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

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