  We report the first comprehensive study of emission line variability in an
accreting young brown dwarf. We have collected 14 high-resolution optical
spectra of 2MASSW J1207334-393254 (M8), a likely member of the nearby
8-million-year-old TW Hydrae association. These spectra show a variety of
emission lines that are commonly seen in classical T Tauri stars. Halpha line
in particular shows dramatic changes in shape and intensity in our dataset,
both on timescales of several weeks and several hours. In spectra from
late-January, the line is relatively weak and only slightly asymmetric. Spectra
from mid- and late-March show intense, broad and asymmetric Halpha emission,
indicative of on-going disk accretion. We estimate that the accretion rate
could have changed by a factor of 5-10 over ~6 weeks in this brown dwarf. March
spectra also reveal significant `quasi-periodic' changes in the Halpha line
profile over the course of a night, from clearly double-peaked to nearly
symmetric. These nightly profile changes, roughly consistent with the brown
dwarf's rotation period, could be the result of a redshifted absorption feature
coming into and out of our line of sight; when the profile is double-peaked we
may be looking into an accretion column, flowing from the inner disk edge on to
the central object, indicating that the accretion is probably channelled along
the magnetic field lines. Our findings provide strong support for the
magnetospheric accretion scenario, and thus for the existence of large-scale
magnetic fields, in the sub-stellar regime (abridged).
