  We study the evolution of the observed photospheric magnetic field and the
modeled global coronal magnetic field during the past 3 1/2 solar activity
cycles observed since the mid-1970s. We use synoptic magnetograms and
extrapolated potential-field models based on longitudinal full-disk
photospheric magnetograms from the NSO's three magnetographs at Kitt Peak, the
Synoptic Optical Long-term Investigations of the Sun (SOLIS) vector
spectro-magnetograph (VSM), the spectro-magnetograph and the 512-channel
magnetograph instruments, and from the U. Stanford's Wilcox Solar Observatory.
The associated multipole field components are used to study the dominant length
scales and symmetries of the coronal field. Polar field changes are found to be
well correlated with active fields over most of the period studied, except
between 2003-6 when the active fields did not produce significant polar field
changes. Of the axisymmetric multipoles, only the dipole and octupole follow
the poles whereas the higher orders follow the activity cycle. All
non-axisymmetric multipole strengths are well correlated with the activity
cycle. The tilt of the solar dipole is therefore almost entirely due to
active-region fields. The axial dipole and octupole are the largest
contributors 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 Event
Detection System (SEEDS), and Nobeyama radioheliograph prominence eruption
catalogs, the rate of solar eruptions is found to be systematically higher for
active years between 2003-2012 than for those between 1997-2002. This behavior
appears to be connected with the weakness of the late-cycle 23 polar fields as
suggested by Luhmann. We see evidence that the process of cycle 24 field
reversal is well advanced at both poles.
