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L dwarfs exhibit low-level, rotationally-modulated photometric variabilitygenerally associated with heterogeneous, cloud-covered atmospheres. Thespectral character of these variations yields insight into the particle sizesand vertical structure of the clouds. Here we present the results of a highprecision, ground-based, near-infrared, spectral monitoring study of twomid-type L dwarfs that have variability reported in the literature, 2MASSJ08354256-0819237 and 2MASS J18212815+1414010, using the SpeX instrument on theInfrared Telescope Facility. By simultaneously observing a nearby referencestar, we achieve <0.15% per-band sensitivity in relative brightness changesacross the 0.9--2.4um bandwidth. We find that 2MASS J0835-0819 exhibitsmarginal (< ~0.5% per band) variability with no clear spectral dependence,while 2MASS J1821+1414 varies by up to +/-1.5% at 0.9 um, with the variabilityamplitude declining toward longer wavelengths. The latter result extends thevariability trend observed in prior HST/WFC3 spectral monitoring of 2MASSJ1821+1414, and we show that the full 0.9-2.4 um variability amplitude spectrumcan be reproduced by Mie extinction from dust particles with a log-normalparticle size distribution with a median radius of 0.24 um. We do not detectstatistically significant phase variations with wavelength. The differentvariability behavior of 2MASS J0835-0819 and 2MASS J1821+1414 suggestsdependencies on viewing angle and/or overall cloud content, underlying factorsthat can be examined through a broader survey.

Spectral Variability of Two Rapidly Rotating Brown Dwarfs: 2MASS J08354256-0819237 and 2MASS J18212815+1414010