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Solar type III radio bursts are excited by electron beams propagating outwardfrom the Sun. The flux of type III radio burst has a time profile of rising anddecay phase at a given frequency, which has been actively studied since 1970s.Several factors that may influence the duration of a type III radio burst hasbeen proposed. However, the major cause of the duration is still an openquestion. In this work, to study the dominant cause of the duration, weinvestigate the source positions of the front edge, the peak, and the tail edgein the dynamic spectrum of a single and clear type III radio burst. Theduration of this type III burst at a given frequency is about 3 second fordecameter wave. The beam-formed observations by the LOw-Frequency ARray (LOFAR)are used, which can provide the radio source positions and the dynamic spectraat the same time. We find that, for this burst, the source positions of thefront edge, the peak, and the tail edge split with each other spatially. Theradial speed of the front edge, the peak, and the tail edge is 0.42 c, 0.25 c,and 0.16 c, respectively. We estimate the influences of the corona densityfluctuation and the electron-velocity dispersion on the duration, and thescattering effect by comparison with a few short-duration bursts from the sameregion. The analysis yields that, in the frequency range of 30 - 41 MHz, theelectron-velocity dispersion is the dominant factor that determines the timeduration of type III radio bursts with long duration, while scattering may playimportant role in the duration of short bursts.

On the Source Position and Duration of a Solar Type III Radio Burst Observed by LOFAR