Evidence for a radiation belt around a brown dwarf.

Ultracool dwarfs are a category of astronomical objects including brown dwarfs and very low-mass stars. Radio observations of ultracool dwarfs have measured their brightness as a function of time (light-curves) and spectral energy distributions, providing insight into their magnetic fields. We present spatially resolved radio observations of the brown dwarf LSR J1835+3259 using very long baseline interferometry, which show extended radio emission. The detected morphology is consistent with the presence of a radiation belt. Comparison with models indicates the radiation belt contains energetic particles confined by magnetic mirroring. We argue that radio emitting ultracool dwarfs possess dipole-ordered magnetic fields with radiation belt-like morphologies and aurorae, similar to those of Jupiter.