How much of the solar irradiance variations is caused by the magnetic field at the solar surface?

The contribution to total solar irradiance variations by the magnetic field at the solar surface is estimated. Detailed models of the irradiance changes on the basis of magnetograms show that magnetic features at the solar surface account for over 90% of the irradiance variations on a solar rotation time scale and at least 70% on a solar cycle time scale. If the correction to the VIRGO record proposed by Fröhlich & Finsterle (2001) is accepted, then magnetic features at the solar surface are responsible for over 90% of the solar cycle irradiance variations as well.     

Modelling solar irradiance variations: Comparison with observations,including line-ratio variations

Solar irradiance variations show a strong temporal and spectral dependence. The progression of the Sun through its activity cycle as well as solar rotation are mirrored in the irradiance variations. The spectral dependence is such that the variations are several magnitudes larger in the EUV than in the visible or infrared.We present a simple 3-component model that is based on the assumption that changes in the solar flux are exclusively due to changes in spot and facular coverage. We compare our model to observations of the spectral solar irradiance variations.Despite its simplicity, we find that the agreement between our model and the observations is surprisingly good. We also explore the reliability and the limitations of our approach by comparing observations of the solar facular contrast and of the changes in spectral line depths with our calculations.     

Reconstruction of Past Solar Irradiance

Accurate measurements of solar irradiance started in 1978, but a much longer time series is needed in order to uncover a possible influence on the Earth's climate. In order to reconstruct the irradiance prior to 1978 we require both an understanding of the underlying causes of solar irradiance variability as well as data describing the state of the Sun (in particular its magnetic field) at the relevant epochs.Evidence is accumulating that on the time-scale of the solar cycle or less, variations in solar irradiance are produced mainly by changes in the amount and distribution of magnetic flux on the solar surface. The main solar features contributing to a darkening of the Sun are sunspots, while active-region faculae and the network lead to a brightening. There is also increasing evidence for secular changes of the solar magnetic field and the associated of solar brightness variability. In part the behavior of sun-like stars is used as a guide of such secular changes.Under the assumption that solar irradiance variations are due to solar surface magnetism on all relevant time scales it is possible to reconstruct the irradiance with some reliability from today to around 1874, and with lower accuracy back to the Maunder minimum. One major problem is the decreasing amount and accuracy of the relevant data with age. In this review the various reconstructions of past solar irradiance are presented and the assumptions underlying them are scrutinized.     

Modelling Short-Term Spectral Irradiance Variations

On time-scales of the solar rotation most of the solar irradiance variations are caused by the changing distribution of solar surface magnetic features. We model these short-term irradiance variations using calculations of sunspot and facular contrasts as a function of wavelength and limb angle on the Sun. The position of active regions on the solar disc is derived from the MDI magnetograms. The reconstructed irradiance variations are compared with total and spectral irradiance measurements obtained by the VIRGO experiment on SOHO.