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Published April 11, 2017 | Published + Accepted Version
Journal Article Open

Conversion of internal gravity waves into magnetic waves


Asteroseismology probes the interiors of stars by studying oscillation modes at a star's surface. Although pulsation spectra are well understood for solar-like oscillators, a substantial fraction of red giant stars observed by Kepler exhibit abnormally low-amplitude dipole oscillation modes. Fuller et al. (2015) suggest this effect is produced by strong core magnetic fields that scatter dipole internal gravity waves (IGWs) into higher multipole IGWs or magnetic waves. In this paper, we study the interaction of IGWs with a magnetic field to test this mechanism. We consider two background stellar structures: one with a uniform magnetic field, and another with a magnetic field that varies both horizontally and vertically. We derive analytic solutions to the wave propagation problem and validate them with numerical simulations. In both cases, we find perfect conversion from IGWs into magnetic waves when the IGWs propagate into a region exceeding a critical magnetic field strength. Downward propagating IGWs cannot reflect into upward propagating IGWs because their vertical wavenumber never approaches zero. Instead, they are converted into upward propagating slow (Alfvénic) waves, and we show they will likely dissipate as they propagate back into weakly magnetized regions. Therefore, strong internal magnetic fields can produce dipole mode suppression in red giants, and gravity modes will likely be totally absent from the pulsation spectra of sufficiently magnetized stars.

Additional Information

© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 December 13. Received 2016 December 12; in original form 2016 October 18. The authors would like to thank Lars Bildsten, Eliot Quataert, Ellen Zweibel, Anna Lieb, Stephane Mathis, Dennis Stello, Rafael Garcia and Frank Timmes for helpful discussions. DL is supported by the Hertz Foundation, a PCTS fellowship, and a Lyman Spitzer Jr fellowship, and would like to thank the University of Sydney School of Mathematics and Statistics for helping fund a visit to Sydney. This work has been carried out in the framework of the Labex MEC (ANR-10-LABX-0092) and of the A*MIDEX project (ANR-11-IDEX-0001-02), funded by the 'Investissements d'Avenir' French Government programme managed by the French National Research Agency (ANR). GMV acknowledges support from the Australian Research Council, project number DE140101960. This research is funded in part by the Gordon and Betty Moore Foundation through Grant GBMF5076 to Lars Bildsten, Eliot Quataert and E. Sterl Phinney. This research was supported in part by the National Science Foundation under Grant No. NSF PHY-1125915. The authors thank KITP for supporting a follow-up meeting where much of this work was initiated. Resources supporting this work were provided by the NASA High-End Computing (HEC) Programme through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center. This project was supported by NASA under the SPIDER TCAN, grant number NNX14AB53G.

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Accepted Version - 1610.08506.pdf


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