Rotation and H-alpha Emission Above and Below the Substellar Boundary
  I present the results of a multiyear survey of very low mass stars and brown dwarfs, at high spectral resolution. The spectra were gathered with the HIRES echelle at the Keck Observatory. Some of these objects are stellar and others are substellar (or ambiguous). Early indications that such objects can be rapidly rotating but display little H-alpha emission turn out to be commonly true. This is the opposite of the relation between rotation and activity in solar-type stars. The H-alpha surface flux drops precipitously at the bottom of the main sequence, and seems to be related to the luminosity or temperature of the objects. There is a general trend to higher rotation velocities as one looks at objects of lower luminosity. I discuss several possible explanations for these results. The dynamos for these objects are probably fully turbulent, driven by convection, and thus more directly related to the object's luminosity. They may be quenched when the rotational velocities become too fast in comparison to the convective velocities   (supersaturation). Another possibility is that the atmospheres of the cooler objects are becoming sufficiently neutral to decouple atmospheric motions from the field. Either of these could explain why young brown dwarfs can be magnetically active while older brown dwarfs are not. A final possibility is that instead of being quenched, the field configuration in rapid rotators changes to a less conducive form for dissipative heating. This could explain why flares are occasionally seen on generally inactive objects. 
Gibor Basri