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Statistics of Local Seismic Emission from the Solar Granulation
We have applied computational seismic holography to SDO/HMI observations of high-frequency helioseismic oscillations in the quiet Sun to locate predominant sources of seismic emission with respect to the structure of the solar granulation. The regions of greatest seismic emission are the edges of photospheric granules, thought to be the source of transonic plumes plummeting into the Sun's interior. This is roughly consistent with our general understanding of the kinetic excitation of sound by turbulence, based upon the strong dependence of acoustic excitation on the local Mach number of the turbulent medium. However, there also appears to be a significant excess of seismic emission from the brightest components of the granulation, few hundred km inside of the edges of granules. These are far from transonic, at least in the photosphere. This statistical anomaly lends strong support to Mark Rast's hypothesis, sixteen years ago, that implosive radiative cooling of granular outflows manifests a significant contribution to seismic emission from the solar granulation.