A Panchromatic View of Brown Dwarf Aurorae

TL;DR

This paper reviews evidence of auroral activity in brown dwarfs, highlighting its manifestations across wavelengths, its potential origins, and the connection to magnetic and satellite interactions, marking a transition from stellar to planetary-like magnetic phenomena.

Contribution

It provides a comprehensive compilation and analysis of multi-wavelength surveys of brown dwarf magnetic activity, emphasizing the auroral phenomena and their underlying physical mechanisms.

Findings

Auroral activity manifests in Hα and radio emissions in brown dwarfs.

Large amplitude near-infrared variability is mainly due to atmospheric clouds, not aurorae.

A connection exists between auroral Hα emission and quiescent radio emission.

Abstract

Stellar coronal activity has been shown to persist into the low-mass star regime, down to late M-dwarf spectral types. However, there is now an accumulation of evidence suggesting that at the end of the main sequence there is a transition in the nature of the magnetic activity from chromospheric and coronal to planet-like and auroral, from local impulsive heating via flares and MHD wave dissipation to energy dissipation from strong large-scale magnetospheric current systems. We examine this transition and the prevalence of auroral activity in brown dwarfs through a compilation of multi-wavelength surveys of magnetic activity, including radio, X-ray, and optical. We compile the results of those surveys and place their conclusions in the context of auroral emission as the consequence of large-scale magnetospheric current systems that accelerate energetic electron beams and drive the particles to impact the cool atmospheric gas. We explore the different manifestation of auroral phenomena in brown dwarf atmospheres, like H$\alpha$, and define their distinguishing characteristics. We conclude that large amplitude photometric variability in the near infrared is most likely a consequence of clouds in brown dwarf atmospheres, but that auroral activity may be responsible for long-lived stable surface features. We report a connection between auroral H$\alpha$ emission and quiescent radio emission in ECMI pulsing brown dwarfs, suggesting a potential underlying physical connection between the quiescent and auroral emissions. We also discuss the electrodynamic engines powering brown dwarf aurorae and the possible role of satellites around these systems to both power the aurorae and seed the magnetosphere with plasma.