Dynamical Tides in Rotating Planets and Stars

TL;DR

This paper revisits a mechanism for tidal dissipation in rotating planets and stars, demonstrating how inertial waves excited at the core can significantly influence planetary evolution, especially in hot Jupiters, through analytical methods.

Contribution

It provides an analytical demonstration of inertial wave excitation at the core via scattering of the equilibrium tide, clarifying the wave excitation process and quantifying the dissipation rate.

Findings

Tidal dissipation scales with the fifth power of core radius.

Tidal Q is approximately ten million for typical hot Jupiter parameters.

Inertial waves can be excited at the core, affecting planetary heat and radius.

Abstract

Tidal dissipation may be important for the internal evolution as well as the orbits of short-period massive planets--hot Jupiters. We revisit a mechanism proposed by Ogilvie and Lin for tidal forcing of inertial waves, which are short-wavelength, low-frequency disturbances restored primarily by Coriolis rather than buoyancy forces. This mechanism is of particular interest for hot Jupiters because it relies upon a rocky core, and because these bodies are otherwise largely convective. Compared to waves excited at the base of the stratified, externally heated atmosphere, waves excited at the core are more likely to deposit heat in the convective region and thereby affect the planetary radius. However, Ogilvie and Lin's results were numerical, and the manner of the wave excitation was not clear. Using WKB methods, we demonstrate the production of short waves by scattering of the equilibrium tide off the core at critical latitudes. The tidal dissipation rate associated with these waves scales as the fifth power of the core radius, and the implied tidal $Q$ is of order ten million for nominal values of the planet's mass, radius, orbital period, and core size. We comment upon an alternative proposal by Wu for exciting inertial waves in an unstratified fluid body by means of compressibility rather than a core. We also find that even a core of rock is unlikely to be rigid. But Ogilvie and Lin's mechanism should still operate if the core is substantially denser than its immediate surroundings.