A Jupiter-like radio aurora in the L3.5 dwarf LSPM J0036+1821

Around 10% of all ultracool dwarfs exhibit pulsed radio emission, reminiscent of the auroral radio emission produced by magnetised planets in the Solar System. At Jupiter, the dominant components are emission from the main oval aurora and from an active magnetic field line powered by its interaction with the moon Io.

Here we present 15 epochs of radio observations of the L3.5 dwarf LSPM J0036+1821 obtained with the VLA, EVN, and VLBA. Using a modelling framework inspired by Jupiter’s auroral morphology (combining a main oval component with emission from an active field line) we are able to reproduce the observed radio variability across the full dataset and constrain the underlying auroral geometry. This approach provides a rare handle on the magnetospheric structure of ultracool dwarfs, which is otherwise extremely difficult to probe observationally.

Additionally, we analyze optical photometry from two TESS sectors to independently determine the rotation period of the dwarf. We refine both the radio and optical rotation periods and find them to be consistent within 2 seconds.