Theoretical Models of Sunspot Structure and Dynamics

TL;DR

This paper reviews recent theoretical models and numerical simulations of sunspot structures, highlighting advances in understanding umbral dots, penumbral filaments, magnetic flux dynamics, and the Evershed flow.

Contribution

It synthesizes recent progress in modeling sunspot features, including realistic MHD simulations and the role of magnetic flux pumping and siphon flows.

Findings

Realistic simulations reproduce umbral dot properties.

Distinct magnetic and flow structures in inner and outer penumbra.

Numerical experiments support magnetic flux pumping and siphon-flow explanations.

Abstract

Recent progress in theoretical modeling of a sunspot is reviewed. The observed properties of umbral dots are well reproduced by realistic simulations of magnetoconvection in a vertical, monolithic magnetic field. To understand the penumbra, it is useful to distinguish between the inner penumbra, dominated by bright filaments containing slender dark cores, and the outer penumbra, made up of dark and bright filaments of comparable width with corresponding magnetic fields differing in inclination by some 30 degrees and strong Evershed flows in the dark filaments along nearly horizontal or downward-plunging magnetic fields. The role of magnetic flux pumping in submerging magnetic flux in the outer penumbra is examined through numerical experiments, and different geometric models of the penumbral magnetic field are discussed in the light of high-resolution observations. Recent, realistic numerical MHD simulations of an entire sunspot have succeeded in reproducing the salient features of the convective pattern in the umbra and the inner penumbra. The siphon-flow mechanism still provides the best explanation of the Evershed flow, particularly in the outer penumbra where it often consists of cool, supersonic downflows.