Predicting frequency changes of global-scale solar Rossby modes due to solar cycle changes in internal rotation

TL;DR

This study predicts how solar cycle-induced internal rotation changes affect the frequencies of global Rossby modes, finding shifts near the detection threshold, which enhances understanding of solar dynamics.

Contribution

The paper introduces a first-order perturbation theory model to quantify frequency shifts of Rossby modes caused by solar cycle variations in internal rotation.

Findings

Frequency shifts are less than 1 nHz for m=1

Frequency shifts reach up to 25 nHz for m=15

Predicted shifts are near the observational detection limit

Abstract

Context. Large-scale equatorial Rossby modes have been observed on the Sun over the last two solar cycles. Aims. We investigate the impact of the time-varying zonal flows on the frequencies of Rossby modes. Methods. A first-order perturbation theory approach is used to obtain an expression for the expected shift in the mode frequencies due to perturbations in the internal rotation rate. Results. Using the time-varying rotation from helioseismic inversions we predict the changes in Rossby mode frequencies with azimuthal orders from m = 1 to m = 15 over the last two solar cycles. The peak-to-peak frequency change is less than 1 nHz for the m = 1 mode, grows with m, and reaches 25 nHz for m = 15. Conclusions. Given the observational uncertainties on mode frequencies due to the finite mode lifetimes, we find that the predicted frequency shifts are near the limit of detectability.