title: A low upper limit on the subsurface rise speed of solar active regions.
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.
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: Aaron C
authors.initials: AC
authors.lastname: Birch
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: Hannah
authors.identifier: 0000-0001-9932-9559
authors.initials: H
authors.lastname: Schunker
authors.affiliation: NorthWest Research Associates, 3380 Mitchell Lane, Boulder, CO 80301, USA.
authors.forename: Douglas C
authors.initials: DC
authors.lastname: Braun
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Cameron
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.; Institut für Astrophysik, Georg-August-Universität Göttingen, 37077 Göttingen, Germany.; Center for Space Science, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE.; National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan.
authors.forename: Laurent
authors.initials: L
authors.lastname: Gizon
authors.affiliation: Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany.; Institut für Astrophysik, Georg-August-Universität Göttingen, 37077 Göttingen, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Löptien
authors.affiliation: National Center for Atmospheric Research, High Altitude Observatory, 3080 Center Green Drive, Boulder, CO 80301, USA.
authors.forename: Matthias
authors.identifier: 0000-0001-5850-3119
authors.initials: M
authors.lastname: Rempel
mesh_terms: D060526:Magnetic Fields; D008962:Models, Theoretical; D019445:Solar Activity
keywords: Physics; astronomy; convection; solar physics