Statistical analysis of acoustic wave power and flows around solar active regions

Abstract

We analyze the effect of a sunspot in its quiet surroundings applying a helioseismic technique on almost three years of Helioseismic and Magnetic Imager (HMI) observations obtained during solar cycle 24 to further study the sunspot structure below the solar surface. The attenuation of acoustic waves with frequencies lower than 4.2 mHz depends more strongly on the wave direction at a distance of $6^o-7^o$ from the sunspot center. The amplification of higher frequency waves is highest 6$^o$ away from the active region and it is largely independent of the wave's direction. We observe a mean clockwise flow around active regions, which angular speed decreases exponentially with distance and has a coefficient close to $-0.7$ degree$^{-1}$. The observed horizontal flow in the direction of the nearby active region agrees with a large-scale circulation around the sunspot in the shape of cylindrical shell. The center of the shell seems to be centered around 7$^o$ from the sunspot center, where we observe an inflow close to the surface down to $\sim$2 Mm, followed by an outflow at deeper layers until at least 7 Mm.