A change in solar radio spectrum during the decay of the Modern Maximum

The Sun experienced a period of unprecedented activity during solar cycle 19 in 1950s and 1960s, now called the Modern Maximum (MM). The decay of the MM has changed the Sun, the heliosphere and the planetary environments in many ways. However, this decay may not have proceeded synchronously in all solar parameters. One of the related key issues is if the relation between the two long parameters of solar activity, sunspot number and the solar 10.7cm radio flux, has remained the same during this decay. While a number of studies agree that this relation has indeed changed, no consensus on its validity exists. A recent study argues that there is an inhomogeneity in the 10.7cm radio flux in 1980, which led to a step-like jump ("1980 jump") in this relation. If true, this would imply that the 10.7cm radio flux is ineligible for long-term studies, which would seriously impede versatile studies of the Sun during the MM. Here we use the 10.7cm radio flux and four other, independent radio flux measurements, the sunspot number, the MgII index and the number of solar active regions in order to study their mutual relations during the decay of MM. We find that all the five radio fluxes depict an increasing trend with respect to the sunspot number from 1970s to 2010s. This excludes the interpretation of the "1980 jump" as an inhomogeneity in the 10.7cm flux, and re-establishes the 10.7cm flux as a reliable and homogeneous long-term measure of solar activity. We find that the fluxes of longer radio waves increased with respect to the shorter waves, which implies a long-term change in the solar spectrum at radio frequencies. We also find that both the MgII index and the number of active regions increased with respect to the sunspot number, indicating a difference in the long-term evolution in chromospheric and photospheric parameters. Our results give evidence for important structural changes in solar magnetic fields and solar atmosphere during the decay of the MM when solar activity weakened considerably. These changes have not been reliably documented so far. We also emphasise that the changing relation between the different (e.g. photospheric and chromospheric) parameters should be taken into account when using sunspot number or any single parameter in long-term studies of solar activity.