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R.K. Ulrich, D.G. Parker, J.M. Pap (Dept. of Physics \& Astronomy, UCLA)
Weak magnetic fields on the solar surface are measured using the Babcock magnetograph system at the 150-foot solar tower on Mt. Wilson. The coverage of the solar surface by weak fields can be quantified in terms of a distribution function which gives the fraction of the solar surface covered by fields in small field strength bands. Observations of these fields over the past solar cycle shows that the gaussian core width of the distribution function decays after the strong magnetic fields have passed their maximum. The delay is approximately 18 months. The new cycle has first appeared as a strengthening of the distribution function wings. This strengthening then migrates to the core and finally increases the core width. This behavior together with the amplitude of core variation suggests that the weak fields arise from the strong fields with a lag time of 10 to 20 years. A forced oscillator model with the strong fields as the driver and a decay time of 15 years yields the conclusion that the weak fields could have systematically increased in strength during the 20th century. Attribution of one third of the solar cycle luminosity variation to this component suggests that the weak fields may have played a role in the global temperature rise since 1910.
This research is supported by NASA, ONR and NSF through a series of grants which began at UCLA in 1986. The 150-foot tower telescope was built on Mt. Wilson under the direction of G.E. Hale. The digital form of magnetograph was developed by R.F. Howard.
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