The Dynamic Polar Magnetic Field Before Its Polarity Reversal

We characterize the magnetic field in the Sun's polar region using the spectropolarimetric measurement from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). With HMI's high cadence, continuous time coverage, and moderate spectral and spatial resolution, we are able to estimate the polar magnetic flux, its latitudinal distribution, and its temporal variation over three years (2010-2012) during Cycle 24's rising phase. A comparison with higher spectral resolution observations from Hinode SOT/SP provides constraints on the flux estimates. The tracking of individual magnetic element movements yields new insight on the polar field's dynamic behavior leading up to the polarity reversal. We compare the result with that from the HMI line-of-sight data, as well as MDI data for Cycle 23. All observations indicate an earlier reversal of the northern hemisphere owing to more solar activity in the rising phase, which resulted in a significant hemispheric asymmetry.