The Bilderberg continuum atmosphere (B.c.a.) model of the Sun (Gingerich & de Jager 1968) has a temperature minimum of 4600 K between the photosphere and low chromosphere, which is based mainly on observations in the ultraviolet. This layer of the solar atmosphere is observable in both the ultraviolet and infrared spectral regions. However, at the time that the B.c.a. model was developed, there were no absolute measurements of the brightness temperature between 12 um and 1 mm although there was some evidence to indicate the shape of the expected minimum in this spectral region. Since then several experiments have been performed from aircraft and high altitude balloons with the object of measuring the brightness temperature of the sun at long infrared wavelengths. The measurement described in this paper made use of a Michelson interferometer employing Fourier transform multiplex techniques and was flown from a balloon to a height of 32.6 km from the N.C.A.R. Balloon Flight Station, Texas, U.S.A., in September 1969. The beam splitter consisted of a stretched film of Melinex 8 um thick and the detector used was a Golay cell. Radiation of wavelengths shorter than about 45 um was completely attenuated by optical filtering with black Melinex, polyethylene loaded with a uranium salt and by the quartz window of the detector. The Michelson interferometer was used with a continuous movement of one of its mirrors at 4 pm s-1 and further shortwave attenuation was achieved by suitable electrical filtering of the signal frequencies in the resulting interferogram.
TRIS. I. Absolute Measurements of the Sky Brightness Temperature at 0.6, 0.82, and 2.5 GHz