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We propose that the 2.7 micron H_2O, 3.3 micron CH_4 and 4.6 micron COabsorption bands can be good tracers of chromospheric activity in brown dwarfs.In our previous study, we found that there are difficulties in explainingentire spectra between 1.0 and 5.0 microns with the Unified Cloudy Model (UCM),a brown dwarf atmosphere model. Based on simple radiative equilibrium,temperature in a model atmosphere usually decreases monotonically with height.However, if a brown dwarf has a chromosphere, as inferred by some observations,the temperature in the upper atmosphere is higher. We construct a simple modelthat takes into account heating due to chromospheric activity by setting atemperature floor in an upper atmosphere, and find that the model spectra of 3brown dwarfs with moderate H-alpha emission, an indicator of chromosphericactivity, are considerably improved to match the AKARI spectra. Because of thehigher temperatures in the upper atmospheres, the amount of CH_4 molecules isreduced and the absorption band strengths become weaker. The strengths of theabsorption bands of H_2O and CO also become weaker. On the other hand, otherobjects with weak H-alpha emission cannot be fitted by our treatment. We alsobriefly discuss magnetic heating processes which possibly operate in upperatmospheres, by extending our numerical simulations for the Sun and stars withsurface convection to brown dwarf atmospheres.

A Signature of Chromospheric Activity in Brown Dwarfs Revealed by 2.5-5.0 Micron AKARI Spectra