Variation of Solar Microwave Spectrum in the Last Half Century

The total solar fluxes at 1, 2, 3.75, and 9.4 GHz were observed continuously from 1957 to 1994 at Toyokawa, and from 1994 until now at Nobeyama, Japan with the current Nobeyama Radio Polarimeters. We examined the multi-frequency and long-term datasets, and found that not only the microwave solar flux but also its monthly standard deviation well indicates the long-term variation of solar activity. Furthermore, we found that the microwave spectra at the solar minima of Cycle 20~24 agree with each other. These results show that the average atmospheric structure above the upper chromosphere in the quiet Sun has not varied for half a century, and suggest that the energy input for atmospheric heating from the sub-photosphere to the corona has not changed in the quiet Sun despite significantly differing strengths of magnetic activity in the last five solar cycles.