A number of fast-rotating ultra cool 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 \sim 1 - 2 mho , either for a closed magnetosphere with a plasma mass outflow rate of \sim 10 ^ { 5 } \mathrm { kg } s ^ { -1 } , i.e .
a factor of \sim 100 larger than that observed at Jupiter ’ s moon Io , or for a dwarf with an open magnetosphere moving through the interstellar medium at \sim 50 \mathrm { km } s ^ { -1 } and a plasma mass outflow rate of \sim 1000 \mathrm { kg } s ^ { -1 } .
The radio luminosity resulting from these mechanisms have opposing dependencies on the magnetic field strength , a point which may be used to discriminate between the two models as more data become available .