The Role of Drag in the Energetics of Strongly Forced Exoplanet Atmospheres

TL;DR

This paper investigates how drag mechanisms can serve as significant heat sources in strongly-forced exoplanet atmospheres, potentially altering circulation patterns and energetics, especially in hot Jupiters and eccentric gas giants.

Contribution

It introduces a modified energy cycle formalism that explicitly accounts for frictional heating and assesses the impact of drag on atmospheric energetics in exoplanets.

Findings

Drag can significantly influence atmospheric circulation and energetics.

Frictional heating may account for 5-10% of stellar irradiation in hot Jupiter models.

Localized drag mechanisms can affect circulation even at low strengths.

Abstract

In contrast to the Earth, where frictional heating is typically negligible, we show that drag mechanisms could act as an important heat source in the strongly-forced atmospheres of some exoplanets, with the potential to alter the circulation. We modify the standard formalism of the atmospheric energy cycle to explicitly track the loss of kinetic energy and the associated frictional (re)heating, for application to exoplanets such as the asymmetrically heated "hot Jupiters" and gas giants on highly eccentric orbits. We establish that an understanding of the dominant drag mechanisms and their dependence on local atmospheric conditions is critical for accurate modeling, not just in their ability to limit wind speeds, but also because they could possibly change the energetics of the circulation enough to alter the nature of the flow. We discuss possible sources of drag and estimate the strength necessary to significantly influence the atmospheric energetics. As we show, the frictional heating depends on the magnitude of kinetic energy dissipation as well as its spatial variation, so that the more localized a drag mechanism is, the weaker it can be and still affect the circulation. We also use the derived formalism to estimate the rate of numerical loss of kinetic energy in a few previously published hot Jupiter models with and without magnetic drag and find it to be surprisingly large, at 5-10% of the incident stellar irradiation.