|Solar radio emissions can be divided into 2 categories;
(i) Emission from the quiet Sun, and
(ii) Emission from the active Sun.
The emission from the active Sun can be further divided into
(i) A slowly varying component (varies over periods of days, weeks, or months) in the 3 - 60 cm range
(ii) A rapidly varying component (varies over periods of seconds, minutes, or hours) characterized by bursts
Intense and complex groups of bursts often follow the appearance of a flare in the solar
The radio emission following a flare is variable. By studying the rapidly variable component over a range of
frequencies one can classify the emissions into five principal types:
(1) Noise-storm bursts (Type I)
(2) Slow-drift bursts (Type II)
(3) Fast-drift bursts (Type III)
(4) Broad-band continuum emission (Type IV)
(5) Continuum emission at meter wavelengths (Type V)
The Type III bursts are short, strong bursts that begin right after a visible flare and move rapidly from around
500MHz to lower frequencies. At low frequencies (less than 25 MHz) the drift is very slow. The bursts
display a slow negative drift rate of 1MHz/sec. These bursts are caused by solar flares which eject high energy electrons into
space zipping away from the sun at about 1/4 the speed of light. These electrons excite radio waves in
space as they move along in the Sun's outer atmosphere. Since the density of the plasma in
space falls off as you move away from the sun, these
zipping electrons cause radio emission at successively lower frequencies.
Radio spectra of a Type III Solar Burst
observed by Ulysses
Radio spectra of a Type II Solar Burst
observed by Ulysses
Information on Solar Bursts at 20MHz by Dr. Dale Gary, New Jersey
Institute of Technology
At 20 MHz the main type of radio-producing event is a beam of electrons that gets accelerated in the solar corona and produces "plasma radiation"
which is a coherent mechanism that can be extremely bright. Individual beams of electrons will last only a 10 seconds or so at 20 MHz, and come from
about 1 solar radius above the surface of the Sun. They sometimes happen in groups, however, and so you may see spikey emission lasting for 1 to 5
minutes. These electron beam bursts are called type III emission.
Another possible type of emission is due to a shock wave, again traveling out through the solar corona but going more slowly. The emission can last
several minutes or more, and will be smoother in time. These shock wave bursts are called type II emission. They are much rarer than type III
Rarely, a type of emission called "noise storm" emission can be produced at 20 MHz. This emission can last for hours or days, and can be rather
spikey. It is much more common at higher frequencies, but can happen occasionally
at 20 MHz. This emission is called type IV emission.