Observations of the Sun at Ultraviolet Wavelengths: 1 to 400 nm
Gary Rottman, Laboratory for Atmospheric and Space Physics


4. Future Observing Programs

We now have a record of the variation of total solar irradiance, and of the variation of its ultraviolet spectral component, over two complete solar cycles. However these observations represent only a snapshot of the long term behavior of the Sun, and it is essential that such measurements be continued. This extended data base will allow us to more fully understand the nature of the Sun and the physically processes underlying its variability. However, the spectral knowledge that we have of solar variability is limited to wavelengths short of 300 nm. In the visible and near infrared we can place upper limits on solar variability, limits set for the most part by observations of total solar irradiance. Since TSI varies only on the order of 0.1%, it is difficult to reconcile spectral changes much different than this value. It is just as unlikely that the spectral character of the variations is "white" and invariant in wavelength. Theories and modeling of solar radiation speak to a structured solar variation, and to address and constrain such theories will require visible and near infrared observations. The reason that more is presently known about the ultraviolet variability is not that the UV measurements are better, but only that their solar variations are so much larger. Past spectral observations have been up to the task of measuring one to a few percent changes, but they not been able to record long term changes at the fraction of a percent level. This is the challenge for future observations, and techniques based on super-sensitive electrical substitution radiometers (ESR's) will soon be applied to these observations. The next generation of irradiance instruments should provide the spectral details underlying the envelope of the TSI.

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