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

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, $\eta =(0,538.018,5.38018\times 10^8,5.38018\times 10^{12})$ 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 $\nabla \cdot {\mathbf{S}}_{\mathrm{total}}$ 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 ${\eta }_{\mathrm{num}}\sim 10^{15}$-$10^{16}$ 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 ($B_{\mathrm{eq}}=0.32$-$1.28$ G), the maximum cyclotron frequencies are ${\nu }_{c,\max }\approx 1.8$-$7.2$ 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.

10 pages, 6 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

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