Issue |
A&A
Volume 652, August 2021
|
|
---|---|---|
Article Number | A96 | |
Number of page(s) | 7 | |
Section | The Sun and the Heliosphere | |
DOI | https://doi.org/10.1051/0004-6361/202141044 | |
Published online | 17 August 2021 |
Detection of Rossby modes with even azimuthal orders using helioseismic normal-mode coupling
1
Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: mandal@mps.mpg.de
2
Tata Institute of Fundamental Research, Mumbai 400005, India
3
Institut für Astrophysik, Georg-August-Universitát Göttingen, 37077 Göttingen, Germany
4
Center for Space Science, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
Received:
10
April
2021
Accepted:
1
June
2021
Context. Retrograde Rossby waves, measured to have significant amplitudes in the Sun, likely have notable implications for various solar phenomena.
Aims. Rossby waves create small-amplitude, very-low frequency motions, on the order of the rotation rate and lower, which in turn shift the resonant frequencies and eigenfunctions of the acoustic modes of the Sun. The detection of even azimuthal orders Rossby modes using mode coupling presents additional challenges and prior work therefore only focused on odd orders. Here, we successfully extend the methodology to measure even azimuthal orders as well.
Methods. We analyze 4 and 8 years of Helioseismic and Magnetic Imager (HMI) data and consider coupling between different-degree acoustic modes (of separations 1 and 3 in the harmonic degree). The technique uses couplings between different frequency bins to capture the temporal variability of the Rossby modes.
Results. We observe significant power close to the theoretical dispersion relation for sectoral Rossby modes, where the azimuthal order is the same as the harmonic degree, s = |t|. Our results are consistent with prior measurements of Rossby modes with azimuthal orders over the range t = 4 to 16 with maximum power occurring at mode t = 8. The amplitudes of these modes vary from 1 to 2 m s−1. We place an upper bound of 0.2 m s−1 on the sectoral t = 2 mode, which we do not detect in our measurements.
Conclusions. This effort adds credence to the mode-coupling methodology in helioseismology.
Key words: Sun: helioseismology / Sun: interior / Sun: oscillations / waves
© K. Mandal et al. 2021
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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