Evolution of solar wind sources and coronal rotation driven by the cyclic variation of the Sun’s large-scale magnetic field

Abstract

Context. The strength and morphology of the Sun’s magnetic field evolve significantly during the solar cycle, with the overall polarity of the Sun’s magnetic field reversing during the maximum of solar activity. Long-term changes are also observed in sunspot and geomagnetic records; however, systematic magnetic field observations are limited to the last four cycles. Aims. Here, we investigate the long-term evolution of the Sun’s magnetic field, and the influence this has on the topology and rotation of the solar corona. Methods. The Sun’s photospheric magnetic field was decomposed into spherical harmonics using synoptic Carrington magnetograms from (1) the Wilcox Solar Observatory, (2) the Michelson Doppler Imager on board the Solar and Heliospheric Observatory, and (3) the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. The time evolution of the spherical harmonic coefficients was used to explore the variation of the Sun’s magnetic field, with a focus on the large-scale modes. Potential field source surface extrapolations of the photospheric field were computed to follow topological changes in the corona. Results. The sources of the Sun’s open magnetic field vary between the polar coronal holes and activity-driven features such as active regions, and equatorial coronal holes. Consequently, the mean rotation rate of the solar wind is modulated during each cycle by the latitudinal variation of open field footpoints, with slower rotation during minima and faster (Carrington-like) rotation during maxima. Conclusions. Coronal rotation is sensitive to cycle to cycle differences in the polar field strengths and hemispherical flux emergence rates. The mean rotation of the corona varies similarly to the ratio of quadrupole to dipole energy. Cycle 23 maintained a larger fraction of quadrupolar energy in the declining phase, which kept the sources of the open magnetic flux closer to the equator, extending the period of faster equator-ward connectivity. The ratio of quadrupole to dipole energy could be a useful proxy when examining the impact of differential rotation on the coronae of other Sun-like stars.