Solar Toroidal Field Evolution Spanning Four Sunspot Cycles Seen by the Wilcox Solar Observatory, the Solar and Heliospheric Observatory/Michelson Doppler Imager, and the Solar Dynamics Observatory/Helioseismic and Magnetic Imager

Forty-four years of Wilcox Solar Observatory, 14 years of Michelson Doppler Imager on the Solar and Heliospheric Observatory, and 11 years of Helioseismic and Magnetic Imager on the Solar Dynamics Observatory magnetic field data have been studied to determine the east–west inclination—the toroidal component—of the magnetic field. Maps of the zonal averaged inclination show that each toroidal field cycle begins at around the same time at high latitudes in the northern and southern hemispheres, and ends at the equator. Observation of these maps also shows that each instance of a dominant toroidal field direction starts at high latitudes near sunspot maximum and is still visible near the equator well past the minimum of its cycle, indicating that the toroidal field cycle spans approximately two sunspot cycles. The length of the extended activity cycle is measured to be approximately 16.8 yr.