Subsurface Velocity of Emerging and Decaying Active Regions

Abstract

We study the temporal variation of subsurface flows of 828 active regions and 977 quiet regions. The horizontal flows cover a range of depths from the surface to about 16 Mm and are determined by analyzing Global Oscillation Network Group high-resolution Doppler data with ring-diagram analyses. The vertical velocity component is derived from the divergence of the measured horizontal flows using mass conservation. For comparison, we analyze Michelson Doppler Imager (MDI) Dynamics Run data covering 68 active regions common to both data sets. We determine the change in unsigned magnetic flux during the disk passage of each active region using MDI magnetograms binned to the ring-diagram grid. We then sort the data by their flux change from decaying to emerging flux and divide the data into five subsets of equal size. We find that emerging flux has a faster rotation than the ambient fluid and pushes it up, as indicated by enhanced vertical velocity and faster-than-average zonal flow. After active regions are formed, downflows are established within two days of emergence in shallow layers between about 4 and 10 Mm. Emerging flux in existing active regions shows a similar scenario, where the upflows at depths greater than about 10 Mm are enhanced and the already established downflows at shallower depths are weakened. When active regions decay, the corresponding flow pattern disappears as well; the zonal flow slows down to values comparable to that of quiet regions and the upflows become weaker at deeper layers. The residual meridional velocity is mainly poleward and shows no obvious variation. The magnitude of the residual velocity, defined as the sum of the squares of the residual velocity components, increases with increasing magnetic flux and decreases with decreasing flux.