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The evolution of the photospheric magnetic field during the declining phaseand minimum of Cycle 23 and the recent rise of Cycle 24 are compared with thebehavior during previous cycles. We used longitudinal full-disk magnetogramsfrom the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-termInvestigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM), theSpectromagnetograph and the 512-Channel Magnetograph instruments, andlongitudinal full-disk magnetograms from the Mt. Wilson 150-foot tower. Weanalyzed 37 years of observations from these two observatories that have beenobserving daily, weather permitting, since 1974, offering an opportunity tostudy the evolving relationship between the active region and polar fields insome detail over several solar cycles. It is found that the annual averages ofa proxy for the active region poloidal magnetic field strength, the magneticfield strength of the high-latitude poleward streams, and the time derivativeof the polar field strength are all well correlated in each hemisphere. Theseresults are based on statistically significant cyclical patterns in the activeregion fields and are consistent with the Babcock-Leighton phenomenologicalmodel for the solar activity cycle. There was more hemispheric asymmetry in theactivity level, as measured by total and maximum active region flux, duringlate Cycle 23 (after around 2004), when the southern hemisphere was moreactive, and Cycle 24 up to the present, when the northern hemisphere has beenmore active, than at any other time since 1974. The active region net proxypoloidal fields effectively disappeared in both hemispheres around 2004, andthe polar fields did not become significantly stronger after this time. We seeevidence that the process of Cycle 24 field reversal has begun at both poles.

Evolution of active and polar photospheric magnetic fields during the rise of Cycle 24 compared to previous cycles