2.4-MU-M SKY BRIGHTNESS AT BALLOON ALTITUDE

The atmospheric airglow is thought to have a narrow window at 2.4 mum, which corresponds to the wavelength gap between the fundamental and the first overtone of the OH vibration bands. In order to confirm this window, we analyzed data of past balloon experiments, and found that the sky brightness, lambda I(lambda), at 2.4 mum amounted to (7+/-2) x 10(-10) W cm-2 sr-1 or approximately 15.5 mag arcsec-2. This level is considerably higher than expected. It can be explained, however, by assuming the nonequilibrium and partially high rotation temperature for OH radicals. This mechanism suggests that the OH window is shifted to longer wavelengths if the emission of other molecules and telescope itself is taken into account.