  Heterogeneous clouds or temperature perturbations in rotating brown dwarfs
produce variability in the observed flux. We report time-resolved simultaneous
observations of the variable T6.5 brown dwarf 2MASSJ22282889-431026 over the
wavelength ranges 1.1-1.7 microns and broadband 4.5 microns. Spectroscopic
observations were taken with Wide Field Camera 3 on board the Hubble Space
Telescope and photometry with the Spitzer Space Telescope. The object shows
sinusoidal infrared variability with a period of 1.4 hr at most wavelengths
with peak-to-peak amplitudes between 1.45% and 5.3% of the mean flux. While the
light curve shapes are similar at all wavelengths, their phases differ from
wavelength to wavelength with a maximum difference of more than half of a
rotational period. We compare the spectra with atmospheric models of different
cloud prescriptions, from which we determine the pressure levels probed at
different wavelengths. We find that the phase lag increases with decreasing
pressure level, or higher altitude. We discuss a number of plausible scenarios
that could cause this trend of light curve phase with probed pressure level.
These observations are the first to probe heterogeneity in an ultracool
atmosphere in both horizontal and vertical directions, and thus are an ideal
test case for realistic three dimensional simulations of the atmospheric
structure with clouds in brown dwarfs and extrasolar planets.
