Resistive MHD Simulations of Stellar Wind-Magnetosphere Coupling in TRAPPIST-1e

TL;DR

This study uses 3D resistive MHD simulations to explore how stellar wind interactions affect TRAPPIST-1e's magnetosphere and potential radio emissions, highlighting the impact of magnetic diffusivity on energy transfer and observability.

Contribution

First detailed resistive MHD simulations of TRAPPIST-1e's magnetosphere under different stellar wind conditions, analyzing energy transport and radio emission proxies.

Findings

Increasing magnetic diffusivity broadens the coupling layer.

Radio-power proxy increases significantly with diffusivity.

Maximum cyclotron frequencies are below ground-based observation range.

Abstract

Close-in terrestrial exoplanets around M dwarfs reside in dense, magnetized winds, where non-ideal plasma coupling can strongly affect how electromagnetic energy is redistributed within the dayside interaction region. We present three-dimensional resistive magnetohydrodynamic simulations of the TRAPPIST-1 wind interacting with a dipolar TRAPPIST-1e magnetosphere for three stellar-wind forcing cases and four prescribed magnetic diffusivities, $\eta=(0,\ 538.018,\ 5.38018\times10^{8},\ 5.38018\times10^{12})$ cm$^{2}$ s$^{-1}$. Energy transport is diagnosed using maps of the total energy density, the magnitude of the total Poynting flux, and the divergence of the total Poynting flux. We further estimate a radio-power proxy from the volume integral of $\nabla\cdot \mathbf{S}_{\rm total}$ over the dayside bow-shock and magnetopause layers. Across all cases, increasing prescribed $\eta$ broadens the coupling layer and shifts the dominant energy-conversion regions from thin, patchy boundary arcs to thicker, more spatially extended structures, with an increasing relative contribution from the magnetopause. The inferred radio-power proxy increases by several orders of magnitude across the explored scan. However, because the estimated numerical magnetic diffusivity in the strongest-gradient regions is $\eta_{\rm num}\sim10^{15}$-$10^{16}$ cm$^{2}$ s$^{-1}$, the present $\eta$ scan is best interpreted as a controlled sensitivity study rather than as a direct constraint on the physical diffusivity of the TRAPPIST-1e environment. For the adopted planetary fields ($B_{\rm eq}=0.32$-$1.28$ G), the maximum cyclotron frequencies are $\nu_{c,\max}\approx1.8$-$7.2$ MHz, below the ground-based window, implying that meaningful radio constraints on TRAPPIST-1e magnetism will require space-based observations below 10 MHz or substantially stronger planetary fields than those assumed here.