Properties of Solar Rossby Waves from Normal Mode Coupling and Characterizing Its Systematics

Abstract

Rossby waves play an important role in mediating the angular momentum of rotating spherical fluids, creating weather on Earth and tuning exoplanet orbits in distant stellar systems. Their recent discovery in the solar convection zone provides an exciting opportunity to appreciate the detailed astrophysics of Rossby waves. Large-scale Rossby waves create subtle drifts in acoustic oscillations in the convection zone, which we measure using helioseismology to image properties of Rossby waves in the interior. We analyze 20 yr of space-based observations, from 1999 to 2018, to measure Rossby-mode frequencies, line widths, and amplitudes. Spatial leakage affects the measurements of normal-mode eigenfunction coupling (which we refer to as "normal-mode coupling" in this paper) and complicates the analysis of separating out specific harmonic degree and azimuthal number of features on the Sun. Here we demonstrate a novel approach to overcome this difficulty and test it by performing synthetic tests. We find that the rms velocity of the modes is of the order of 0.5 m s^-1 at the surface.