Detection of Rossby modes with even azimuthal orders using helioseismic normal-mode coupling

TL;DR

This paper extends helioseismic mode coupling techniques to detect even azimuthal order Rossby modes in the Sun, analyzing several years of data to confirm their presence and characteristics, thus advancing understanding of solar dynamics.

Contribution

The study introduces a method to detect even azimuthal order Rossby modes using helioseismic mode coupling, expanding previous focus on odd orders and validating the approach with extensive data analysis.

Findings

Detected Rossby modes with azimuthal orders 4 to 16.

Maximum power observed at mode t=8.

Upper bound set for t=2 mode at 0.2 m/s.

Abstract

Retrograde Rossby waves, measured to have significant amplitudes in the Sun, likely have notable implications for various solar phenomena. Rossby waves create small-amplitude, very-low frequency motions (on the order of the rotation rate and lower), which in turn shift the resonant frequencies and eigenfunctions of the acoustic modes of the Sun. The detection of even azimuthal orders Rossby modes using mode coupling presents additional challenges and prior work therefore only focused on odd orders. Here, we successfully extend the methodology to measure even azimuthal orders as well. We analyze 4 and 8 years of Helioseismic and Magnetic Imager (HMI) data and consider coupling between different-degree acoustic modes (of separations 1 and 3 in harmonic degree). The technique uses couplings between different frequency bins to capture the temporal variability of the Rossby modes. We observe significant power close to the theoretical dispersion relation for sectoral Rossby modes (where the azimuthal order is same as harmonic degree, s = |t|). Our results are consistent with prior measurements of Rossby modes with azimuthal orders over the range t = 4 to 16 with maximum power occurring at mode t = 8. The amplitudes of these modes vary from 1 to 2 m/s. We place an upper bound of 0.2 m/s on the sectoral t = 2 mode, which we do not detect in our measurements. This effort adds credence to the mode-coupling methodology in helioseismology