6.5.4 Conclusions

In this paper we have applied a new method, following the original idea of Vigouroux and Delache (1994), to evaluate and understand the uncertainties in solar total and UV irradiance at 280 nm represented by the Mg c/w ratio, and magnetic activity indices, such as the PSI function and the full disk magnetic flux. It has been shown that study of the scatter plot diagrams between daily solar irradiance measurements and their standard deviations provides an important tool to separate the random fluctuations related to instrumental effects from real solar variations.

Our results show that the standard deviations of total solar irradiance depend on the phase of the solar cycle. We have found that the values of the standard deviations in the case of the Nimbus-7/ERB and SMM/ACRIM I total irradiances are larger during maximum solar activity conditions. In contrast, during solar minimum the standard deviations are small and their scatter represents the intrinsic precision of the measurements. The larger values of the standard deviations of total irradiance during solar maximum are attributed to the large daily irradiance fluctuations related to the evolution of active regions.

Our results show that the dispersion diagrams of total irradiance and its surrogates provide a new approach to study the long-term changes over the solar cycle. A cluster structure of these diagrams has been discovered and it is shown that the shape and the compression of these clusters change as a function of the solar cycle. These investigations may provide an important new technique in predicting the time behavior of solar activity indices and estimating the occurrence of the maximum and minimum of solar cycle.

Our results indicate that the length of the solar minimum is different for solar total irradiance and for the Mg c/w proxy of UV irradiance and various measures of solar magnetic activity, such as the full disk integrated magnetic flux and the Photometric Sunspot Index. It has been shown that there is a phase shift between the changes in the solar magnetic flux and solar irradiance at the beginning and the end of solar minimum. In addition, both total solar and UV irradiances start to rise about 8 to 10 months prior to the magnetic flux and PSI at the beginning of the ascending phase of solar cycle 22. The origin of this phase shift is not yet understood. It is an open question whether the current magnetic field measurements cannot recognize faint solar features, such as the network and intranetwork elements, or the discovered phase shift is related to non-linear coupling between the subphotospheric, photospheric and chromospheric layers.

Acknowledgements: The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology and University of California, Los Angeles, under a contract with the National Aeronautics and Space Administration (J.M. Pap) and by the laboratoire Cassini, associé au C.N.R.S. (U.R.A. 1362) of the Observatoire de la Côte d'Azur (A. Vigouroux). The SMM/ACRIM I data used in this study have been produced by the Active Cavity Radiometer Irradiance Monitoring Group at JPL. The Nimbus-7/ERB data were kindly provided by Dr. D. Hoyt. We acknowledge the NASA/GSFC Ozone Processing Team (OPT) for the Nimbus-7 SBUV/1 data and NOAA/NESDIS for the SBUV/2 data, which were kindly provided by L. Puga. The NSO/Kitt Peak magnetic data used here are produced cooperatively by NSF/NOAO, NASA/GSFC and NOAA/SEL and provided by courtesy of Dr. J. Harvey. The useful comments of Drs. R.F. Donnelly, E. Fossat, M. Neugebauer, L. Puga as well as an unknown referee are highly appreciated.

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