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

TL;DR

This study compares the evolution of photospheric magnetic fields during Solar Cycle 24 with previous cycles, revealing hemispheric asymmetries, the disappearance of active region proxy fields, and early signs of polar field reversal, supporting the Babcock-Leighton model.

Contribution

It provides a comprehensive 37-year analysis of magnetic field evolution across multiple solar cycles, highlighting hemispheric asymmetries and early polar field reversal signs during Cycle 24.

Findings

Hemispheric asymmetry in activity levels increased after 2004.

Active region proxy poloidal fields disappeared around 2004.

Evidence of beginning polar field reversal in Cycle 24.

Abstract

The evolution of the photospheric magnetic field during the declining phase and minimum of Cycle 23 and the recent rise of Cycle 24 are compared with the behavior during previous cycles. We used longitudinal full-disk 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 Spectromagnetograph and the 512-Channel Magnetograph instruments, and longitudinal full-disk magnetograms from the Mt. Wilson 150-foot tower. We analyzed 37 years of observations from these two observatories that have been observing daily, weather permitting, since 1974, offering an opportunity to study the evolving relationship between the active region and polar fields in some detail over several solar cycles. It is found that the annual averages of a proxy for the active region poloidal magnetic field strength, the magnetic field strength of the high-latitude poleward streams, and the time derivative of the polar field strength are all well correlated in each hemisphere. These results are based on statistically significant cyclical patterns in the active region fields and are consistent with the Babcock-Leighton phenomenological model for the solar activity cycle. There was more hemispheric asymmetry in the activity level, as measured by total and maximum active region flux, during late Cycle 23 (after around 2004), when the southern hemisphere was more active, and Cycle 24 up to the present, when the northern hemisphere has been more active, than at any other time since 1974. The active region net proxy poloidal fields effectively disappeared in both hemispheres around 2004, and the polar fields did not become significantly stronger after this time. We see evidence that the process of Cycle 24 field reversal has begun at both poles.