Simulations of Electron Beam Interactions in Brown Dwarf Atmospheres

Abstract

Over two decades ago, the first detection of electron cyclotron maser instability (ECMI) radio emission from a brown dwarf confirmed the presence of aurorally precipitating electrons on these objects. This detection established that brown dwarfs can exhibit magnetic activity that is planetary and auroral, rather than stellar in nature. This discovery motivated ongoing observational searches for the corresponding optical, ultraviolet (UV), and infrared (IR) auroral emission expected based on solar system analogs. The continuing nondetection of such auroral emission indicates important differences exist between auroral processes on brown dwarfs and solar system planets. In this work, we implement a Monte Carlo simulation of monoenergetic electron beams interacting with brown dwarf atmospheres, as a step towards understanding the physics of brown dwarf auroral emission. We detail the algorithm and underlying assumptions, and validate against previously published Jovian results (Hiraki et al. 2008). Our results agree well with literature, with some discrepancy from our updated interaction cross sections. We demonstrate the applicability of our simulation across the range of surface gravities and effective temperatures of radio-emitting brown dwarfs. We present an analytic parameterization of interaction rates based on our finding that atmospheric column density governs the interaction profiles. We apply this parameterization to calculate the total volumetric interaction rates and energy deposition rate for representative electron beam energy spectra enabling future predictions for spectra of aurorally emitting brown dwarfs. Simulations of high energy electron interactions with substellar hydrogen-dominated atmospheres will guide observational searches for multi-wavelength auroral features beyond the solar system.