| Scientists monitor the structure of the solar corona, the outer most regions of the
Sun’s atmosphere, using radio waves. The surface of the Sun is 6,000 degrees Kelvin,
while the high corona can reach several million degrees Kelvin. Solar radio emissions
at different frequencies allow us to observe radiation from different heights in the
atmosphere. The lower the frequency, the higher the height of origin. The frequency,
like the electron density, decreases uniformly outwards: 245 MHz originates high in
the corona, while 15,400 MHz originates in the low corona. The 5 MHz emission corresponds
to about 10 solar radii height. For a detailed review, see McLean and Labrum (1985)Solar
Radiophysics.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.
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