Abstract
The microwave flux of the Sun is responsive to the same conditions that
produce
magnetically-structured radiation at visible and X-ray wavelengths, and
so the solar flux at high
radio frequencies (e.g. 2.8 GHz, or 10.7 cm) has, since the 1940's, been
used as a proxy for solar
optical variations. More recently, but still dating from the 1950's, the
Toyokawa/Nobeyama
measurements of flux at 1.0, 2.0, 3.75, and 9.4 GHz have supplemented
the 2.8 GHz time series.
Other synoptic measurements at lower and higher frequencies greatly add
to the spectral
information that is available on timescales ranging from days to
decades.
From the multi-frequency synoptic measurements in the 1.0-9.4 GHz range,
it has been found
that the the microwave flux time series show significant spectral
variations that in addition to the
brightness variations, provide useful proxy information for estimating
properties of the solar
atmosphere such as spot/plage fractions and total irradiance. An
important new concept in the
analysis of synoptic radio data is the definition of the
``rotationally-modulated microwave
component", which differs from the old ``S-component". This
``rotationally-modulated"
component of the emission contains a large, often dominant, contribution
from gyroresonance
emission, whereas the ``S-component", defined by the excess above the
cycle minimum level,
usually does not. This component distinguishes plage-associated emission
(due to coronal
bremsstrahlung) from spot-associated emission (gyroresonance) in the
time series of microwave
flux.
In some cases, the calibration of the ground-based radio measurements
has proven to be so
stable that it may be possible to ``bridge" short gaps in measurements
of total solar irradiance,
should that become necessary due to launch failures or satellite
malfunctions.
We will present a brief, but necessarily incomplete, history of
long-timescale synomptic radio
observations, with several of the more interesting results of recent
years.