On the asymmetry of velocity oscillation amplitude in bipolar active regions

The velocity field in the lower solar atmosphere undergoes strong interactions with magnetic fields. Many authors have pointed out that power is reduced by a factor between two and three within magnetic regions, depending on frequency, depth, the radius, and the magnetic strength of the flux tube. Many mechanisms have been proposed to explain the observations. In this work, Solar Dynamics Observatory (SDO) dopplergrams and magnetograms of 12 bipolar active regions (βARs) at a 45-s cadence are used to investigate the relation between velocity fluctuations and magnetic fields. We show that there is an asymmetry within βARs, with the velocity oscillation amplitude being more suppressed in the leading polarities than in the trailing polarities. Also, the strongest magnetic fields do not completely suppress the five-minute oscillation amplitude, even in the spot's innermost umbrae.