Correlation of spectral solar irradiance with solar activity as measured by VIRGO

Abstract

Context. The variability of solar spectral irradiance (SSI) over the rotational period and its trend over the solar activity cycle are important for understanding the Sun-Earth connection as well as for observational constraints for solar models. Recently the Spectral Irradiance Monitor (SIM) experiment on the Solar Radiation and Climate Experiment (SORCE) has published an unexpected negative correlation with total solar irradiance (TSI) of the visible spectral range. It is compensated by a strong and positive variability of the near UV range. Aims. We aim to verify whether the anti-correlated SIM-trend in the visible can be confirmed by independent observations of the Variability of solar IRadiance and Gravity Oscillations (VIRGO) experiment on the SOlar and Heliospheric Observatory (SOHO) satellite. The challenge of all space experiments measuring solar irradiance are sensitivity changes of their sensors due to exposure to intense UV radiation, which are difficult to assess in orbit. Methods. We exclude the first six years prior to 2002 where one or more fast processes contributed to instrumental changes and analyse a ten-year timeseries of VIRGO sun photometer data between 2002 and 2012. The variability of SSI is correlated with the variability of the TSI, which is taken as a proxy for solar activity. Results. Observational evidence indicates that after six years only one single long-term process governs the degradation of the backup sun photometer in VIRGO which is operated once a month. This degradation can be well approximated by a linear function over ten years. The analysis of the residuals from the linear trend yield robust positive correlations of spectral irradiance at 862, 500, and 402 nm with total irradiance. In the analysis of annual averages of these data the positive correlations change into weak negative correlations, but with little statistical significance for the 862 nm and 402 nm data. At 500 nm the annual spectral data are still positively correlated with TSI. The persisting positive correlation at 500 nm is in contradiction to the SIM results.

Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI, and SDO/HMI observations

“…As visibly evident in the figure, SIM SSI does not exhibit any consistent relation to the solar cycle in any of the wavelength intervals, in conflict with the solar cycle modulation evident in the reconstruction, certain records of ultraviolet solar irradiance (see e.g., DeLand & Cebula 2008, and Fig. 15) and VIRGO SPM photometry (Wehrli et al 2013).…”

“…In the case of SSI, gaping disagreements between models and observations persist, exemplified in the current debate on the overall trends in the SORCE/SIM record Lean & DeLand 2012;Unruh et al 2012;Wehrli et al 2013;Ermolli et al 2013). SIM registered a decline in ultraviolet flux (200 to 400 nm) between 2004 to 2008 that is a factor of two to six greater than in other measurements and models.…”

Solar irradiance observations with PREMOS filter radiometers on the PICARD mission: In-flight performance and data release

“…11-year solar cycle) is highly uncertain because of the accumulated uncertainty. There are indeed conflicting trends between SIM observations and VIRGO data (Wehrli et al 2013) and between SSI observations and semi-empirical models, such as SATIRE-S, over the whole spectrum (see Yeo et al 2014, for a review).…”

Solar rotational modulations of spectral irradiance and correlations with the variability of total solar irradiance

“…The variability of SSI and its connection with TSI were explored with satellite measurements and models (Deland & Cebula 2012;Lean & Deland 2012;Wehrli et al 2013;Ball et al 2011Ball et al , 2014and references therein). Some models (e.g.…”

“…Although solar variability in many different timescales is documented (Lean 1991(Lean , 1997Fröhlich & 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. In particular, significant differences in SSI variability between observations and models (Ermolli et al 2013) indicate that there is still uncertainty in our knowledge of how the SSI varies over different timescales and wavelengths.…”