum interval between Solar Cycles (SCs) 23 and 24, variations in the TSI are found to be related to variations both in the photometric index Σ K , calculated from Ca ii K-line photometric sums and in the magnetic flux in the solar activity latitudinal band (as found in previous work). This suggests that the TSI changes during the minimum are caused by the reduced line-blanketing effect of diffused magnetic field.