Models for the long-term variations of solar activity

TL;DR

This paper reviews dynamo models explaining long-term solar cycle variations, such as grand minima and maxima, analyzing their causes and comparing model results with observational data.

Contribution

It provides a comprehensive review of dynamo models for long-term solar cycle modulations and analyzes their strengths, weaknesses, and relation to observed phenomena.

Findings

Identifies magnetic feedback, stochastic forcing, and time delays as key causes of cycle modulation.

Demonstrates illustrative models for each cause and discusses their implications.

Highlights current gaps in matching models with detailed observational features.

Abstract

One obvious feature of the solar cycle is its variation from one cycle to another. In this article, we review the dynamo models for the long-term variations of the solar cycle. By long-term variations, we mean the cycle modulations beyond the 11-year periodicity and these include, the Gnevyshev-Ohl/Even-Odd rule, grand minima, grand maxima, Gleissberg cycle, and Suess cycles. After a brief review of the observed data, we present the dynamo models for the solar cycle. By carefully analyzing the dynamo models and the observed data, we identify the following broad causes for the modulation: (i) magnetic feedback on the flow, (ii) stochastic forcing, and (iii) time delays in various processes of the dynamo. To demonstrate each of these causes, we present the results from some illustrative models for the cycle modulations and discuss their strengths and weakness. We also discuss a few critical issues and their current trends. The article ends with a discussion of our current state of ignorance about comparing detailed features of the magnetic cycle and the large-scale velocity from the dynamo models with robust observations.