Statistical Analysis of Decimetric Type III Bursts |
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Authors: | Y Ma R X Xie M Wang |
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Institution: | (1) National Astronomical Observatories/Yunnan Observatory, CAS, Yunnan, China |
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Abstract: | Detailed statistics and analysis of 264 type III bursts observed with the 625–1500 MHz spectrograph during the 23rd solar
cycle (from July 2000 to April 2003) are carried out in the present article. The main statistical results are similar to those
of microwave type III bursts presented in the literature cited, such as the correlation between type III bursts and flares,
polarization, duration, frequency drift rate (normal and reverse slopes), distribution of type III bursts and frequency bandwidth.
At the same time, the statistical results also point out that the average values of the frequency drift rates and degrees
of polarization increase with the increase in frequency and the average value of duration decreases with the increase in frequency.
Other statistical results show that the starting frequencies of the type III bursts are mainly within the range from 650 to
800 MHz, and most type III bursts have an average bandwidth of 289 MHz. The distributions imply that the electron acceleration
and the place of energy release are within a limited decimetric range. The characteristics of the narrow bandwidth possibly
involve the magnetic configuration at decimetric wavelengths, the location of electron acceleration in the magnetic field
nearto the main flare, the relevant runaway or trapped electrons, or the coherent radio emission produced by some secondary
shock waves. In addition, the number of type III bursts with positive frequency drift rates is almost equal to that with negative
frequency drift rates. This is probably explained by the hypothesis that an equal number of electron beams are accelerated
upwards and downwards within the range of 625 to 1500 MHz. The radiation mechanism of type III bursts at decimetric wavelengths
probably includes these microwave and metric mechanisms and the most likely cause of the coherent plasma radiation are the
emission processes of the electron cyclotron maser. |
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