Thermalization of <formula alphabet="latin">O([SUP]1[/SUP]D) atoms in the thermosphere

Measurements of the Doppler width of the 6300 Å airglow emission line have been extensively used to determine the thermospheric temperature. This technique is based on the assumption that the bulk of the emitting O([SUP]1[/SUP]D) atoms are thermalized in the region of the airglow source (200-300 km). A Monte Carlo stochastic model is used to calculate the energy distribution function of O([SUP]1[/SUP]D) atoms in the daytime and nighttime thermosphere. Hot O([SUP]1[/SUP]D) atoms are produced by exothermic processes and their thermalization is controlled by the competition between radiation, collisional quenching, and relaxation. It is found that the O([SUP]1[/SUP]D) temperature departs from the background gas temperature not only in the upper thermosphere but also in the region of the bulk 6300 Å emission. At 300 km for low solar activity conditions, the model predicts an excess O([SUP]1[/SUP]D) temperature of ~180 K during daytime and ~950 K at night. The temperature departure persists at lower altitudes as a result of the major contribution of the O[SUB]2[/SUB][SUP]+[/SUP] dissociative recombination source of hot [SUP]1[/SUP]D atoms. Experimental evidence based on the Fabry-Perot interferometer measurements on board the Dynamics Explorer satellite confirms the existence of an O([SUP]1[/SUP]D) temperature excess over the mass spectrometer/incoherent scatter (MSIS) value. It is concluded that temperatures deduced from the 6300 Å airglow line width may significantly exceed the ambient gas temperature in a way depending on solar activity, local time, and observation geometry.