Numerical determination of the cutoff frequency in solar models. (arXiv:2006.00526v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Felipe_T/0/1/0/all/0/1">T. Felipe</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sangeetha_C/0/1/0/all/0/1">C. R. Sangeetha</a>
In stratified atmospheres, acoustic waves can only propagate if their
frequency is above the cutoff value. Different theories provide different
cutoff values. We developed an alternative method to derive the cutoff
frequency in several standard solar models, including various quiet-Sun and
umbral atmospheres. We performed numerical simulations of wave propagation in
the solar atmosphere. The cutoff frequency is determined from the inspection of
phase difference spectra computed between the velocity signal at two
atmospheric heights. The process is performed by choosing pairs of heights
across all the layers between the photosphere and the chromosphere, to derive
the vertical stratification of the cutoff in the solar models. The cutoff
frequency predicted by the theoretical calculations departs significantly from
our measurements. In quiet-Sun atmospheres, the cutoff shows a strong
dependence on the magnetic field for adiabatic wave propagation. When radiative
losses are taken into account, the cutoff frequency is greatly reduced and the
variation of the cutoff with the strength of the magnetic field is lower. The
effect of the radiative losses in the cutoff is necessary to understand recent
quiet-Sun and sunspot observations. In the presence of inclined magnetic
fields, our numerical calculations confirm the reduction of the cutoff
frequency due to the reduced gravity experienced by waves propagating along
field lines. An additional reduction is also found in regions with significant
changes in the temperature, due to the lower temperature gradient along the
path of field-guided waves. Our results show that the cutoff values are not
correctly captured by theoretical estimates. In addition, most of the
widely-used analytical cutoff formulae neglect the impact of magnetic fields
and radiative losses, whose role is critical to determine the evanescent or
propagating nature of the waves.
In stratified atmospheres, acoustic waves can only propagate if their
frequency is above the cutoff value. Different theories provide different
cutoff values. We developed an alternative method to derive the cutoff
frequency in several standard solar models, including various quiet-Sun and
umbral atmospheres. We performed numerical simulations of wave propagation in
the solar atmosphere. The cutoff frequency is determined from the inspection of
phase difference spectra computed between the velocity signal at two
atmospheric heights. The process is performed by choosing pairs of heights
across all the layers between the photosphere and the chromosphere, to derive
the vertical stratification of the cutoff in the solar models. The cutoff
frequency predicted by the theoretical calculations departs significantly from
our measurements. In quiet-Sun atmospheres, the cutoff shows a strong
dependence on the magnetic field for adiabatic wave propagation. When radiative
losses are taken into account, the cutoff frequency is greatly reduced and the
variation of the cutoff with the strength of the magnetic field is lower. The
effect of the radiative losses in the cutoff is necessary to understand recent
quiet-Sun and sunspot observations. In the presence of inclined magnetic
fields, our numerical calculations confirm the reduction of the cutoff
frequency due to the reduced gravity experienced by waves propagating along
field lines. An additional reduction is also found in regions with significant
changes in the temperature, due to the lower temperature gradient along the
path of field-guided waves. Our results show that the cutoff values are not
correctly captured by theoretical estimates. In addition, most of the
widely-used analytical cutoff formulae neglect the impact of magnetic fields
and radiative losses, whose role is critical to determine the evanescent or
propagating nature of the waves.
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