Magnetic flux generation and transport in cool stars

TL;DR

This paper models magnetic flux generation and transport in cool stars, linking deep dynamo processes with surface magnetic activity, and explores how stellar rotation affects magnetic flux emergence and activity cycles.

Contribution

It integrates a comprehensive physical model of the stellar dynamo, flux buoyancy, and surface transport to explain magnetic activity patterns across different stellar rotation rates.

Findings

Surface magnetic flux patterns reflect deep dynamo waves for slow rotators.

Rapid rotation causes high-latitude flux emergence due to Coriolis effects.

Fast-rotating subgiants exhibit noisy activity cycles with mixed-polarity fields.

Abstract

The Sun and other cool stars harbouring outer convection zones manifest magnetic activity in their atmospheres. The connection between this activity and the properties of a deep-seated dynamo generating the magnetic flux is not well understood. By employing physical models, we study the spatial and temporal characteristics of the observable surface field for various stellar parameters. We combine models for magnetic flux generation, buoyancy instability, and transport, which encompass the entire convection zone. The model components are: (1) a thin-layer alpha-Omega dynamo at the base of the convection zone; (2) buoyancy instabilities and the rise of flux tubes through the convection zone in 3D, which provides a physically consistent determination of emergence latitudes and tilt angles; and (3) horizontal flux transport at the surface. For solar-type stars and rotation periods longer than about 10 days, the latitudinal dynamo waves generated by the deep-seated alpha-Omega dynamo are faithfully reflected by the surface distribution of magnetic flux. For rotation periods of the order of two days, however, Coriolis acceleration of rising flux loops leads to surface flux emergence at much higher latitudes than the dynamo waves at the bottom of the convection zone reach. A similar result is found for a K0V star with a rotation period of two days. In the case of a rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy activity cycles and mixed-polarity fields at high latitudes.