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GJ 398 is a highly active mid-M dwarf (M4V) located at ≍15 pc, with a long history of reported X-ray and radio activity. Lying near the transition between partially and fully convective interiors, it occupies a key regime within the active-M-dwarf population and serves as a benchmark target for understanding multiwavelength magnetic energy release in active mid-M dwarfs. We analyzed the flaring activity of GJ 398 to investigate its stellar atmosphere and magnetic properties using 2 minute short-cadence photometry from the Transiting Exoplanet Survey Satellite and radio observations from the upgraded Giant Metrewave Radio Telescope (uGMRT), noting that the radio and photometric datasets were obtained at different epochs. Interestingly, we report for the first time specifically on GJ 398 a high super-flare event of 2.6 × 10<SUP>34</SUP> erg with a duration of ≍90 minutes, for which solar-based scaling relations suggest an associated magnetic field strength of at least 362 G to power it. We correlate flare duration with its energy, <inline-formula> <mml:math><mml:mo>△</mml:mo><mml:mi>t</mml:mi><mml:mo>∝</mml:mo><mml:msubsup><mml:mi>E</mml:mi><mml:mrow><mml:mspace></mml:mspace><mml:mtext>bol</mml:mtext><mml:mspace></mml:mspace></mml:mrow><mml:mrow><mml:mn>0.51</mml:mn><mml:mo>±</mml:mo><mml:mn>0.03</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula>, indicating a characteristic coronal magnetic field of ≍60 G for GJ 398, with localized &gt;200 G fields required only to power the most energetic superflare. This reveals a discrepancy with solar flares in that, despite a solar-like duration─energy scaling, stellar superflares require substantially stronger localized magnetic fields, suggesting a difference in the magnetic environment. There was no detectable radio emission during uGMRT observations, which consisted of 56 minutes of on-source integration in both Bands 4 and 5. We cannot draw a definitive conclusion regarding the cause of nondetection without more dedicated observing time.

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.