Latitudinal Propagation of Thermal Rossby Waves in Stellar Convection Zones

TL;DR

This paper develops an analytic model to study thermal Rossby waves in stellar convection zones, revealing how their propagation and trapping depend on frequency, wave direction, and entropy gradients, with implications for solar observations.

Contribution

It provides a novel analytic derivation of eigenfrequencies for trapped thermal Rossby waves in stellar atmospheres, considering effects of stratification and wave direction.

Findings

Prograde thermal Rossby waves are trapped radially above a critical frequency.

A continuous spectrum of inertial waves exists below the threshold frequency.

Entropy gradients can trap inertial waves in stellar convection zones.

Abstract

Using an analytic model, we derive the eigenfrequencies for thermal Rossby waves that are trapped radially and latitudinally in an isentropically stratified atmosphere. We ignore the star's curvature and work in an equatorial f-plane geometry. The propagation of inertial waves is found to be sensitive to the relative direction of the wave vector to the zonal direction. Prograde propagating thermal Rossby waves are naturally trapped in the radial direction for frequencies above a critical threshold, which depends on the angle of propagation. Below the threshold frequency, there exists a continuous spectrum of prograde and retrograde inertial waves that are untrapped in an isentropic atmosphere, but can be trapped by gradients in the specific entropy density such as occurs in a stellar convection zone. Finally, we discuss the implications of these waves on recent observations of inertial oscillations in the Sun, as well as in numerical simulations.