Quasi-biennial variations in helioseismic frequencies: can the source of the variation be localized?
Article first published online: 1 DEC 2011
© 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
Monthly Notices of the Royal Astronomical Society
Volume 420, Issue 2, pages 1405–1414, February 2012
How to Cite
Broomhall, A.-M., Chaplin, W. J., Elsworth, Y. and Simoniello, R. (2012), Quasi-biennial variations in helioseismic frequencies: can the source of the variation be localized?. Monthly Notices of the Royal Astronomical Society, 420: 1405–1414. doi: 10.1111/j.1365-2966.2011.20123.x
- Issue published online: 24 JAN 2012
- Article first published online: 1 DEC 2011
- Accepted 2011 November 2. Received 2011 November 1; in original form 2011 August 5
- methods: data analysis;
- Sun: helioseismology;
- Sun: oscillations
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.