Quasi-Biennial variations in helioseismic frequencies: Can the source of the variation be localized?

TL;DR

This study analyzes the spherical harmonic degree dependence of the quasi-biennial oscillation in solar p-mode frequencies, suggesting it originates in shallow interior layers and is influenced by the 11-year solar cycle, especially at low latitudes.

Contribution

It provides new insights into the latitudinal and depth dependence of the seismic QBO, linking it to shallow interior magnetic flux and the 11-year solar cycle.

Findings

Seismic QBO amplitude modulated by the 11-year cycle, largest at solar maximum.

Greater QBO amplitude observed in l=2 and 3 modes than in l=0 and 1.

QBO likely originates in shallow interior layers near the solar surface.

Abstract

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 (BiSON) 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 5per 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 degrees), 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.