Evidence for CH⁴ 7.6 μ non-local thermodynamic equilibrium emission in the mesosphere

We present unequivocal evidence for CH₄ 7.6 μm non-local thermodynamic equilibrium (non-LTE) mesospheric emissions as measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment on board the ESA Envisat satellite. We have employed non-LTE radiative transfer models, already used and validated in studies of other atmospheric gases, in order to quantify the magnitude and extent of the CH₄ non-LTE deviation in the Earth's atmosphere. The observed daytime enhancement signatures apparent in the measured radiances cannot be attributed to enhanced daytime temperatures and/or CH₄ abundance variations and are reproduced successfully with a dedicated CH₄ non-LTE radiative transfer model. We conclude that depending on the upper stratospheric/mesospheric temperature structure, the departure from LTE might commence at an altitude as low as 45 km, reaching limb radiance deviations of 20% at 60 km and quickly rising up to 60% at 70 km, for the 7.6 μm CH₄ lines.