Thermo-Resistive Instability of Hot Planetary Atmospheres

TL;DR

This paper investigates a thermal instability in hot planetary atmospheres caused by ohmic dissipation, which could explain observed thermal inversions in hot Jupiters, highlighting the need for advanced modeling.

Contribution

It identifies the conditions under which thermo-resistive instability occurs in hot exoplanet atmospheres and discusses its potential role in explaining thermal inversions.

Findings

Instability occurs at specific pressure and temperature ranges.

High non-thermal photoionization suppresses the instability.

Global non-linear models are needed for further understanding.

Abstract

The atmospheres of hot Jupiters and other strongly-forced exoplanets are susceptible to a thermal instability in the presence of ohmic dissipation, weak magnetic drag and strong winds. The instability occurs in radiatively-dominated atmospheric regions when the ohmic dissipation rate increases with temperature faster than the radiative (cooling) rate. The instability domain covers a specific range of atmospheric pressures and temperatures, typically P ~ 3-300 mbar and T ~ 1500-2500K for hot Jupiters, which makes it a candidate mechanism to explain the dayside thermal "inversions" inferred for a number of such exoplanets. The instability is suppressed by high levels of non-thermal photoionization, in possible agreement with a recently established observational trend. We highlight several shortcomings of the instability treatment presented here. Understanding the emergence and outcome of the instability, which should result in locally hotter atmospheres with stronger levels of drag, will require global non-linear atmospheric models with adequate MHD prescriptions.