  Brown dwarfs emit bursts of Halpha, white light flares, and show radio flares
and quiescent radio emission. They are suggested to form Aurorae, similar to
planets in the solar system but much more energetic. All these processes
require a source gas with an appropriate degree of ionisation which, so far, is
mostly postulated to be sufficient. We aim to demonstrate that the galactic
environment influences atmospheric ionisation, and that it hence amplifies or
enables the magnetic coupling of the atmospheres of ultra-cool objects, like
brown dwarfs and free-floating planets. We consider the effect of
photoionisation by Lyman continuum radiation in three different environments:
the InterStellar Radiation Field (ISRF), O and B stars in star forming regions,
and also for white dwarf companions in binary systems. We apply our Monte Carlo
radiation transfer code to investigate the effect of Lyman continuum
photoionisation for prescribed atmosphere structures for very low-mass objects.
The external radiation environment plays an important role for the atmospheric
ionisation of very low-mass, ultra-cool objects. Lyman continuum irradiation
greatly increases the level of ionisation in the uppermost atmospheric regions.
Our results suggest that a shell of an almost fully ionised atmospheric gas
emerges for brown dwarfs in star forming regions and brown dwarfs in white
dwarf binary systems. As a consequence, brown dwarf atmospheres can be
magnetically coupled which is the presumption for chromospheric heating to
occur and for Aurorae to emerge. First tests for assumed chromosphere-like
temperature values suggest that the resulting free-free X-ray luminosities are
comparable with those observed from non-accreting brown dwarfs in star forming
regions.
