A submillimeter heterodyne receiver with an open structure mixer and a laser local oscillator was used to detect the CO (J = 6 ← 5) absorption transition at 691.473 GHz in the earth's upper atmosphere. Spectra were taken toward the sun with a 1024‐channel acousto‐optical spectrometer. The resolution was 250 kHz. The CO volume mixing ratio profile was calculated from the spectral line shape. It is shown that the CO (J = 6 ← 5) measurement results in a well‐determined CO profile for the lower mesospheric layers, as compared with the results of other authors for observations of CO transitions with lower J values.
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