Critical Velocity Ionization in Substellar Atmospheres

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

doi:10.3847/1538-4357/ab5800

Abstract

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.

Highlights

As well as emitting in the near-IR, substellar objects exhibit radio (e.g., Williams et al 2015, 2017; Route & Wolszczan 2016), X-ray (e.g., Audard et al 2007; Berger et al 2010), and optical Hα (e.g., Schmidt et al 2007; Pineda et al 2016) emission
Several ionization processes occur in substellar atmospheres that generate regions of plasma, giving a source of the energetic, nonthermal processes required to generate the observed emission, including, thermal ionization (RodríguezBarrera et al 2015), lightning discharges (Helling et al 2013), cosmic-ray ionization (Rimmer & Helling 2013), photoionization (Rodríguez-Barrera et al 2018), turbulence-induced dust– dust collisions (Helling et al 2011), and critical velocity ionization (CVI; Stark et al 2013)
This paper presents the novel coupling of a gas–magnetohydrodynamics interactions code (GMIC) with a substellar global circulation model (GCM) to investigate the impact of CVI on substellar environments, for the first time

Summary

Introduction

As well as emitting in the near-IR, substellar objects exhibit radio (e.g., Williams et al 2015, 2017; Route & Wolszczan 2016), X-ray (e.g., Audard et al 2007; Berger et al 2010), and optical Hα (e.g., Schmidt et al 2007; Pineda et al 2016) emission. CVI can produce degrees of ionization ranging from ≈10−6 to 1, and is expected to be most effective where pgas ≈ 10−5–10−15 bar, where flow speeds are sufficiently high, » (1–10 km s-1) (Stark et al 2013). 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. The aim of this paper is to simulate CVI in substellar atmospheres to quantify the spatial extent of the resulting plasma regions, their degree of ionization, and their lifetime for a typical substellar atmosphere.

A Fluid Description of CVI

Global Circulation Model

Incorporating CVI

Recombination

Time-dependent Ionization

Conclusions