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| Paper: |
Acoustic Power Reduction and Phase Difference in Sunspots and Quiet Sun from Hankel Transform of Solar Dynamo Observatory Data |
| Volume: |
479, Progress in Physics of the Sun and Stars |
| Page: |
409 |
| Authors: |
Couvidat, S. |
| Abstract: |
The Helioseismic & Magnetic Imager and Atmospheric Imaging Assembly instruments produce Doppler velocity and continuum intensity at 6173 Å, as well as intensity maps at 1600 Å and 1700 Å, usable for helioseismic studies at different heights in the solar photosphere. We perform an Hankel-Fourier analysis around sunspots and quiet-Sun regions, and estimate the change in power and phase of waves crossing them. We find a dependence of power-reduction coefficients α on measurement height: Sunspots reduce the power of outgoing waves with ν ≤ 4.5 mHz at all heights, but enhance the power of waves in the range ν ≈ 4.5–5.5 mHz toward chromospheric heights, which is likely to be the signature of acoustic glories (halos). Maximum power reduction seems to occur near the continuum level and to decrease with altitude. Sunspots also impact the frequencies of outgoing waves in an altitude-dependent fashion. Conversely, the scattering phase shifts do not show any significant dependence on height, and the regions of acoustic scattering and power reduction are located at different depths. The quiet Sun behaves like a strong power reducer for outgoing f- and p-modes at continuum level, with α ≈ 15–20%, and like a weak power enhancer for p-modes higher in the atmosphere. It is speculated that the surprising power reduction at the continuum is related to granulation. |
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