High Altitude Observatory, Boulder, USA
"Irradiance Variability due to Magnetic Activity"
Part of our process to understand solar radiative variability is the construct of a physics-based scheme for computing contemporary irradiances as well as those in the past and, perhaps, in the future. Our observational picture of the Sun and the changes in its atmosphere point to reasonable approaches to a relatively intractable problem. Properties of the Sun's radiative variability from the XUV to the IR will be reviewed. The relationship of this spectrum of variability to the spatial and thermodynamic properties of different parts of the solar atmosphere demonstrates the need to use quite different approaches to
computation of the output radiation from the photosphere as compared to the outer layers. Variation of spatial structure from the photosphere into the corona limits the use of plane parallel model atmospheres to computation of photospheric and chromospheric radiation. Current work using a composite solar
atmosphere containing observation-based models sunspot, faculae, network, and cell regions gives very reasonable estimates of absolute intensities from the photosphere and chromosphere. I stress the importance of the “continuum” spectrum in understanding photospheric radiation as the dominant
contributor to the total solar irradiance. Computations of the continuum spectrum give insight into how different solar structures associated with activity affect the output spectrum and, hence, the total irradiance. In this approach, the distribution of active regions around the Sun as it rotates results in radiative variability comparable to that observed. Current controversies about “dark” faculae, variability of the quiet Sun, and modeling the inhomogeneous, dynamic solar atmosphere will be discussed. Since
irradiances are measures of solar radiation in the plane of the ecliptic, the more global question is : How does the Sun’s luminosity vary?