A coupled $2\times2$D Babcock-Leighton solar dynamo model. II. Reference dynamo solutions

TL;DR

This paper presents a hybrid 2x2D flux transport dynamo model of the solar cycle, integrating surface flux transport and internal dynamo processes, calibrated with observed data, and capable of reproducing key solar cycle features and variability.

Contribution

It introduces a coupled 2x2D Babcock-Leighton dynamo model with a calibrated emergence function, capturing solar cycle characteristics and stochastic amplitude fluctuations.

Findings

Reproduces solar cycle features such as hemispheric coupling and phase relationships.

Models cycle amplitude saturation via BMR tilt quenching.

Simulates solar cycle variability, including Dalton-like epochs.

Abstract

In this paper we complete the presentation of a new hybrid $2\times2$D flux transport dynamo (FTD) model of the solar cycle based on the Babcock-Leighton mechanism of poloidal magnetic field regeneration via the surface decay of bipolar magnetic regions (BMRs). This hybrid model is constructed by allowing the surface flux transport (SFT) simulation described in Lemerle et al. 2015 to provide the poloidal source term to an axisymmetric FTD simulation defined in a meridional plane, which in turn generates the BMRs required by the SFT. A key aspect of this coupling is the definition of an emergence function describing the probability of BMR emergence as a function of the spatial distribution of the internal axisymmetric magnetic field. We use a genetic algorithm to calibrate this function, together with other model parameters, against observed cycle 21 emergence data. We present a reference dynamo solution reproducing many solar cycle characteristics, including good hemispheric coupling, phase relationship between the surface dipole and the BMR-generating internal field, and correlation between dipole strength at cycle maximum and peak amplitude of the next cycle. The saturation of the cycle amplitude takes place through the quenching of the BMR tilt as a function of the internal field. The observed statistical scatter about the mean BMR tilt, built into the model, acts as a source of stochasticity which dominates amplitude fluctuations. The model thus can produce Dalton-like epochs of strongly suppressed cycle amplitude lasting a few cycles and can even shut off entirely following an unfavorable sequence of emergence events.