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Cosmic rays provide an important source for free electrons in the Earth'satmosphere and also in dense interstellar regions where they produce aprevailing background ionization. We utilize a Monte Carlo cosmic ray transportmodel for particle energies of 1 MeV < E < 1 GeV, and an analytic cosmic raytransport model for particle energies of 1 GeV < E < 1 TeV in order toinvestigate the cosmic ray enhancement of free electrons in substellaratmospheres of free-floating objects. The cosmic ray calculations are appliedto Drift-Phoenix model atmospheres of an example brown dwarf with effectivetemperature Teff = 1500 K, and two example giant gas planets (Teff = 1000 K,1500 K). For the model brown dwarf atmosphere, the electron fraction isenhanced significantly by cosmic rays when the pressure pgas < 10^-2 bar. Ourexample giant gas planet atmosphere suggests that the cosmic ray enhancementextends to 10^-4 - 10^-2 bar, depending on the effective temperature. For themodel atmosphere of the example giant gas planet considered here (Teff = 1000K), cosmic rays bring the degree of ionization to fe ~ 10^-8 when pgas < 10^-8bar, suggesting that this part of the atmosphere may behave as a weakly ionizedplasma. Although cosmic rays enhance the degree of ionization by over threeorders of magnitude in the upper atmosphere, the effect is not likely to besignificant enough for sustained coupling of the magnetic field to the gas.

Ionization in atmospheres of Brown Dwarfs and extrasolar planets IV. The Effect of Cosmic Rays