Article

Observed Effect of Magnetic Fields on the Propagation of Magnetoacoustic Waves in the Lower Solar Atmosphere

We study Hinode/SOT-FG observations of intensity fluctuations in Ca ii H-line and G-band image sequences and their relation to simultaneous and co-spatial magnetic field measurements. We explore the G-band and H-line intensity oscillation spectra both separately and comparatively via their relative phase differences, time delays and cross-coherences. In the non-magnetic situations, both sets of fluctuations show strong oscillatory power in the 3 – 7 mHz band centered at 4.5 mHz, but this is suppressed as magnetic field increases. A relative phase analysis gives a time delay of H-line after G-band of 20±1 s in non-magnetic situations implying a mean effective height difference of 140 km. The maximum coherence is at 4 – 7 mHz. Under strong magnetic influence the measured delay time shrinks to 11 s with the peak coherence near 4 mHz. A second coherence maximum appears between 7.5 – 10 mHz. Investigation of the locations of this doubled-frequency coherence locates it in diffuse rings outside photospheric magnetic structures. Some possible interpretations of these results are offered.

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