Type III Radio Bursts
A major goal of the URAP experiment is to study
the characteristics of solar type III radio bursts, which are the most
frequently observed solar radio burst at frequencies < 1 MHz. In the
Figure, type III bursts start at approximately 4:00, 14:30, 15:15, and
22:00, at the highest frequency. These bursts are generated when
suprathermal electrons (velocity ~ 0.05 to 0.3 c, where c is the speed
of light) are ejected from solar active regions and then travel outward
along open magnetic field lines through the corona and interplanetary
medium. Along their path these electrons excite plasma oscillations
(Langmuir waves) at the electron plasma frequency (fpe); these
electrostatic waves are then partially converted into radio waves at
fpe and/or 2fpe. Thus the progress of the electrons into regions of
decreasing density produces radio waves progressing to successively
lower frequencies. The absolute low-frequency limit of the radio waves
is imposed by the local fpe, below which the radio waves cannot
propagate. In general, however, the type III frequencies do not
descend to fpe.
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Updated 2005-10-17