||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.