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The Solar Brightness Temperature in the Far Infrared

1977; Springer Nature (Netherlands); Linguagem: Inglês

10.1007/978-94-010-1211-9_19

2214-7985

Edith A. Müller, Peter Stettler, J. Rast, Fritz Kurt Kneubühl, D. Huguenin,

Infrared Target Detection Methodologies

Measurements of the solar brightness temperature in the far infrared give information on the radial temperature distribution in the transition region between the photosphere and the chromosphere. Since the source of opacity in this wavelength range is due to the free-free absorption of the H-ion, which is well understood, the absolute solar brightness measurements can readily be interpreted in terms of the temperature variation with height. For the purpose of measuring the absolute solar brightness over a wide range of far-infrared and submillimeter wavelengths, a lamellar-grating interferometer was developed at the ETH Zürich. Incorporated in the Geneva Observatory gondola it was launched to an altitude of about 35 km from the balloon launching station of the CNES in Aire-sur-1'Adour, France. The tracking system of the Geneva Observatory gondola is described by D. Huguenin (1974) and the optical system of the balloon-borne lamellar grating interferometer is discussed in detail by Stettler et al. (1975). The experiment covering the wavelength range 200–600 μ revealed that at 200 μ the solar brightness temperature may reach the value of about 4100°K. Müller et al. (1975) mentioned that this result is not incompatible with the higher values of the temperature minimum derived from ultraviolet observations around λ1600Å. The emergent radiation in the 100–200 μ range originates at the same height as that of around λ1600Å. However in the far infrared the continuum source function S is equal to the Planck function B whereas in the ultraviolet S>B; thus the ultraviolet observations are expected to yield higher values of the temperature minimum than infrared measurements (Vernazza et al., 1976). Recently an improved lamellar-grating interferometer was constructed and launched on September 19, 1975 to an altitude of 34.5 km. It recorded seven solar interferograms in the wavelength range of 80-250 y. The resulting brightness temperature is constant between about 95 µ and 200 µ, thus revealing the flat temperature minimum of the photosphere/chromosphere transition region. Due to some calibration problems the temperature scale was adapted to the absolute calibration of previous flights covering the 200–600 µ range. Further absolute measurements are planned for the 80–250 µ wavelength range in order to establish precisely the value of the temperature minimum in the solar atmosphere. Four of the seven spectra secured with the new lamellar-grating interferometer have a spectral resolution better than 0.5 cm-1. This resolution allows the detection of a number of stratospheric molecular absorption lines (H2O, O2, O3). This research was supported by the "Fonda national suisse de la recherche scientifique."