THE MICROWAVE BRIGHTNESS TEMPERATURE SPECTRUM OF THE QUIET SUN

We present new measurements of the microwave brightness temperature spectrum of the center of the quite Sun, acquired at Owens Valley over several months during the 1986-1987 sunspot minimum. Observations were made simultaneously with a single dish using a frequency-agile receiver at 20 frequencies over the range 1.4 to 18 GHz. The Moon was used as a primary brightness temperature standard with appropriate corrections for the known lunar limb darkening. The resulting brightness temperature spectra are consistent with previous data, but exhibit much less frequency-to-frequency scatter. The corona is fitted well by an optically thin source at temperature of 10(6) K, scale height H = 5 x 10(9), and density of 3.2 x 10(8) cm-3, and the chromosphere, an optically thick source at around 11,000 K. No evidence of a contribution from the transition zone was found. Thus the emission measure of that region must be 5-10 times less than published models.