First Observation of the Solar Type III Burst Decay and Its Interpretation

The decay of a Type III burst into two Type III bursts was registered during solar observations by GURT and URAN-2 radio telescopes on 2017 April 18. This was the first time such a phenomenon was observed. Newborn Type III bursts have drift rates smaller than that of a decaying Type III burst. Such decays of Type III bursts were predicted by a gas-dynamic theory of high-energy electron beams propagating through the thermal background plasma. In the frame of this theory Type III sources are beam-plasma structures moving with constant velocities. In our case the sum of velocities of newborn Type III sources equals the velocity of a decaying Type III source. The last one is 0.33c in the case of fundamental radio emission and 0.2c at the harmonic radio emission of a Type III burst. The density ratio of slow and fast newborn Type III sources is about 3.