Simulations of Electron Beam Interactions in Brown Dwarf Atmospheres

TL;DR

This work develops a Monte Carlo framework to model how energetic electron beams precipitate into hydrogen-dominated brown dwarf atmospheres, aiming to explain the elusive UV and IR auroral signatures associated with ECMI radio emissions. Validated against Jupiter, the model extends across brown dwarfs with varying gravity and effective temperature, and yields an analytic parameterization of the ionization profile $q_{ion}$ as a function of incident energy and atmospheric density via a Moyal distribution in $-\,\ln(N_{ion})$. The parameterization enables fast computation of total ionization rates $Q_{ion}$ and energy-deposition profiles for arbitrary beam spectra, facilitating predictions of UV/IR auroral features and guiding observational campaigns (e.g., JWST). The results suggest that higher-energy electron beams deposit energy deeper in the atmosphere, potentially suppressing UV/IR emission, and provide a robust framework for interpreting substellar aurorae across the brown dwarf parameter space.

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.

Figures (17)

• Figure 1: A schematic of the emission processes involved in electron-beam aurorae.
• Figure 2: Temperature profiles for the objects considered in this work. For Jupiter, the isothermally extended profile is shown.
• Figure 3: Density profiles for the objects considered in this work. For Jupiter, the isothermally extended profile is shown.
• Figure 4: Our simulated ionization profile (solid) for Jupiter using the density profile used by Hiraki2008, compared with the parameterized ionization profile presented in that paper (dashed). Note that in this and other figures, partial pressure of H$_2$ is reported rather than full atmospheric pressure. This is because in extending the density profile of H$_2$ isothermally, we do not calculate density profiles or pressures for other species and thus for consistency with the isothermal region report partial pressure across the atmosphere. However, this is a small effect on the value of the pressure axis reported, as the mixing ratio of H$_2$ is several orders of magnitude larger than any other species throughout the atmospheres which we consider.
• Figure 5: Ionization rate per incident electron as a function of altitude or pressure for Jupiter using the density profile derived from custom PICASO model outputs and Galileo measurements, for a range of incident electron energies. As described in Section \ref{['subsec:params']}, the relationship between altitude and pressure is determined from the outputs of a custom PICASO model plus an isothermal extension.