Simulations of the onset and dynamical evolution of inertial waves in solar/stellar interior

TL;DR

This study uses global numerical simulations to explore how inertial modes are excited and evolve in the Sun and stellar interiors, revealing their origins, dynamics, and impact on differential rotation.

Contribution

It demonstrates the nonlinear excitation and evolution of inertial modes driven by baroclinic instability in stellar interiors, with implications for solar and stellar dynamics.

Findings

High-latitude inertial modes form retrograde polar vortices.

Equatorial Rossby modes are less intense but still excited.

Increased shear modifies differential rotation via Reynolds stresses.

Abstract

Inertial modes have been recently detected in the Sun via helioseismology, yet their origin, evolution, and role in the dynamics of the solar plasma and magnetic field remain poorly understood. In this study, we employ global numerical simulations to investigate the excitation mechanisms and dynamical consequences of inertial modes in the Sun and stellar interiors. We validate first our numerical setup by analyzing the evolution of sectoral and tesseral perturbations imposed on a rigidly rotating sphere. The results confirm that a perturbation of a given mode can excite neighboring modes with both smaller and larger wavenumbers along the dispersion relation of Rossby waves. Subsequently, we use a physically motivated forcing to impose differential rotation with varying shear amplitudes, and examine the spontaneous onset and nonlinear evolution of inertial modes. The simulations reveal that the growth of velocity perturbations is primarily driven by baroclinic instability. It gives rise to high-latitude inertial modes in the form of retrograde polar vortices whose properties depend on the imposed shear. Equatorial Rossby modes are also excited, albeit with lower intensity than their high-latitude counterpart. Perturbations with arbitrary azimuthal wavenumbers lead to the excitation of Rossby modes for all available wave numbers, sustained by both direct and inverse energy cascades. In simulations with stronger shear, the high latitude modes produce Reynolds stresses able to modify the imposed differential rotation and accelerate the rotation of the poles.