Acoustic-Power Maps of Solar Active Regions with Direction Filters and Phase-Velocity Filters

Abstract

We study the properties of power maps of solar acoustic waves filtered with direction filters and phase-velocity filters. A direction filter is used to isolate acoustic waves propagating in a narrow range of directions. The acoustic-power map of the waves filtered with a direction filter shows extended reduced-power features behind magnetic regions with respect to the wave direction. A phase-velocity filter is further applied to isolate waves with similar wave paths. In the power maps of the waves filtered with both a direction filter and a phase-velocity filter, a reduced-power image of a sunspot appears behind the sunspot with respect to the wave direction. The distance between the sunspot and the secondary image is consistent with the one-skip travel distance of the wave packet associated with the phase-velocity filter. The waves filtered with direction and phase-velocity filters at the location of the secondary image could be used to probe the sunspot. In the quiet Sun, spatial fluctuations exist in any acoustic-power map. These fluctuations are mainly caused by interference among modes with the same frequency. The fluctuations are random with two properties: They change rapidly with time, and their magnitude decreases with the square root of the number of frames used in computing the acoustic-power map.