Critical velocity ionisation in substellar atmospheres

A. D. Wilson*, I. Dobbs-Dixon, D. A. Diver, C. R. Stark

*Corresponding author for this work

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Abstract

The observation of radio, X-ray and Hα emission from substellar objects indicates the presence of plasma regions and associated high-energy processes in their surrounding envelopes. This paper numerically simulates and characterises Critical Velocity Ionisation, a potential ionisation process, that can efficiently generate plasma as a result of neutral gas flows interacting with seed magnetized plasmas. By coupling a Gas-MHD interactions code (to simulate the ionisation 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 ionisation and their lifetime for a typical substellar atmosphere. It is found that the typical average ionisation fraction reached at equilibrium (where the ionisation 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 ionisation fraction with decreasing atmospheric pressure. The ionisation fractions reached as a result of Critical Velocity Ionisation are sufficient to allow magnetic fields to couple to gas flows in the atmosphere.
Original languageEnglish
Article number138
Number of pages8
JournalAstrophysical Journal
Volume887
Issue number2
Early online date17 Dec 2019
DOIs
Publication statusPublished - 17 Dec 2019

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