Variation of Solar Microwave Spectrum
in the Last Half Century
Masumi Shimojo1, Kazumasa Iwai2, Ayumi Asai3, Satoshi Nozawa4,
Tetsuhiro Minamidani5, and Masao Saito5
1
2
National Astronomical Observatory of Japan, Japan
National Institute of Information and Communications Technology, Japan
3
Unit of Synergetic Studies for Space, Kyoto University, Japan
4
Department of Science, Ibaraki University, Japan
5
Nobeyama Radio Observatory, Japan
masumi.shimojo@nao.ac.jp
The total solar fluxes at 1, 2, 3.75, and 9.4 GHz have been observed continually
from 1957 to 1994 at Toyokawa, from 1994 until now at Nobeyama, Japan with the
current Nobeyama Radio Polarimeters. We examined the multi-frequencies and longterm dataset, and found that a monthly standard deviation of the total solar flux in
microwave can indicate an actual minimum state of the solar activity in one solar cycle
better than the sunspot number. Therefore, we define ”radio solar minimum” from the
behavior of the monthly standard deviation of the total solar flux. Furthermore, we
found that the microwave spectra of the radio solar minima in Cycle 20~24 agree with
each other within seven percent. This result shows that the average atmospheric
structure above the transition region in the quiet sun did not vary for half a century, and
suggests that the energy input for atmospheric heating from the sub-photosphere to the
corona did not change in the quiet sun despite significantly different strength of the last
five solar cycles.