Measuring solar active region inflows with local correlation tracking of granulation

¹ Institut für Astrophysik, Georg-August Universität Göttingen, 37077 Göttingen, Germany
e-mail: loeptien@mps.mpg.de
² Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

Received: 28 April 2017
Accepted: 9 June 2017

Abstract

Context. Sixteen years ago local helioseismology detected spatially extended converging surface flows into solar active regions. These flows play an important role in flux-transport models of the solar dynamo.

Aims. We aim to validate the existence of the inflows by deriving horizontal flow velocities around active regions with local correlation tracking of granulation.

Methods. We generate a six-year time series of full-disk maps of the horizontal velocity at the solar surface by tracking granules in continuum intensity images provided by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO).

Results. On average, active regions are surrounded by inflows extending up to 10° from the center of the active region of magnitudes of 20–30 m/s, reaching locally up to 40 m/s, which is in agreement with results from local helioseismology. By computing an ensemble average consisting of 243 individual active regions, we show that the inflows are not azimuthally symmetric, but converge predominantly towards the trailing polarity of the active region with respect to the longitudinally and temporally averaged flow field.