J/MNRAS/xx/yy/zz (Swayne+, 2026) ================================================================================ Plasma chemistry and electron-moderated pathways in substellar atmospheres: a new perspective on the L/T transition Swayne M. I., Stark C. R., Diver D. A. =Submitted for pub. in MNRAS ================================================================================ Keywords: (stars:) brown dwarfs -- stars: atmospheres -- planets and satellites: atmospheres -- plasma -- molecular processes Abstract: The long‑standing puzzle of the CO/CH$_{4}$ transition in brown dwarfs endures because conventional chemistry lacks a crucial driver. We argue that cloud‑driven electrical activation, from 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 exttt{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 $T_{ m gas} \in [700, 1600]\,\mathrm{K}$ and $T_{e} \in [2, 5]\,\mathrm{eV}$. We simulate a 1 microsecond pulse, representing a short dart-stepped leader; and, two pulsed systems with $(t_{ m on}, t_{ m off}) = (10^{-8}~\mathrm{s}, 10^{-6}~\mathrm{s})$ and $(10^{-9}~\mathrm{s}, 10^{-9}~\mathrm{s})$, 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. Description: Record contains 9 folders displaying records of SPARCKS run. Folders display the simulation data for all three ran scenarios. These include the single-pulse run for one microsecond and both scenarios involving intermittent pulsing, scenario 1 involving a 10 ns on-pulse and a 990 ns off-pulse performed four times for 4 microseconds and scenario 2 involving a 1 ns on-pulse and 1 ns off-pulse, 500 times for 1 microsecond. The single-pulse runs and on-pulse times are held at a high electron temperature. This electron temperature is signified for each file by the first number in the file name. 200 = 2.0 eV and so on. The second number represents the constant gas temperature for the run. There are three folders for each scenario as explained below. All the files in total contain the simulation runs for the electron temperature distribution shown in Figures 6-8 of the paper, displaying the most notable changes in behaviour in species population for our model. Each file begins with a key of the different temperatures in the simulation along with the timestep. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . this file ndsp/* . 102 Individual number density files for selected range of electron and gas temperatures (single pulse) dtsp/* . 102 Individual rate of change of no. density files for selected range of electron and gas temperatures (single pulse) rrsp/* . 104 Individual reaction rate coefficients for selected range of electron and gas temperatures (single pulse) nds1/* . 102 Individual number density files for selected range of electron and gas temperatures (MP scenario 1) dts1/* . 102 Individual rate of change of no. density files for selected range of electron and gas temperatures (MP scenario 1) rrs1/* . 104 Individual reaction rate coefficients for selected range of electron and gas temperatures (MP scenario 1) nds2/* . 102 Individual number density files for selected range of electron and gas temperatures (MP scenario 2) dts2/* . 102 Individual rate of change of no. density files for selected range of electron and gas temperatures (MP scenario 2) rrs2/* . 104 Individual reaction rate coefficients for selected range of electron and gas temperatures (MP scenario 2) -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: ndsp/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 19 --- time(s) timestep of simulation in seconds 20-707 --- n[sp] normalised number density of labelled species 708-723 --- driver Column recording added electrons each step (not used) -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: dtsp/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 15 --- time(s) timestep of simulation in seconds 20-555 --- n[sp] rate of change of number density of labelled species 556-571 --- driver Column recording added electrons each step (not used) -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: rrsp/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 3601 --- [no.] Reaction rate of indicated reaction for each timestep -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: nds1/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 19 --- time(s) timestep of simulation in seconds 20-707 --- n[sp] normalised number density of labelled species 708-723 --- driver Column recording added electrons each step (not used) -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: dts1/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 15 --- time(s) timestep of simulation in seconds 20-555 --- n[sp] rate of change of number density of labelled species 556-571 --- driver Column recording added electrons each step (not used) -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: rrs1/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 3601 --- [no.] Reaction rate of indicated reaction for each timestep -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: nds2/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 19 --- time(s) timestep of simulation in seconds 20-707 --- n[sp] normalised number density of labelled species 708-723 --- driver Column recording added electrons each step (not used) -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: dts2/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 15 --- time(s) timestep of simulation in seconds 20-555 --- n[sp] rate of change of number density of labelled species 556-571 --- driver Column recording added electrons each step (not used) -------------------------------------------------------------------------------- Byte-by-byte Description of files in folder: rrs2/* -------------------------------------------------------------------------------- Bytes Units Label Explanations -------------------------------------------------------------------------------- 1- 3601 --- [no.] Reaction rate of indicated reaction for each timestep Acknowledgements: ================================================================================ (prepared by author / pyreadme )