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Modeling the variations.

  Figure:   The total irradiance measurement for the Sun from the ACRIM I satellite (solid) together with proxy models (broken) which account for sunspot deficits (over the relevant period), faculae excesses, and other effects. Note how poorly the proxy model performs at solar maximum - the most crucial period for energy output variations.

The first modeling efforts have focused on the total solar irradiance, the quantity that has been measured by satellites for a number of years. Because the largest contribution to the total irradiance comes from the visible wavelength bands of the electromagnetic spectrum, and since the visible radiation arises from on or below the solar surface, then changes in features on the surface of the Sun will contribute most directly to variability. The variability that occurs on time scales of days to months has been successfully explained in terms of the effects of active regions (a term which denotes any region of larger-than-average magnetic field on the solar surface).

Modeling variability on the decadal solar cycle time scale, in addition to the active-region effect, requires another component associated with global faculae, the global magnetic network, and the like. The explanation of this component is more controversial, and the existent observations do not fully resolve this controversy. In any event, the amplitude of this effect, just like the short-time-scale activity-related modulation, is small, not exceeding the 0.1% level in the total radiation variance referred to above. At present these models, while reproducing the overall variation in the total solar irradiance record, still contain large discrepancies with the measurements, especially when the Sun is most active.

If the Sun does have a definitive role to play in influencing climate, it should be in terms of a third mechanism operating on still longer time scales (perhaps centuries). Unlike the variations which reside on the solar surface, this variation will be more deeply seated within the Sun, and the resulting variability will likely affect all global parameters (e.g., luminosity and radius). While the direct record of solar irradiance is currently inadequate to show this longer variation, if it exists, it is likely to be far more significant for global climate change on the Earth.

An equally important modeling effort is that of variations of the irradiance as a function of wavelength: the spectral irradiance. The ultraviolet and extreme ultraviolet regions of the spectrum are of fundamental importance to studies of the Earth's upper atmosphere and its chemistry (particularly ozone production and destruction). At present, spectral irradiance studies are limited by incomplete datasets, often with either large uncertainties or poor wavelength coverage or both.

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Approved by Peter Fox
Last revised: Wed May 17 11:28:21 MDT 2000