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Using a novel wide-slit, multi-object approach with the GMOS spectrograph onthe 8-meter Gemini South telescope, we have obtained precise time-seriesspectrophotometry of the binary brown dwarf Luhman 16 at optical wavelengthsover two full nights. The B component of this binary system is known to bevariable in the red optical and near-infrared with a period of 5 hr and anamplitude of 5--20%. Our observations probe its spectrally-resolved variabilityin the 6000--10000 Angstrom range. At wavelengths affected by the extremelystrong, broadened spectral lines of the neutral alkali metals (the potassiumdoublet centered near 7682 Angstroms and the sodium doublet at 5893 Angstroms),we see photometric variations that differ strikingly from the those of the8000--10000 Angstrom `red continuum' that dominates our detected flux. On UT2014 February 24, these variations are anticorrelated with the red continuum,while on Feb 25 they have a large relative phase shift. The extent to which thewavelength-dependent photometric behavior diverges from that of the redcontinuum appears to correlate with the strength of the alkali absorption. Weconsider but ultimately reject models in which our observations are explainedby lightning or auroral activity. A more likely cause is cloud-correlated,altitude-dependent variations in the gas-phase abundances of sodium andpotassium, which are in chemical equilibrium with their chlorides in browndwarf atmospheres. Clouds could influence these chemical equilibria by changingthe atmospheric temperature profile and/or through cloud particles acting aschemical catalysts.

Weather on Other Worlds. VI. Optical Spectrophotometry of Luhman 16B Reveals Large-amplitude Variations in the Alkali Lines