Auroral radio emission from ultracool dwarfs: a Jovian model

A number of fast-rotating ultracool dwarfs (UCDs) emit pulsed coherent radiation, attributed to the electron–cyclotron maser instability, a phenomenon that occurs in the Solar system at planets with strong auroral emission. In this paper, we examine magnetosphere–ionosphere coupling currents in UCDs, adopting processes used in models of Jovian emission. We consider the angular velocity gradient arising from a steady outward flux of angular momentum from an internal plasma source, as analogous to the Jovian main oval current system, as well as the interaction of a rotating magnetosphere with the external medium. Both of these mechanisms are seen in the Solar system to be responsible for the production of radio emission. We present the results of an investigation over a range of relevant plasma and magnetosphere–ionosphere coupling parameters to determine regimes consistent with observed UCD radio luminosities. Both processes are able to explain observed UCD luminosities with ionospheric Pedersen conductances of ∼1–2 mho, either for a closed magnetosphere with a plasma mass outflow rate of ∼105 kg s−1, i.e. a factor of ∼100 larger than that observed at Jupiter's moon Io, or for a dwarf with an open magnetosphere moving through the interstellar medium at ∼50 km s−1 and a plasma mass outflow rate of ∼1000 kg s−1. The radio luminosity resulting from these mechanisms has opposing dependencies on the magnetic field strength, a point that may be used to discriminate between the two models as more data become available.