A research group led by Masumi Shimojo (Assistant Professor at NAOJ Chile Observatory), including members from Nagoya University, Kyoto University, and Ibaraki University, analyzed the more than 60 years of solar microwave data from these telescopes. They found that microwave intensities and spectra at the minimums of the latest five cycles were the same every time. In contrast, during the periods of maximum solar activity, both the intensity and spectrum varied from cycle to cycle.
The Sun goes through cycles of active and quiet periods approximately every 11 years. This solar cycle is often associated with the number of sunspots, but there are other types of solar activity as well. So simply counting the number of sunspots is insufficient to understand the solar activity conditions. The Japanese research, using continuous four-frequency solar microwave observations (1, 2, 3.75 and 9.4 GHz) began in 1957 at the Toyokawa Branch of the Research Institute of Atmospherics, Nagoya University. In 1994 the telescopes were relocated to NAOJ Nobeyama Campus, where they have continued observations up to the present.
Microwaves are another indicator of solar activity. Microwaves have the advantage that, unlike sunspots, they can be observed on cloudy days. Also, monitoring multiple frequencies of microwaves makes it possible to calculate the relative strength at each frequency - this is called the spectrum.
Source: This research was published in the Astrophysical Journal on October 10, 2017. Provided by Nobeyama Radio Observatory. Credit - https://watchers.news/2017/11/18/solar-minimum-surprisingly-constant/ - Posted by Steven Young