  Recently, studies of brown dwarfs have demonstrated that they possess strong
magnetic fields and have the potential to produce radio and optical auroral
emissions powered by magnetospheric currents. This emission provides the only
window on magnetic fields in the coolest brown dwarfs and identifying
additional benchmark objects is key to constraining dynamo theory in this
regime. To this end, we conducted a new red optical (6300 - 9700 Angstrom)
survey with the Keck telescopes looking for H-alpha emission from a sample of
late L dwarfs and T dwarfs. Our survey gathered optical spectra for 29 targets,
18 of which did not have previous optical spectra in the literature, greatly
expanding the number of moderate resolution (R~2000) spectra available at these
spectral types. Combining our sample with previous surveys, we confirm an
H-alpha detection rate of 9.2 (+3.5/-2.1) % for L and T dwarfs in the optical
spectral range of L4 - T8. This detection rate is consistent with the recently
measured detection rate for auroral radio emission from Kao et al. (2016),
suggesting that geometrical selection effects due to the beaming of the radio
emission are small or absent. We also provide the first detection of H-alpha
emission from 2MASS 0036+1821, previously notable as the only electron
cyclotron maser radio source without a confirmed detection of H-alpha emission.
Finally, we also establish optical standards for spectral types T3 and T4,
filling in the previous gap between T2 and T5.
