This work reports a peculiar and interesting train of microwave type III pairbursts in the impulsive rising phase of a solar flare on 2011 September 26. Theobservations include radio spectrometers at frequency of 0.80 - 2.00 GHz, hardX-ray (RHESSI and FERMI), EUV images of SWAP/PROBA-2 and magnetogram ofHMI/SDO. By using a recently developed method (Tan et al. 2016a), we diagnosedthe plasma density, temperature, plasma beta, magnetic field near the sourceregion, the energy of energetic electrons and the distance between theacceleration region and the emission start sites of type III bursts. From thediagnostics, we find that: (1) The plasma density, temperature, magnetic field,and the distance between the acceleration region and the emission start sitesalmost have no obvious variations during the period of type III pair trains,while the energy of electrons has an obvious peak value which is consistent tothe hard X-ray emission. (2) The plasma beta is much higher than an unityshowing a highly dynamic process near the emission start site of type IIIbursts. (3) Although the reversed-slope type III branches drift slower at oneorder of magnitude than that of the normal type III branches, the relateddowngoing and upgoing electrons still could have same order of magnitude ofenergy. These facts indicate that both of the upgoing and downgoing electronsare possibly accelerated by similar mechanism and in a small source region.This diagnostics can help us to understand the microphysics in the sourceregion of solar bursts.
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