Abstract:
The well-observed acoustic halo is an enhancement in time-averaged Doppler velocity and intensity power with
respect to quiet-Sun values that is prominent for the weak and highly inclined field around the penumbra of
sunspots and active regions. We perform 3D linear wave modeling with realistic distributed acoustic sources in a
magnetohydrostatic sunspot atmosphere and compare the resultant simulation enhancements with multiheight SDO
observations of the phenomenon. We find that simulated halos are in good qualitative agreement with observations.
We also provide further proof that the underlying process responsible for the halo is the refraction and return of fast
magnetic waves that have undergone mode conversion at the critical a=c atmospheric layer. In addition, we also
find strong evidence that fastAlfvén mode conversion plays a significant role in the structure of the halo, taking
energy away from photospheric and chromospheric heights in the form of field-aligned Alfvén waves. This
conversion process may explain the observed “dual-ring” halo structure at higher (>8 mHz) frequencies.
