Dynamo Model for North-South Asymmetry of Solar Activity

TL;DR

This paper introduces a dynamo model explaining the long-term North-South asymmetry in solar activity by superposing dipolar and quadrupolar magnetic fields, with results supported by numerical simulations and analytical examples.

Contribution

It presents a novel dynamo model that accounts for long-term solar activity asymmetry through superposition of magnetic modes and random fluctuations in the $oldsymbol{ extalpha}$-effect.

Findings

Model confirms excitation of quadrupolar mode by dipolar mode.

Long-term asymmetry arises from superposition and fluctuations.

Phase locking explains observed asymmetry states.

Abstract

Observations reveal a relatively small but statistically significant North-South (NS) asymmetry in sunspot activity varying on a time scale 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 time scale of several (about 6 in the present model) solar cycles resulting from random variations in the $\alpha$-effect on a time scale 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.