  We use microwave imaging observations from the Nobeyama Radioheliograph at 17
GHz for long-term studies of solar activity. In particular, we use the polar
and low-latitude brightness temperatures as proxies to the polar magnetic field
and the active-regions, respectively. We also use the location of prominence
eruptions as a proxy to the filament locations as a function of time. We show
that the polar microwave brightness temperature is highly correlated with the
polar magnetic field strength and the fast solar wind speed. We also show that
the polar microwave brightness at one cycle is correlated with the low latitude
brightness with a lag of about half a solar cycle. We use this correlation to
predict the strength of the solar cycle: the smoothed sunspot numbers in the
southern and northern hemispheres can be predicted as 89 and 59, respectively.
These values indicate that cycle 25 will not be too different from cycle 24 in
its strength. We also combined the rush to the pole data from Nobeyama
prominences with historical data going back to 1860 to study the north-south
asymmetry of sign reversal at solar poles. We find that the reversal asymmetry
has a quasi-periodicity of 3-5 cycles.
