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.
Highlights
Rossby waves are global-scale waves of vorticity which arise due to the conservation of the radial component of the absolute vorticity for rotating fluids
We investigate the impact of the time-varying zonal flows on the frequencies of Rossby modes
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
Summary
Rossby waves are global-scale waves of vorticity which arise due to the conservation of the radial component of the absolute vorticity for rotating fluids They are most commonly discussed in the context of planetary atmospheres and oceans All data sets revealed changes in the rotation rate that are associated with the solar cycle These time-varying zonal flows or torsional oscillations were first noted at the surface (Howard & Labonte 1980) and have since been shown to extend throughout most of the convection zone (Vorontsov et al 2002), they weaken with depth In the convection zone SC24 has higher rotation rates at low latitudes, but lower rotation rates at high latitudes This asymmetry is not due to the inconsistencies between the MDI and HMI data, and is clearly seen in GONG data (Basu & Antia 2019)
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