Critical Velocity Ionization in Substellar Atmospheres

Abstract

The observation of radio, X-ray, and H α emission from substellar objects indicates the presence of plasma regions and associated high-energy processes in their surrounding envelopes. This paper numerically simulates and characterizes critical velocity ionization (CVI), a potential ionization process, that can efficiently generate plasma as a result of neutral gas flows interacting with seed magnetized plasmas. By coupling a gas–magnetohydrodynamic (MHD) interactions code (to simulate the ionization mechanism) with a substellar global circulation model (to provide the required gas flows), we quantify the spatial extent of the resulting plasma regions, their degree of ionization, and their lifetime for a typical substellar atmosphere. It is found that the typical average ionization fraction reached at equilibrium (where the ionization and recombination rates are equal and opposite) ranges from 10−5 to 10−8, at pressures between 10−1 and 10−3 bar, with a trend of increasing ionization fraction with decreasing atmospheric pressure. The ionization fractions reached as a result of CVI are sufficient to allow magnetic fields to couple to gas flows in the atmosphere.

Evolution of spheroidal dust in electrically active sub-stellar atmospheres

“…Alfvén ionization can create regions with degrees of ionization ranging from 10 −6 −1 and is most effective where p gas ≈ 10 −5 −10 −15 bar and where flow speeds are of the order of 1 − 10 kms −1 (Stark et al 2013). Wilson et al (2019) found that for HD189733b (T eff = 1117 K) the average ionization fraction at equilibrium as the result of Alfvén ionization ranges from 10 −5 − 10 −8 for pressures between 10 −1 − 10 −3 bar. Cosmic-ray ionization is another non-thermal process that can significantly enhance the degree of ionization for atmospheric pressures p gas < 10 −2 bar.…”