Long-Term Global Solar Activity Observed by the Nobeyama Radioheliograph

The Nobeyama Radioheliograph has been observing the Sun at a frequency of 17 GHz regularly since 1992, providing synthesized full-disk images. This long period of continuous and consistent operation, providing well-calibrated data of a uniform standard, makes possible long-term studies of solar activity, from full-disk down to the angular resolution of the instrument. By using about 7200 daily, full-disk images, it has been possible to generate a radio version of the butterfly diagram, which differs significantly from the sunspot butterfly diagram. The polar regions are bright at 17 GHz, with their brightness well-correlated with the polar magnetic field strengths. Both are anti-correlated with activity at low latitudes, such as active regions and solar flares. The 17 GHz butterfly diagram shows both high and low-latitude activity. The brightness of both these facets of solar activity shows a significant decline over the 20+ years observations that have been made. In the northern hemisphere, the radio brightnesses at low and high latitudes are strongly anti-correlated. However, this anti-correlation is weak in the southern hemisphere. We find a weakening of the synchronization of activity between the northern and southern hemispheres, and also between high and low latitude activity in the southern hemisphere. Possible causes of polar brightening and the meaning with respect to the general scenario of solar activity are discussed.