  We report the statistics of the number of active regions (NAR) observed at 17
GHz with the Nobeyama Radioheliograph between 1992, near the maximum of cycle
22, and 2013, that also includes the maximum of cycle 24, and we compare with
other activity indexes. We find that NAR minima are shorter than those of the
sunspot number (SSN) and radio flux at 10.7 cm (F10.7). This shorter NAR minima
could reflect the presence of active regions generated by faint magnetic fields
or spotless regions, which were a considerable fraction of the counted active
regions. The ratio between the solar radio indexes F10.7/NAR shows a similar
reduction during the two minima analyzed, which contrasts with the increase of
the ratio of both radio indexes in relation to the SSN during the minimum of
cycle 23-24. These results indicate that the radio indexes are more sensitive
to weaker magnetic fields than those necessary to form sunspots, of the order
of 1500 G. The analysis of the monthly averages of the active region brightness
temperatures shows that its long term variation mimics the solar cycle,
although, due to the gyro-resonance emission, a great number of intense spikes
are observed in the maximum temperature study. The decrease, in number, of
these spikes is also evident during the current cycle 24, a consequence of the
sunspot magnetic field weakening in the last years.
