Letter to the Editor

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

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: crgoddard10@gmail.com, gizon@mps.mpg.de
² Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Center for Space Science, NYUAD Institute, New York University Abu Dhabi, Abu Dhabi, UAE

Received: 30 May 2020
Accepted: 13 July 2020

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.