Interaction of acoustic oscillations with an active region on the Sun

Abstract

Based on MDI data, we constructed acoustic maps of the high-degree solar oscillations as they interacted with the active region NOAA 7978 using the acoustic imaging technique. We analyze the reconstructed power maps for the incoming and outgoing oscillations, as well as the phase-shift maps and the envelope-shift maps of wave packets in the frequency range 3.0–5.0 mHz. We perform a cross-correlation analysis of the time series for the acoustic oscillations before and after their interaction with the active region and analyze direct observational data. Our results point to a difference between the phase and envelope shifts. Thus, for example, the phase and group velocities of the oscillations increase as they pass through a sunspot, with the increase in group velocity being more significant. We found a phase-shift difference between the inward and outward propagating oscillations, ~0.4–0.5 min. This difference is interpreted as the effect of subsurface flow from the active region.