The solar spectrum at 8 mm

Abstract

THE brightness temperature of the quiet Sun has been measured over almost the complete radio spectrum. It ranges from approximately 6,000 K at millimetre wavelengths to over 10⁶ K at metre wavelengths. Various compilations of the disk temperatures^1–3 had at centimetre and millimetre wavelengths from 10 to 100 GHz (λ=3 cm to 3 mm) (Fig. 1). A monotonically decreasing brightness temperature presented by Shimabukuro and Stacey¹ from a model by van de Hulst⁴ is also shown in Fig. 1. Zheleznyakov⁵ has pointed out that the apparent dip in the observed disk temperatures near 50 GHz and at the apparent peak near 70 GHz, if real, would indicate an effective temperature inversion in the solar chromosphere from 3,000 to 3,300 km above the photosphere. Reber³, however, measured the slope of the brightness against frequency curve at 50 GHz and found that the disk temperature is, in fact, decreasing with increasing frequency in nearly perfect agreement with the van de Hulst⁴ model. Reber's method does not depend on an absolute measurement of the brightness temperature but rather on a relative measurement between two closely spaced frequencies, with a single instrument, thereby giving the slope with more accuracy than can be obtained by comparing a number of independent, absolute measurements.