A low upper limit on the subsurface rise speed of solar active regions

TL;DR

This study combines surface flow measurements and simulations to show that the subsurface rise speed of magnetic flux in solar active regions is very low, indicating convection controls flux emergence.

Contribution

It provides the first observational constraint on the subsurface rise speed of magnetic flux, challenging standard flux tube models.

Findings

Rise speed is no larger than 150 m/s at 20 Mm depth.

Observed flows are consistent with convective velocities.

Convective flows dominate flux emergence dynamics.

Abstract

Magnetic field emerges at the surface of the Sun as sunspots and active regions. This process generates a poloidal magnetic field from a rising toroidal flux tube, it is a crucial but poorly understood aspect of the solar dynamo. The emergence of magnetic field is also important because it is a key driver of solar activity. We show that measurements of horizontal flows at the solar surface around emerging active regions, in combination with numerical simulations of solar magnetoconvection, can constrain the subsurface rise speed of emerging magnetic flux. The observed flows imply that the rise speed of the magnetic field is no larger than 150 m/s at a depth of 20 Mm, that is, well below the prediction of the (standard) thin flux tube model but in the range expected for convective velocities at this depth. We conclude that convective flows control the dynamics of rising flux tubes in the upper layers of the Sun and cannot be neglected in models of flux emergence.