A Critical Review of Measurements and Models of Composite Total Solar Irradiance I
Presiding: R C Willson, Columbia University; N Scafetta, Duke University
SH22B-01 10:40h
Review of the current state of theoretical modeling of the total solar irradiance
We review the present state and latest results for theoretical models of the total solar irradiance and certain important irradiance bands. We highlight the ability, or lack thereof, of solar proxy data and models to both represent the irradiance as well as its variability on timescales ranging from days to multiple solar cycles. We also contrast the theoretical models with other types of models that may or may not have a physical basis for the form of the model. We also comment on the degree of redundancy in parts of the solar spectrum and how this could affect what wavelengths and bandpasses in the spectrum are important for observing programs.
SH22B-02 11:00h
Can Changing Sunspot and Facular Areas Reproduce the Amplitude of Total Irradiance Variations?(Look,Mom; No Free Parameters!)
Empirical models of total solar irradiance variation demonstrate a high correlation between observed irradiance fluctuations and the changing areas of spots and faculae. However, the contrast of these structures (especially the faculae) in integrated light is still uncertain. Consequently, the agreement in amplitude of the measured and modeled irradiance time series remains poorly known. Recently, the first measurements of facular contrast in broad - band integrated light were obtained using the balloon -borne Solar Bolometric Imager (Foukal et al., Ap.J. Letts 611,57,2004). These measurements, obtained over approximately the same wavelength range accepted by radiometers such as VIRGO or ACRIM, enable the first reconstruction of the total irradiance expected from spots and faculae, with no free parameters. We compare this reconstruction with the radiometric record to determine whether other contributions besides the darkness of spots and brightness of faculae are required to explain solar irradiance variation, at least over rotational time scales.
SH22B-03 INVITED 11:15h
Non Magnetic Changes in the Total Solar Irradiance
From the available total, spectral and spatial observations of solar irradiance, it is known that the major contributions to the irradiance variations on timescales longer than one day come from dark sunspots and bright features on the solar disc. Both the sunspots and the bright features are characterised by an increased magnetic field strength compared to the solar disc background, hence they can be called magnetic features. The current TSI proxy models use the dark sunspots and the bright features as the two 'magnetic' ingredients to reconstruct the measured TSI variation. Accurate measurements with the DIARAD/VIRGO radiometer of the current solar activity cycle show an increase of the TSI at the beginning of the activity cycle which is much stronger than what is predicted by the TSI proxy models. This indicates that an essential 'non magnetic' ingredient is missing in the TSI proxy models. In this presentation we will quantify the missing term from all available TSI measurements.
SH22B-04 11:30h
Climate Sensitivity of Earth to Solar Irradiance: update II
This paper is a continuation of a study by Douglass, Clader and Knox (DCKI) [1]. In that paper we determined the solar effect on the lower tropospheric global temperature T using the Total Solar Irradiance (TSI) of Frohlich and Lean (FL)[2]. The sensitivity k, determined primarily by the 11 year activity cycle, was found to be twice that expected from a no-feedback Stefan-Boltzmann radiation balance model implying positive feedback. A linear trend of 77mK/decade was also found from that analysis. Since DCKI it has come to our attention that there is another construction of TSI by Willson and Mordvinov (WM)[3]. The WM TSI shows the familiar 11 year cycle but differs from FL in that they find a positive trend for TSI while FL find a negative trend. We now do a new analysis on T using the TSI of WM to determine the differences. We expect the sensitivity k to be nearly the same. However, the linear trend estimate could be significantly reduced. 1. D. H. Douglass, B. D. Clader, and R. S. Knox, Paper presented at 2004 Solar Radiation and Climate (SORCE) meeting on Decade Variability in the Sun and the Climate. See http://arXiv.org/abs/physics/0411002. 2.C. Fröhlich and J. Lean, Geophys. Res. Lett. 25, 4377-4380 (1998). Version 18: http://www.obsun.pmodwrc.ch 3.R. C. Willson and A. V. Mordvinov. Geophys. Res. Lett. 30(5), article 1199 (2003)
SH22B-05 11:45h
Helioseismic Limits on Irradiance Variations
Changes in the frequencies of solar oscillations are the most precise probe of irradiance variations over the solar cycle. Using MDI data, Dziembowski and Goode (2005) showed that f-mode changes arise from the direct effect of the evolving magnetic activity, while p-mode changes are due to small, activity induced changes in convective flows very near to the solar surface (turbulent pressure). The f-modes also sharply limit the allowed field growth with activity, and the limit is consistent with the observations of Lin and Rimmele (1999). Combining MDI data with BBSO Ca II K, we find the Sun is smooth at activity minimum and becomes increasingly corrugated with rising activity. The overall physical picture is one in which the Sun is hottest and smoothest at activity minimum, and becomes cooler, more corrugated and irradiant with rising activity. These results place a lower limit on irradiance variations and are roughly consistent with a picture of Spruit (2000).