Decameter Type III Bursts with Changing Frequency Drift-Rate Signs

We discuss properties of type III bursts that change the sign of their drift rate from negative to positive and vice versa. Moreover, these bursts may change the sign of their drift rates more than once. These particular type III bursts were observed simultaneously by the radio telescopes UTR-2 (Ukrainian T-shaped Radio telescope, Kharkov, Ukraine), URAN-2 (Ukrainian Radio telescope of the Academy of Sciences, Poltava, Ukraine), and by the NDA (Nançay Decametric Array, Nancay, France) in the frequency range 8 – 41 MHz. The negative drift rates of these bursts are similar to those of previously reported decameter type III bursts and vary from −0.7 MHz s−1 to −1.7 MHz s−1, but their positive drift rates vary in a wider range from 0.44 MHz s−1 to 6 MHz s−1. Unlike inverted U-bursts, the tracks of these type III bursts have C- or inverted C-shapes. Our basic explanation of the positive drift rate of these type III bursts differs from the common assumption that positive drift rates of type III bursts are connected with electron beam propagation toward the Sun. We propose that, even if electron beams move outward from the Sun, they can generate type III bursts with positive drift rates if in some regions of the solar corona the group velocities of type III radio emissions are lower than the velocities of the electron beams.