Magnetospheric Structure and Atmospheric Joule Heating of Habitable Planets Orbiting M-dwarf Stars

TL;DR

This study models the magnetospheric structure and atmospheric Joule heating of habitable planets around M-dwarf stars, revealing extreme space environments and significant atmospheric heating that impact planetary habitability.

Contribution

It introduces a coupled MHD model to analyze magnetospheric dynamics and Joule heating for planets in M-dwarf habitable zones, highlighting the importance of sub- and super-Alfvenic conditions.

Findings

Magnetospheres change dramatically with bow shocks forming in super-Alfvenic sectors.

Most of the time, planets are in sub-Alfvenic sectors with poor atmospheric protection.

Joule heating accounts for 0.1-3% of stellar irradiation, increasing by 50% in time-dependent scenarios.

Abstract

We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic (MHD) models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvenic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvenic sectors, while no bow shock forms in the sub-Alfvenic sectors. The planets reside most of the time in the sub-Alfvenic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the planetary interaction with the stellar wind. For the steady-state solution, the heating is about 0.1-3\% of the total incoming stellar irradiation, and it is enhanced by 50\% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport.