Large-scale horizontal flows in the solar photosphere

II. Long-term behaviour and magnetic activity response

¹ Astronomical Institute, Academy of Sciences of the Czech Republic (v. v. i.), Fričova 298, 251 65 Ondřejov, Czech Republic e-mail: michal@astronomie.cz; [mklvana;msobotka;bumba]@asu.cas.cz
² Astronomical Institute, Charles University in Prague, V Holešovičkách 2, 180 00 Prague 8, Czech Republic

Received: 25 April 2007
Accepted: 6 October 2007

Abstract

We have developed a method to map large-scale horizontal velocity fields in the solar photosphere. The method was developed, tuned, and calibrated using synthetic data. Now, we apply the method to the series of Michelson Doppler Imager (MDI) Dopplergrams covering almost one solar cycle to generate the information about the long-term behaviour of surface flows. Our method clearly reproduces the widely accepted properties of mean flow field components, such as torsional oscillations and a pattern of meridional circulation. We also performed a period analysis, however due to the data series length and large gaps we did not detect any significant periods. The relation between the magnetic activity and the mean zonal motion is studied. We found evidence that the emergence of compact magnetic regions locally accelerates the rotation of the supergranular pattern in their vicinity and that the presence of magnetic fields generally decelerates the rotation in the equatorial region. Our results show that active regions in the equatorial region emerge, exhibiting a constant velocity (faster by  m s^-1 than the Carrington rate), suggesting that they emerge from the base of the surface radial shear at , disconnect from their magnetic roots, and slow down during their evolution.