Acoustic-Power Maps of Solar Active Regions with Direction Filters and Phase-Velocity Filters |
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| Authors: | Dean-Yi Chou Zhi-Chao Liang Ming-Hsu Yang Ming-Tsung Sun |
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| Institution: | (1) Department of Physics, National Tsing Hua University, Hsinchu, 30013, Taiwan, ROC;(2) Department of Mechanical Engineering, Chang-Gung University, Kwei-San, 33333, Taiwan, ROC |
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| 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. |
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| Keywords: | Oscillations solar Helioseismology observations Active regions magnetic fields Waves acoustic |
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