Critical Velocity Ionization in Substellar Atmospheres
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
... 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. Unified Astronomy Thesaurus concepts: Magnetohydrodynamics (1964); Ionization (2068); Brown dwarfs (185); Extrasolar gas giants (509); Astrophysical fluid dynamics (101) - 1 10 km s 1 ) . However, to fully characterize CVI in substellar atmospheres and to determine the distribution and spatial extent of the generated plasma regions, including their degree of ionization as a function of position, the role of the global circulatory system and the resulting atmospheric flows must be investigated.