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

Abstract

The long-standing puzzle of the CO/CH$_{4}$ 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-CH$_{4}$ electron-moderated chemistry across the parameter ranges $T_{\rm gas} \in [700, 1600]$ K} and $T_{e} \in [2, 5]$ eV. We simulate a 1 microsecond pulse, representing a short dart-stepped leader; and, two pulsed systems with $(t_{\rm on}, t_{\rm off})$ = ($10^{-8}$ 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 CH$_{4}$/CO ratio in our sample atmosphere within one microsecond. Beyond the CO-CH$_{4}$ system, electron-moderated plasma chemistry exerts a far-reaching influence on substellar atmospheric composition.