Observed Power and Frequency Variations of Solar Rossby Waves with Solar Cycles

TL;DR

This study analyzes 12 years of helioseismic data to examine how the power and frequency of solar Rossby waves vary with the solar cycle, revealing cycle-dependent changes that could inform solar interior dynamics.

Contribution

It provides the first systematic analysis of the cycle dependence of solar Rossby waves using subsurface velocity data from multiple helioseismic methods.

Findings

Rossby wave power peaks during solar maximum

Wave frequency decreases during active years

Cycle dependence suggests interaction with solar magnetic activity

Abstract

Several recent studies utilizing different helioseismic methods have confirmed the presence of large-scale vorticity waves known as solar Rossby waves within the Sun. Rossby waves are distinct from acoustic waves, typically with longer periods and lifetimes; and their general properties, even if only measured at the surface, may be used to infer properties of the deeper convection zone, such as the turbulent viscosity and entropy gradients which are otherwise difficult to observe. In this study, we utilize $12~$years of inverted subsurface velocity fields derived from the SDO/HMI's time--distance and ring-diagram pipelines to investigate the propoerty of the solar equatorial Rossby waves. By covering the maximum and the decline phases of Solar Cycle 24, these datasets enable a systematic analysis of any potential cycle dependence of these waves. Our analysis provides evidence of a correlation between the average power of equatorial Rossby waves and the solar cycle, with stronger Rossby waves during the solar maximum and weaker waves during the minimum. Our result also shows that the frequency of the Rossby waves is lower during the magnetic active years, implying a larger retrograde drift relative to the solar rotation. Although the underlying mechanism that enhances the Rossby wave power and lowers its frequency during the cycle maximum is not immediately known, this observation has the potential to provide new insights into the interaction of large-scale flows with the solar cycle.