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We study the evolution of the observed photospheric magnetic field and themodeled global coronal magnetic field during the past 3 1/2 solar activitycycles observed since the mid-1970s. We use synoptic magnetograms andextrapolated potential-field models based on longitudinal full-diskphotospheric magnetograms from the NSO's three magnetographs at Kitt Peak, theSynoptic Optical Long-term Investigations of the Sun (SOLIS) vectorspectro-magnetograph (VSM), the spectro-magnetograph and the 512-channelmagnetograph instruments, and from the U. Stanford's Wilcox Solar Observatory.The associated multipole field components are used to study the dominant lengthscales and symmetries of the coronal field. Polar field changes are found to bewell correlated with active fields over most of the period studied, exceptbetween 2003-6 when the active fields did not produce significant polar fieldchanges. Of the axisymmetric multipoles, only the dipole and octupole followthe poles whereas the higher orders follow the activity cycle. Allnon-axisymmetric multipole strengths are well correlated with the activitycycle. The tilt of the solar dipole is therefore almost entirely due toactive-region fields. The axial dipole and octupole are the largestcontributors to the global field except while the polar fields are reversing.This influence of the polar fields extends to modulating eruption rates.According to the Computer Aided CME Tracking (CACTus), Solar Eruptive EventDetection System (SEEDS), and Nobeyama radioheliograph prominence eruptioncatalogs, the rate of solar eruptions is found to be systematically higher foractive years between 2003-2012 than for those between 1997-2002. This behaviorappears to be connected with the weakness of the late-cycle 23 polar fields assuggested by Luhmann. We see evidence that the process of cycle 24 fieldreversal is well advanced at both poles.

Solar magnetic activity cycles, coronal potential field models and eruption rates