Two-dimensional partially ionized magnetohydrodynamic turbulence

TL;DR

This study investigates the turbulent dynamics of partially ionized magnetohydrodynamic flows in planetary atmospheres using a two-fluid model, revealing how collisions influence energy dissipation and the coupling of neutral and ionized components.

Contribution

The paper introduces a 2D partially-ionized MHD simulation framework and derives an expression for collision-induced energy loss, enhancing understanding of planetary atmospheric turbulence.

Findings

Collision effects significantly influence energy dissipation.

Neutral fluid couples to ions in the strongly collisional regime.

Neutral and ionized components behave as a single MHD fluid under certain conditions.

Abstract

Ionization occurs in the upper atmospheres of hot Jupiters and in the interiors of Gas Giant Planets, leading to magnetohydrodynamic (MHD) effects which couple the momentum and the magnetic field, thereby significantly altering the dynamics. In regions of moderate temperatures the gas is only partially ionized, which also leads to interactions with neutral molecules. To explore the turbulent dynamics of these regions we utilize Partially-Ionized MHD (PIMHD), a two-fluid model -- one neutral and one ionized -- coupled by a collision term proportional to the difference in velocities. Motivated by planetary settings where rotation constrains the large-scale motions to be mostly two-dimensional, we perform a suite of simulations to examine the parameter space of 2D PIMHD turbulence and pay particular attention to collisions and their role in the dynamics, dissipation, and energy exchange between the two species. We arrive at, and numerically confirm, an expression for the energy loss due to collisions in both the weakly and strongly collisional limits, and show that, in the latter limit, the neutral fluid couples to the ions and behaves as an MHD fluid. Finally, we discuss some implications of our findings to current understanding of gas giant planet atmospheres.