Variability from thermo-resistive instability in the atmospheres of hot jupiters

TL;DR

This paper introduces a dynamic model of thermo-resistive instability in hot Jupiter atmospheres, revealing periodic magnetic oscillations driven by temperature-dependent electrical conductivity, which may cause atmospheric variability.

Contribution

It presents a new time-dependent solution showing oscillatory behavior due to temperature-dependent conductivity, highlighting the importance of this factor in hot Jupiter atmospheric models.

Findings

Instability occurs at 0.1-1 bar pressures and 1300-1800 K temperatures.

Weaker magnetic fields lead to stronger instability.

Variability is likely in magnetized hot Jupiter atmospheres at intermediate temperatures.

Abstract

The atmosphere of a hot jupiter may be subject to a thermo-resistive instability, in which the increasing electrical conductivity with temperature leads to runaway Ohmic heating. We introduce a simplified model of the local dynamics in the equatorial region of a hot jupiter that incorporates the back reaction on the atmospheric flow as the increasing electrical conductivity leads to flux freezing, which in turn quenches the flow and therefore the Ohmic heating. We demonstrate a new time-dependent solution that emerges for a temperature-dependent electrical conductivity (whereas a temperature-independent conductivity always evolves to a steady-state). The periodic cycle consists of bursts of Alfven oscillations separated by quiescent intervals, with the magnetic Reynolds number alternating between values smaller than and larger than unity, maintaining the oscillation. We investigate the regions of pressure and temperature in which the instability operates. For the typical equatorial accelerations seen in atmospheric models, we find instability at pressures $\sim 0.1$--$1\ {\rm bar}$ and temperatures $\approx 1300$--$1800\ {\rm K}$ for magnetic fields $\sim 10\ {\rm G}$. Unlike previous studies based on a constant wind velocity, we find that the instability is stronger for weaker magnetic fields. Our results add support to the idea that variability should be a feature of magnetized hot jupiter atmospheres, particularly at intermediate temperatures. The temperature-dependence of the electrical conductivity is an important ingredient that should be included in MHD models of hot jupiter atmospheric dynamics.