The origin of parity changes in the solar cycle

Abstract

ABSTRACT Although sunspots have been systematically observed on the Sun’s surface over the last four centuries, their magnetic properties have been revealed and documented only since the early 1900s. Sunspots typically appear in pairs of opposite magnetic polarities which have a systematic orientation. This polarity orientation is opposite across the equator – a trend that has persisted over the last century. Taken together with the configuration of the global poloidal field of the Sun – this phenomena is consistent with the dipolar parity state of an underlying magnetohydrodynamic dynamo. Although transient hemispheric asymmetry in sunspot emergence is observed, a global parity shift has never been observed. We simulate hemispheric asymmetry through introduction of random fluctuations in a computational dynamo model of the solar cycle and demonstrate that changes in parity are indeed possible in long-term simulations covering thousands of years. Quadrupolar modes are found to exist over significant fraction of the simulated time. In particular, we find that a parity shift in the underlying nature of the sunspot cycle is more likely to occur when sunspot activity dominates in any one hemisphere for a time which is significantly longer than the cycle period. We establish causal pathways connecting hemispheric asymmetry to parity flips mediated via a decoupling of the dynamo cycle period across the two solar hemispheres. Our findings indicate that the solar cycle may have resided in quadrupolar parity states in the past, and provides a possible pathway for predicting parity flips in the future.