Journal Article

Quasi-biennial variations in helioseismic frequencies: can the source of the variation be localized?

A.-M. Broomhall, W. J. Chaplin, Y. Elsworth and R. Simoniello

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 420, issue 2, pages 1405-1414
Published in print February 2012 | ISSN: 0035-8711
Published online January 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.20123.x
Quasi-biennial variations in helioseismic frequencies: can the source of the variation be localized?

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We investigate the spherical harmonic degree (l) dependence of the ‘seismic’ quasi-biennial oscillation (QBO) observed in low-degree solar p-mode frequencies, using Sun-as-a-star Birmingham Solar Oscillations Network data. The amplitude of the seismic QBO is modulated by the 11-yr solar cycle, with the amplitude of the signal being largest at solar maximum. The amplitude of the signal is noticeably larger for the l= 2 and 3 modes than for the l= 0 and 1 modes. The seismic QBO shows some frequency dependence but this dependence is not as strong as observed in the 11-yr solar cycle. These results are consistent with the seismic QBO having its origins in shallow layers of the interior (one possibility being the bottom of the shear layer extending 5 per cent below the solar surface). Under this scenario the magnetic flux responsible for the seismic QBO is brought to the surface (where its influence on the p modes is stronger) by buoyant flux from the 11-yr cycle, the strong component of which is observed at predominantly low latitudes. As the l= 2 and 3 modes are much more sensitive to equatorial latitudes than the l= 0 and 1 modes the influence of the 11-yr cycle on the seismic QBO is more visible in l= 2 and 3 mode frequencies. Our results imply that close to solar maximum the main influence of the seismic QBO occurs at low latitudes (<45°), which is where the strong component of the 11-yr solar cycle resides. To isolate the latitudinal dependence of the seismic QBO from the 11-yr solar cycle we must consider epochs when the 11-yr solar cycle is weak. However, away from solar maximum, the amplitude of the seismic QBO is weak making the latitudinal dependence hard to constrain.

Keywords: methods: data analysis; Sun: helioseismology; Sun: oscillations

Journal Article.  7478 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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