Forty years of solar radio observations at Metsähovi Radio Observatory

The Aalto University Metsähovi Radio Observatory in Finland began regular solar radio observations over 40 years ago, in 1978. It is one of the longest solar radio data collections at millimeter wavelengths, and the data have been widely used in the field of solar radio physics over the years. The main research topics have been microwave solar radio bursts, oscillations, and pulsations in various solar features, for example, radio brightenings and the overall long‑term solar radio variability. Currently, Metsähovi has three different solar radio instruments, operating at wavelengths from 8 mm to 6 m. Most of the observations have been made with the 13.7‑m antenna at 8 mm, but data have also been collected at 3 mm and 1.3 cm. Radio brightenings at 8 mm are a good indicator of the general solar activity level. They show the phase of the solar cycle equally accurately as the traditional solar sunspot number. In this paper, we compare the daily value of the maximum radio brightness temperature and the daily sunspot number in 1989–2017, Solar Cycles 21–24. A more detailed analysis and comparison is performed for Solar Cycle 24 from 2008 onward. We also compare our radio observations to the daily 10.7 cm total radio flux values in Solar Cycle 24.