Magneto-gravity wave packet dynamics in strongly magnetised cores of evolved stars. (arXiv:2002.11130v1 [astro-ph.SR])

Magneto-gravity wave packet dynamics in strongly magnetised cores of evolved stars. (arXiv:2002.11130v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Loi_S/0/1/0/all/0/1">Shyeh Tjing Loi</a>

Magnetic fields are believed to be generated in the cores of massive main
sequence stars, and these may survive on to later stages of evolution.
Observations of depressed dipole modes in red giant stars have been touted as
evidence for these fields, but the predictions of existing magnetic theories
have difficulty accommodating several aspects, including the need to return a
fraction of wave energy from the core to the envelope, and the persistent
gravity-like character of affected modes. In this work we perform a Hamiltonian
ray tracing study investigating the dynamics of magneto-gravity waves in full
spherical geometry, using realistic stellar models and magnetic field
configurations. This technique applies in the limit where wavelengths are much
shorter than scales of background variation. We conduct a comprehensive
exploration of parameter space, examining the roles of wave frequency,
spherical harmonic degree, wavevector polarisation, incoming latitude, field
strength, field radius, and evolutionary state. We demonstrate that even in the
presence of a strong field, there exist trajectories where waves remain
predominantly gravity-like in character, and these are able to undergo
reflection out of the core much like pure gravity waves. The remaining
trajectories are ones where waves acquire significant Alfven character,
becoming trapped and eventually dissipated. Orientation effects, i.e.
wavevector polarisation and incoming latitude, are found to be crucial factors
in determining the outcome (trapped versus reflected) of individual wave
packets. The allowance for partial energy return from the core offers a
solution to the conundrum faced by the magnetic hypothesis.

Magnetic fields are believed to be generated in the cores of massive main
sequence stars, and these may survive on to later stages of evolution.
Observations of depressed dipole modes in red giant stars have been touted as
evidence for these fields, but the predictions of existing magnetic theories
have difficulty accommodating several aspects, including the need to return a
fraction of wave energy from the core to the envelope, and the persistent
gravity-like character of affected modes. In this work we perform a Hamiltonian
ray tracing study investigating the dynamics of magneto-gravity waves in full
spherical geometry, using realistic stellar models and magnetic field
configurations. This technique applies in the limit where wavelengths are much
shorter than scales of background variation. We conduct a comprehensive
exploration of parameter space, examining the roles of wave frequency,
spherical harmonic degree, wavevector polarisation, incoming latitude, field
strength, field radius, and evolutionary state. We demonstrate that even in the
presence of a strong field, there exist trajectories where waves remain
predominantly gravity-like in character, and these are able to undergo
reflection out of the core much like pure gravity waves. The remaining
trajectories are ones where waves acquire significant Alfven character,
becoming trapped and eventually dissipated. Orientation effects, i.e.
wavevector polarisation and incoming latitude, are found to be crucial factors
in determining the outcome (trapped versus reflected) of individual wave
packets. The allowance for partial energy return from the core offers a
solution to the conundrum faced by the magnetic hypothesis.

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