Cosmic rays provide an important source for free electrons in the Earth ’ s atmosphere and also in dense interstellar regions where they produce a prevailing background ionization .
We utilize a Monte Carlo cosmic ray transport model for particle energies of 10 ^ { 6 } eV < E < 10 ^ { 9 } eV , and an analytic cosmic ray transport model for particle energies of 10 ^ { 9 } eV < E < 10 ^ { 12 } eV in order to investigate the cosmic ray enhancement of free electrons in substellar atmospheres of free-floating objects .
The cosmic ray calculations are applied to Drift-Phoenix model atmospheres of an example brown dwarf with effective temperature T _ { eff } = 1500 K , and two example giant gas planets ( T _ { eff } = 1000 K , 1500 K ) .
For the model brown dwarf atmosphere , the electron fraction is enhanced significantly by cosmic rays when the pressure p _ { gas } < 10 ^ { -2 } bar .
Our example giant gas planet atmosphere suggests that the cosmic ray enhancement extends to 10 ^ { -4 } -10 ^ { -2 } bar , depending on the effective temperature .
For the model atmosphere of the example giant gas planet considered here ( T _ { eff } = 1000 K ) , cosmic rays bring the degree of ionization to f _ { e } \gtrsim 10 ^ { -8 } when p _ { gas } < 10 ^ { -8 } bar , suggesting that this part of the atmosphere may behave as a weakly ionized plasma .
Although cosmic rays enhance the degree of ionization by over three orders of magnitude in the upper atmosphere , the effect is not likely to be significant enough for sustained coupling of the magnetic field to the gas .