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

TL;DR

This study models how cosmic rays increase free electron levels in brown dwarf and exoplanet atmospheres, revealing significant ionization effects at certain pressures but limited magnetic coupling potential.

Contribution

It introduces a combined Monte Carlo and analytic cosmic ray transport model to quantify ionization in substellar atmospheres, a novel approach for this context.

Findings

Cosmic rays significantly enhance electron fractions at low pressures.

Ionization levels suggest weak plasma behavior in upper atmospheres.

Magnetic coupling remains limited despite increased ionization.

Abstract

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 1 MeV < E < 1 GeV, and an analytic cosmic ray transport model for particle energies of 1 GeV < E < 1 TeV 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 Teff = 1500 K, and two example giant gas planets (Teff = 1000 K, 1500 K). For the model brown dwarf atmosphere, the electron fraction is enhanced significantly by cosmic rays when the pressure pgas < 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 (Teff = 1000 K), cosmic rays bring the degree of ionization to fe ~ 10^-8 when pgas < 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.