Resolved imaging confirms a radiation belt around an ultracool dwarf

Abstract

Radiation belts are present in all large-scale Solar System planetary magnetospheres: Earth, Jupiter, Saturn, Uranus, and Neptune¹. These persistent equatorial zones of relativistic particles up to tens of MeV in energy can extend farther than 10 times the planet’s radius, emit gradually varying radio emissions^2–4 and impact the surface chemistry of close-in moons⁵. Recent observations demonstrate that very low mass stars and brown dwarfs, collectively known as ultracool dwarfs, can produce planet-like radio emissions such as periodically bursting aurorae^6–8 from large-scale magnetospheric currents^9–11. They also exhibit slowly varying quiescent radio emissions^7,12,13 hypothesized to trace low-level coronal flaring^14,15 despite departing from empirical multi-wavelength flare relationships^8,15. Here we present high resolution imaging of the ultracool dwarf LSR J1835+3259 at 8.4 GHz demonstrating that its quiescent radio emission is spatially resolved and traces a double-lobed and axisymmetric structure similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf radii separate the two lobes, which are stably present in three observations spanning more than one year. For plasma confined by the magnetic dipole of LSR J1835+3259, we estimate 15 MeV electron energies consistent with Jupiter’s radiation belts⁴. Our results confirm recent predictions of radiation belts at both ends of the stellar mass sequence^8,16–19 and support broader re-examination of rotating magnetic dipoles in producing non-thermal quiescent radio emissions from brown dwarfs⁷, fully convective M dwarfs²⁰, and massive stars^18,21.