A Solar Dynamo Model Driven by Mean-Field Alpha and Babcock-Leighton Sources: Fluctuations, Grand-Minima-Maxima and Hemispheric Asymmetry in Sunspot Cycles

摘要

Extreme solar activity fluctuations and the occurrence of solar grand minimaand maxima episodes, are well established, observed features of the solarcycle. Nevertheless, such extreme activity fluctuations and the dynamics of thesolar cycle during Maunder minima-like episodes remain ill-understood. Weexplore the origin of such extreme solar activity fluctuations and the role ofdual poloidal field sources, namely the Babcock-Leighton mechanism and themean-field alpha effect in the dynamics of the solar cycle. We mainlyconcentrate on entry and recovery from grand minima episodes such as theMaunder minimum and the dynamics of the solar cycle. We use a kinematic solardynamo model with a novel set-up in which stochastic perturbations force twodistinct poloidal field alpha effects. We explore different regimes ofoperation of these poloidal sources with distinct operating thresholds, toidentify the importance of each. The perturbations are implementedindependently in both hemispheres which allows one to study the level ofhemispheric coupling and hemispheric asymmetry in the emergence of sunspots.From the simulations performed we identify a few different ways in which thedynamo can enter a grand minima episode. While fluctuations in any of the$\alpha$ effects can trigger intermittency we find that the mean-field alphaeffect is crucial for the recovery of the solar cycle from a grand minimaepisode which a Babcock-Leighton source alone, fails to achieve. Oursimulations also demonstrate other cycle dynamics. We conclude that stochasticfluctuations in two interacting poloidal field sources working with distinctoperating thresholds is a viable candidate for triggering episodes of extremesolar activity and that the mean-field alpha effect capable of working on weak,sub-equipartition fields is critical to the recovery of the solar cyclefollowing an extended solar minimum.