||Scientists monitor the structure of the solar corona, the outer most regions of the
Sun's atmosphere, using radio waves (100???s of MHz to 10???s of GHz). Variations
in the radiowave spectrum reveal characteristics of the corona and upper chromosphere
in terms of altitude profile for the local plasma temperature, density and magnetic
field. Typically, the lower the frequency then the higher the height of origin. The
frequency, like the solar electron density, decreases uniformly outwards with 245
MHz originating high in the corona whereas 15,400 MHz originates in the low corona.
Radio bursts are associated with solar flares. The delay at Earth of the different
radio frequencies during burst events is due to the outward movement of the source.
Bursts can have temperatures of 10xE12 degrees Kelvin. Large bursts last 10 to 20
minutes on average. Longer radio noise storms of persistent and variable high levels
of radiation originate in sunspot groups, areas of large, intense magnetic fields.
These storms are strongly circularly polarized due to the intense magnetic fields.
The microwave wavelength 2800 MHz daily radio flux correlates highly with the daily
sunspot number and the two databases are used interchangeably. The 2800 MHz, or 10.7
cm, responds to the same conditions that produce changes in the visible and X-ray
wavelengths. Schmahl and Kundu (1995) find that the solar radio fluxes in the spectral
range 1000-9400 MHz correlate well with the total solar irradiance. The intermediate
frequencies (at 2800 and 3750 MHz) are produced mainly by free-free gyroresonance
emission from sunspot structures, while 1000 and 9400 MHz flux are produced mainly
by free-free processes from structures associated with plages. They can distinguish
plage-associated emission from spot-associated emission in the time series of microwave
flux, both contributing opposing effects on the total solar irradiance.