Spatio-temporal analysis of helioseismic quasi-biennial oscillations

TL;DR

This study analyzes helioseismic data to understand the spatial and temporal characteristics of quasi-biennial oscillations (QBOs) across solar cycles, revealing their weak latitudinal dependence and partial independence from the solar cycle amplitude.

Contribution

It provides new insights into the latitudinal variation, amplitude behavior, and cycle relationship of QBOs using wavelet analysis of helioseismic frequency shifts.

Findings

QBO periods are nearly constant at ~3 years at higher latitudes.

QBO amplitudes increase with mode frequency and are higher at low latitudes.

QBO amplitude ratios differ between Cycles 23 and 24, with a linear relation to cycle amplitude.

Abstract

Quasi-biennial oscillations (QBOs) are shorter-term periodic signals that occur alongside the dominant 11-year solar cycle. In this study, we examine the spatial and temporal evolution of QBOs using helioseismic p-mode frequency shifts from the Global Oscillation Network Group (GONG) across solar Cycles 23 and 24 and the ascending phase of Cycle 25. By applying wavelet analysis to frequency shifts, we studied the changes in QBO periodicities to determine whether the QBO period and amplitude vary with latitude. Our results show that QBO periods exhibit a weak latitudinal dependence, with shorter and less persistent signals at low latitudes, while at higher latitudes the periods are nearly constant at $\sim$3 years. Cycle 24 tends to display slightly longer periods than Cycle 23, though within uncertainties. At all latitudes, QBO amplitudes increase with mode frequency, which is consistent with previous studies. Higher amplitude QBOs are found at low latitudes, reflecting the distribution of surface magnetic activity. The ratio of QBO to cycle amplitude is systematically higher in Cycle 24 than in Cycle 23, and above $20^\circ$ latitude the amplitude ratio is nearly uniform in Cycle 23 but shows modest variations in Cycle 24. A linear relation between QBO amplitude and cycle amplitude is found in both cycles, but with significantly different slopes, indicating that QBO amplitudes are not wholly governed by the solar cycle strength and are at least partially decoupled from it. Finally, we find no evidence that QBO period depends on QBO amplitude, consistent with a linear oscillation regime.