Resolving a radiation belt around an ultracool dwarf

Radiation belts are present in all large-scale Solar System planetary magnetospheres. Ultracool dwarfs, i.e., very low mass stars and brown dwarfs, exhibit a radio emission component long hypothesized to trace stellar coronal activity or more recently extrasolar radiation belt analogs. Here we present High Sensitivity Array (a VLBI array consisting of the Very Long Baseline Array, Very Large Array, Greenbank Telescope and Effelsberg Telescope) imaging of the ultracool dwarf LSR J1835+3259 demonstrating that this radio emission is spatially resolved and traces a long-lived and double-lobed plasma structure with ∼10 stellar radii separating the two lobes. This structure is stably present in three observations spanning about 1 year. Comparing lobe geometry to the relative location of this object’s aurora suggests an equatorial distribution confined by its magnetic dipole. Our results support broader re-examination of radiation belts as a possible radio emission source from fully convective M dwarfs and young stars, for which growing evidence points to the prevalence of large-scale magnetospheres.