tokens: et al. ( 2012 ) applied the one-dimensional scan observation at two different frequency channels, and estimated that the temporal change of plasma density contributed 4 GHz and 6 GHz to the frequency drifts in two microwave type III bursts, respectively. However, in our work we found that most microwave type III pairs have frequency drifts 4 GHz . Only 3 among the 11 events have absolute frequency drifts >4 GHz ; see in Table 1 . Actually, in the framework of the PE mechanism the separate frequency ( f x ) reflects the plasma density in the flare energy release site, and the temporal variation of f x may reveal the plasma density change during the microwave type III burst pair train. Table 1 lists the temporal change of separate frequencies, and most of them are very slow, even close to zero. Among the 11 type III pair events, the maximum occurred in the event on 2006 December 13, which is about 10 MHz s −1 , and s −1 . Then s −1 . The observation shows that the relative frequency drift () is from 0.3 s −1 to 5.3 s −1 , which is three orders bigger than that caused by the plasma density temporal variation. It seems that the plasma density temporal variation does not contribute to the formation of frequency drift in type III pairs as much as in microwave type III bursts reported by Altyntsev et al. ( 2007 ) and Meshalkina et al. ( 2012 ). The plasma density gradient dominates the frequency drifts of type III pairs. Therefore, we have the relations and . With these approximations, the velocity can be expressed as The frequency gap () between the normal and RS branches is seldom mentioned in the previous literature. This work shows that the observed results of are ranging from 10 MHz to 1700 MHz, and are different from event to event. Then, what factors do dominate the frequency gap? Li et al. ( 2011 ) simulated the generation and propagation of type III bursts based on the assumption of a power-law model (PL; Allen 1947 ) and offset power-law model (OPL; Aschwanden Benz 1995 ), respectively. They find that frequency gaps may be very large ( MHz) when electron beams propagate in offset power law plasma, and relatively small ( MHz) in power law plasma. It seems that the frequency gap may be related to the distribution of plasma density. According to the PE mechanism, the frequency gap means a density difference between the start sites of normal and RS type III bursts. With a density scale length H n , a space length can be derived: L c can be regarded as an estimation of the length of acceleration regions. The electrons are accelerated in this region and get a relatively high energy, then trigger microwave type III bursts outside this region. It is possible that L c is only an upper limit of the acceleration region. Panel (4) of Figure 5 show that the frequency gap has no dependence on the separate frequency. This fact implies that the length of the acceleration region is possibly irrelevant to the plasma density. It is possible that the
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