The microwave spectrum of oxygen in the Earth's atmosphere

Abstract

Space probe techniques open the possibility of radio and microwave spectroscopic investigations of planetary atmospheres through the study of resonant transitions in gaseous constituents. Computations were undertaken to determine the opacity and the thermal emission produced by the millimeter-wavelength complex of oxygen lines in the earth's atmosphere. The calculations predict line profiles of individual oxygen transitions in emission or in absorption against the sun. Potential experimental observation heights were assumed: sea level, aircraft heights (5 and 8 km), a typical high-altitude balloon height (30 km), and earth-satellite heights. The computed spectrums are based on the Van Vleck-Weisskopf theory of oxygen absorption combined with the ARDC model atmosphere. The transfer equation for the oxygen line complex is complicated by the Zeeman splitting produced by the earth's magnetic field, but this effect is significant only in spectrums predicted for satellite observations and is neglected here. Computed values of the zenith opacity from the earth's surface are in good agreement with measurements that extend down to about 1 Gc/s. The depths of absorption lines observed against the sun are predicted to be greater by a factor of 20 than the intensity of the same lines in emission. Satellite observations of microwave brightness temperature as a function of frequency around 60 Gc/s can yield information about the vertical thermal structure of the atmosphere; this technique, undertaken with a polar satellite, offers the possibility of a global determination of the approximate vertical temperature distribution in the earth's atmosphere. The relationship between the emission spectrum and the temperature as a function of height demonstrates that the emission at a given frequency represents the average temperature in a layer of air roughly 10 km deep. The mean height of these layers depends on frequency and varies between 0 and 40 km. It may be possible to increase this height considerably by taking the Zeeman effect into account. Specifications are estimated for the aircraft, balloon, and satellite radiometric systems that would be required to undertake the research program defined in this paper.