  Aurorae are detected from all the magnetized planets in our Solar System,
including Earth. They are powered by magnetospheric current systems that lead
to the precipitation of energetic electrons into the high-latitude regions of
the upper atmosphere. In the case of the gas-giant planets, these aurorae
include highly polarized radio emission at kilohertz and megahertz frequencies
produced by the precipitating electrons, as well as continuum and line emission
in the infrared, optical, ultraviolet and X-ray parts of the spectrum,
associated with the collisional excitation and heating of the
hydrogen-dominated atmosphere. Here we report simultaneous radio and optical
spectroscopic observations of an object at the end of the stellar main
sequence, located right at the boundary between stars and brown dwarfs, from
which we have detected radio and optical auroral emissions both powered by
magnetospheric currents. Whereas the magnetic activity of stars like our Sun is
powered by processes that occur in their lower atmospheres, these aurorae are
powered by processes originating much further out in the magnetosphere of the
dwarf star that couple energy into the lower atmosphere. The dissipated power
is at least four orders of magnitude larger than what is produced in the Jovian
magnetosphere, revealing aurorae to be a potentially ubiquitous signature of
large-scale magnetospheres that can scale to luminosities far greater than
those observed in our Solar System. These magnetospheric current systems may
also play a part in powering some of the weather phenomena reported on brown
dwarfs.
