  A key first step to constrain the impact of energetic particles in exoplanet
atmospheres is to detect the chemical signature of ionisation due to stellar
energetic particles and Galactic cosmic rays. We focus on GJ$\,$436, a
well-studied M dwarf with a warm Neptune-like exoplanet. We demonstrate how the
maximum stellar energetic particle momentum can be estimated from the stellar
X-ray luminosity. We model energetic particle transport through the atmosphere
of a hypothetical exoplanet at orbital distances between $a=0.01-0.2\,$au from
GJ$\,$436, including GJ$\,$436$\,$b's orbital distance (0.028$\,$au). For these
distances we find that, at top-of-atmosphere, stellar energetic particles
ionise molecular hydrogen at a rate of $\zeta_{\rm StEP,H_2} \sim
4\times10^{-10}-2\times10^{-13}\,\mathrm{s^{-1}}$. In comparison, Galactic
cosmic rays alone lead to $\zeta_{\rm GCR, H_2}\sim2\times 10^{-20}-10^{-18}
\,\mathrm{s^{-1}}$. At 10au we find that ionisation due to Galactic cosmic rays
equals that of stellar energetic particles: $\zeta_{\rm GCR,H_2} = \zeta_{\rm
StEP,H_2} \sim 7\times10^{-18}\,\rm{s^{-1}}$ for the top-of-atmosphere
ionisation rate. At GJ$\,$436$\,$b's orbital distance, the maximum ion-pair
production rate due to stellar energetic particles occurs at pressure $P\sim
10^{-3}\,$bar while Galactic cosmic rays dominate for $P>10^2\,$bar. These high
pressures are similar to what is expected for a post-impact early Earth
atmosphere. The results presented here will be used to quantify the chemical
signatures of energetic particles in warm Neptune-like atmospheres.
