Coriolis force acting on near-surface horizontal flows during simulations of flux emergence produces a tilt angle consistent with Joy's law on the Sun

TL;DR

This study demonstrates through 3D magnetohydrodynamic simulations that the Coriolis force acting on horizontal flows near the Sun's surface can produce tilt angles of bipolar active regions consistent with Joy's law, supporting its role in solar dynamo models.

Contribution

The paper models the Coriolis force's effect on rising flux tubes and shows it can produce Joy's law tilt angles at realistic solar rotation rates.

Findings

Coriolis force causes tilt angles matching Joy's law in simulations.

Tilt angles remain stable after flux emergence.

Simulation at enhanced rotation rate scaled to solar conditions.

Abstract

Joy's law describes the tilt of bipolar active regions on the Sun away from an east-west orientation, where the flux of the polarity concentrated at the prograde side tends to be closer to the equator than the polarity on the retrograde side. Joy's law is attributed to the Coriolis force because of the observed increase in tilt angle at higher latitudes. This tilt plays a crucial role in some solar dynamo models. Our goal is to model the effects of the Coriolis force on a flux tube as it rises through the near-surface convection zone. We use a three-dimensional Cartesian magnetohydrodynamic simulation of an untwisted flux tube ascending from a depth of 11 Mm. We model the Coriolis effect using the f-plane approximation, that only considers and acts on horizontal flows. On the Sun, Joy's law is weak and is only evident as an average over many active regions. To achieve a measurable effect in a single simulation, we consider a rotation rate 110 times faster than the Sun. The simulation shows that the flux tube emerges at the surface with a tilt angle consistent with Joy's law when scaled to the Sun's slower rotation, and the tilt angle does not substantially change after emergence. This shows that the Coriolis force acting on flows horizontal to the surface within the near-surface convection zone is consistent with Joy's law.