Abstract
Aims: We characterize the solar rotational modulations of spectral solar irradiance (SSI) and compare them with the corresponding changes of total solar irradiance (TSI). Solar rotational modulations of TSI and SSI at wavelengths between 120 and 1600 nm are identified over one hundred Carrington rotational cycles during 2003-2013. Methods: The SORCE (Solar Radiation and Climate Experiment) and TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)/SEE (Solar EUV Experiment) measured and SATIRE-S modeled solar irradiances are analyzed using the EEMD (Ensemble Empirical Mode Decomposition) method to determine the phase and amplitude of 27-day solar rotational variation in TSI and SSI. Results: The mode decomposition clearly identifies 27-day solar rotational variations in SSI between 120 and 1600 nm, and there is a robust wavelength dependence in the phase of the rotational mode relative to that of TSI. The rotational modes of visible (VIS) and near infrared (NIR) are in phase with the mode of TSI, but the phase of the rotational mode of ultraviolet (UV) exhibits differences from that of TSI. While it is questionable that the VIS to NIR portion of the solar spectrum has yet been observed with sufficient accuracy and precision to determine the 11-year solar cycle variations, the temporal variations over one hundred cycles of 27-day solar rotation, independent of the two solar cycles in which they are embedded, show distinct solar rotational modulations at each wavelength.
Highlights
While it is questionable that the VIS to near infrared (NIR) portion of the solar spectrum has yet been observed with sufficient accuracy and precision to determine the 11-year solar cycle variations, the temporal variations over one hundred cycles of 27-day solar rotation, independent of the two solar cycles in which they are embedded, show distinct solar rotational modulations at each wavelength
While continued calibration and validation is needed in order to improve the accuracy and precision to determine low frequency of solar variations, i.e., the 11-year solar cycle, the amplitudes and phases of 27-day solar rotational variation can be determined in a relatively short period compared to the 11-year solar cycle
The SORCE SOLSTICE is a follow-on to the SOLSTICE on the Upper Atmospheric Research Satellite (UARS; Rottman 1999; Floyd et al 2003) and it measures the daily solar spectral irradiance in UV from 115 to 320 nm with a resolution of 0.1 nm, an absolute accuracy of better than 5%, and a relative accuracy of 0.5% per year (McClintock et al 2005a, 2005b; Snow et al 2005).The SOLSTICE measurements are made using a pair of identical spectrometers, SOLSTICE A and SOLSTICE B
Summary
Solar variability in many different timescales is documented (Lean 1991, 1997; Fröhlich & Lean 2004; Solanki et al 2005; Unruh et al 2008; Domingo et al 2009; Krivova et al 2011; Lean & DeLand 2012; Wehrli et al 2013; Woods et al 2015; and references therein), the exact magnitude and the spectral dependence of variations relative to that of total solar irradiance (TSI) are still in debate. While continued calibration and validation is needed in order to improve the accuracy and precision to determine low frequency of solar variations, i.e., the 11-year solar cycle, the amplitudes and phases of 27-day solar rotational variation can be determined in a relatively short period compared to the 11-year solar cycle. These values are less sensitive to instrument degradation uncertainties. To determine and compare the TSI and SSI variation on rotational timescales, we apply Ensemble Empirical Mode Decomposition (EEMD; Huang et al 1998) and extract the temporal variation of total and spectral irradiance
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