Critical velocity ionization in substellar atmospheres

Wilson, A.D., Dobbs-Dixon, I., Diver, D.A. and Stark, C.R. (2019) Critical velocity ionization in substellar atmospheres. Astrophysical Journal, 887(2), 138. (doi: 10.3847/1538-4357/ab5800)

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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.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Stark, Mr Craig and Diver, Professor Declan and Wilson, Dr Alasdair
Authors: Wilson, A.D., Dobbs-Dixon, I., Diver, D.A., and Stark, C.R.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Astrophysical Journal
Publisher:American Astronomical Society
ISSN:0004-637X
ISSN (Online):1538-4357
Published Online:17 December 2019
Copyright Holders:Copyright © 2019 The American Astronomical Society
First Published:First published in Astrophysical Journal 887(2): 138
Publisher Policy:Reproduced under a Creative Commons License

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
172170Evolution and dynamics of pellets and dust in dynamic gas-plasma systemsDeclan DiverEngineering and Physical Sciences Research Council (EPSRC)EP/N018117/1P&S - Physics & Astronomy