On the rotational distribution of the 5.3‐μm “thermal” emission from nitric oxide in the nighttime terrestrial thermosphere

Abstract

The rotational distribution of the “thermal” emission from the v = 1 vibrational level of NO, resulting from impacts of NO in the ground vibrational level (v=0) with oxygen atoms, is examined using the cryogenic infrared radiance for shuttle (CIRRIS‐1A) database. A block of NO quiescent (nonauroral) nighttime limb radiances observed by the CIRRIS ‐1A interferometer and radiometer around 5.3 μm are inverted to obtain the local rotational envelopes of the 1→0 vibrational transition as functions of altitude for both spin components. It is found possible to describe these local rotational envelopes by Maxwell‐Boltzmann distributions and to obtain rotational temperatures for each spin component of the vibrationally excited NO. The two spin components, within the accuracy of the measurements, are described by the same rotational temperature, which differs, however, from the mass spectrometer incoherent scatter (MSIS) model temperature at most altitudes. At low altitudes (≤110 km), the rotational temperature and the temperature describing the relative population of the spin states of the vibrationally excited NO approach each other, indicating the onset of thermodynamic equilibrium for the spin and the rotational degrees of freedom.