Polar Photospheric Magnetic Field Evolution and Global Flux Transport

The Sun's polar magnetic fields are of paramount importance in structuring the heliosphere and in seeding the activity cycle, but they are difficult to observe from the ecliptic plane. However, they are formed by observable active-region magnetic-flux decay and transport processes. We give an observational description of photospheric flux transport and polar field evolution using the > 40 yr series (1974 - 2017) of full-disk line-of-sight photospheric magnetograms from the NSO Kitt Peak Vacuum Telescope (KPVT) 512-channel and Spectromagnetograph (SPMG) and the Synoptic Optical Long-term Investigation of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM). An analytical technique introduced by Durrant, Turner, and Wilson (Solar Physics, 222, 345, 2004). is adapted to analyze time series of full-disk magnetograms to estimate global photospheric flux transport self-consistently. The major hemispheric flux changes, due to cancellation or transport of leading-polarity flux across the equator, occur around activity maximum in each cycle, when peak levels of flux emerge = 10(?) from the equator. This highlights the efficiency of flux cancellation and transport within each hemisphere, allowing peak levels of leading-polarity flux to reach the equator at this time, letting peak trailing-polarity flux surge poleward. Polar flux evolution follows the hemispheric flux closely each cycle, but with temporal variations dampened by lower-latitude flux cancellation. Major net hemispheric flux change ceased early during Cycle 23, producing weakened polar fields. Cycle 24 was distinguished by large hemispheric flux fluctuations forming a three-part structure: (i) dominant northern activity and north polar flux decrease toward zero; (ii) dominant southern activity peak, swift south polar field decrease, polarity reversal, and growth while the north pole was stalled; and (iii) southern activity decline, north again dominant, north polar flux growth with reversed polarity. Recent global photospheric field evolution and polar field changes are described using Global Oscillations Network Group (GONG, 2006 - present) and Solar Dynamics Observatory Helioseismic and Magnetic Imager (SDO/HMI) line-of-sight magnetograms.