Dynamo Model for North-South Asymmetry of Solar Activity

Observations reveal a relatively small but statistically significant north-south (NS) asymmetry in sunspot activity varying on a timescale of several solar cycles. This paper proposes a dynamo model for the phenomenon of long-term NS asymmetry. The model separates dynamo equations for magnetic fields of dipolar and quadrupolar equatorial parity. The NS asymmetry results from the superposition of dipolar and quadrupolar fields. Model computations confirm the formerly proposed excitation of the quadrupolar dynamo mode by a dominant dipolar mode mediated by the equator-symmetric fluctuations in the alpha-effect as a mechanism for the long-term NS asymmetry. An analytically solvable example of oscillations excited by short-term random forcing is given to justify the numerical result of NS asymmetry coherent on a timescale of several (about six in the present model) solar cycles resulting from random variations in the alpha-effect on a timescale of one solar rotation. The model computations show the phase-locking phenomenon of dipolar and quadrupolar fields oscillating predominantly in phase (northern type asymmetry) or in antiphase (southern type asymmetry) with relatively short irregular transitions between these two states. Large asymmetry in the simulated Grand minima is found and explained by weak magnetic quenching of the alpha-effect during the minima. The possibility of polar field asymmetry in activity minima as a precursor of sunspot asymmetry in the following activity cycles is discussed based on the dynamo model and observations.

Automated summary

This study proposes a dynamo model to explain the phenomenon of long-term north-south asymmetry in sunspot activity, showing how the superposition of dipolar and quadrupolar fields leads to this asymmetry. The model computations reveal the phase-locking and irregular transitions of dipolar and quadrupolar fields oscillations. Additionally, the possibility of polar field asymmetry as a precursor of sunspot asymmetry in following activity cycles is discussed based on the dynamo model and observations.