Plasma chemistry and electron-moderated pathways in substellar atmospheres: a new perspective on the L/T transition

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ID: 320747
2026
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Abstract
Abstract The long-standing puzzle of the CO/CH4 transition in brown dwarfs endures. Although the bulk spectral evolution across an atmosphere can be accounted for through thermal equilibrium cloud models, the behaviour in the NIR remains unaccounted for, indicating that additional, non-thermal processes may influence atmospheric chemistry alongside conventional pathways. We explore cloud-driven electrical activation, where low-energy sparks to full lightning discharges, unlocks non-equilibrium reaction pathways inaccessible under thermal conditions alone. To quantify this, the aim of this paper is to model the electron-moderated atmospheric chemistry with SPARCKS, a bespoke zero-dimensional code that solves the coupled set of particle balance equations for substellar plasma activation and reaction kinetics, focusing on the key CO–CH4 electron-moderated chemistry across the parameter ranges Tgas ∈ [700, 1600] K and Te ∈ [2, 5] eV. We simulate a 1 microsecond pulse, representing a short dart-stepped leader; and, two pulsed systems with (ton, toff) = (10−8 s, 10−6 s) and (10−9 s, 10−9 s), representing small-scale inter-grain discharges, consistent with a typical characteristic substellar atmosphere. Our results show that even modest, physically plausible energies can strongly perturb atmospheric composition: an electron energy of 3.0 eV is sufficient to halve the CH4/CO ratio in our sample atmosphere within one microsecond. Beyond the CO–CH4 system, electron-moderated plasma chemistry exerts a far-reaching influence on substellar atmospheric composition.
Reference Key
openalex_W7168079962 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors M I Swayne, D. A. Diver, C. R. Stark
Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1322
URL
https://doi.org/10.1093/mnras/stag1322
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