The Quasi-Biennial Periodicity (QBP) in velocity and intensity helioseismic observations

TL;DR

This study detects a quasi-biennial periodicity in helioseismic data, revealing latitudinal and activity-dependent variations that suggest a distinct solar dynamo mechanism beyond the 11-year cycle.

Contribution

It provides the first comprehensive analysis of QBP signatures across multiple helioseismic datasets, highlighting latitudinal dependence and independence from surface activity indices.

Findings

QBP signatures are present at all activity levels in modes sensitive to higher latitudes.

Signal strength of QBP increases with latitude and is modulated by the 11-year cycle.

Seismic QBP is persistent and differs from surface activity variations.

Abstract

We looked for signatures of Quasi-Biennial Periodicity (QBP) over different phases of solar cycle by means of acoustic modes of oscillation. Low-degree p-mode frequencies are shown to be sensitive to changes in magnetic activity due to the global dynamo. Recently have been reported evidences in favor of two-year variations in p-mode frequencies. Long high-quality helioseismic data are provided by BiSON (Birmingham Solar Oscillation Network), GONG (Global Oscillation Network Group), GOLF (Global Oscillation at Low Frequency) and VIRGO (Variability of Solar IRradiance and Gravity Oscillation) instruments. We determined the solar cycle changes in p-mode frequencies for spherical degree l=0, 1, 2 with their azimuthal components in the frequency range 2.5 mHz < nu < 3.5 mHz. We found signatures of QBP at all levels of solar activity in the modes more sensitive to higher latitudes. The signal strength increases with latitude and the equatorial component seems also to be modulated by the 11-year envelope. The persistent nature of the seismic QBP is not observed in the surface activity indices, where mid-term variations are found only time to time and mainly over periods of high activity. This feature together with the latitudinal dependence provides more evidences in favor of a mechanism almost independent and different from the one that brings up to the surface the active regions. Therefore, these findings can be used to provide more constraints on dynamo models that consider a further cyclic component on top of the 11-year cycle.