Solar Radiation at a Wavelength of 3.18 Centimetres

Abstract

Solar radiation at a wavelength of 3.18 cm. has been measured over a period of three months. The received intensity was found to vary from day to day and the changes are shown to be closely associated with sunspots. The equivalent black-body temperature of the sun over this period, in the absence of sunspots, was 19,300 °K., with a probable error of ±7 per cent. The temperature increased by 8 °K. per unit increase of sunspot area (one unit equals 10^-5 times the area of the sun's visible disk). This increase is much less than that at longer microwavelengths. Sudden increases of radiation at 3.18 cm., caused by disturbed conditions in the sun, were found to be rare. A number of bursts were observed and a comparison is made with records of longer wave solar radiation and other phenomena of solar origin. Observations were made during the solar eclipse of November 1, 1948 and the results are consistent with either of two simple brightness distributions on the sun's disk. In the first of these, 74 per cent. of the energy is emitted uniformly by the sun's visible disk and the remaining 26 per cent. by a bright ring around the circumference ; in the second, the whole of the radiation comes from a uniform disk of diameter 1.1 times that of the visible sun.