Understanding how human activity alters the Earth's atmosphere requires the ability to distinguish between 'forced' and natural drivers of climate change. Changes in the solar spectral irradiance, i.e. changes in the Sun's brightness at different wavelengths, are significant due to their influence on the chemistry of the Earth's upper atmosphere.

The solar surface magnetic field, which is responsible for solar irradiance variations, manifests itself in e.g. sunspots and faculae. While the effects of sunspots and faculae are reasonably well modelled and dominate solar irradiance changes on timescales of days to weeks, the most important outstanding issues are the contribution of small-scale magnetic elements (thought to be responsible for long-term irradiance changes) and a physics-based parameterisation of the contrast of magnetic features with magnetic flux. The direct measurement of flux contrasts of small-scale magnetic elements is very difficult, and is, even for space instruments, affected by scattered light and instrument defocus.

We therefore use an approach via simulations: as no observed contrasts for small-scale magnetic flux elements are available, we start from 3-D MHD numerical simulations of the solar atmosphere that include the interaction between convection and magnetic flux tubes (MURaM). To improve SSI reconstructions, the primary objective of this project is the determination of the brightness of small-scale magnetic elements (network and faculae) as a function of magnetic flux, wavelength, and position on the Sun. To obtain the network and facular brightness, we calculate emergent intensities from the MURaM simulations using the spectral synthesis code ATLAS9 (Kurucz 1992). This is done along different sightlines that represent different solar disk positions. An example for simulations at different solar disk positions is given in Figure 1, where the effect of facular brightening towards the limb is illustrated.

Fig. 1: Facular contrast at 270 nm for a MURaM simulation (with average magnetic field of 50G): The contrast increases towards the limb (i.e. towards the left in the figure, note the foreshortening effects) since the faculae appear brightest near the limb, where the hot walls are well seen.

We validate the simulated results by comparison with observations. Highly spatially resolved filtergrams are obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the HINODE spacecraft. Preliminary results at disk centre look promising (Afram et al. 2010, arXiv:0910.0976v1), see Figure 2.

Fig.2: MURaM simulation snapshot (at 450nm, average magnetic field of 50G, limb angle μ=0.9) with original resolution (left) and after degrading with the PSF of the optical system (middle), and comparison with an extract of a (quiet Sun) Hinode observation (right),  observed at 450nm and at a limb angle of μ=0.9. The approximate image size is 7.5 arcsec x 8.3 arcsec.

Text by Nadine Afram.