Decametric type III bursts with variable sign of frequency drift rate

Abstract

We report the observations of decameter type III bursts whose frequency drift rates change their sign. Moreover, some bursts change the sign several times. This occurs in spite of that the expected radiation source moves from the Sun, thus providing the radiation drift from the high frequencies to the low ones. The values of positive drift rates for individual type III bursts vary from 0.44 to 12 MHz/s. At the same time, the values of the negative frequency drift rates of these bursts are within the range of usual type III bursts. A possible interpretation of these bursts is discussed in the frame of plasma emission mechanism. It consists in that in some frequency range the group velocity of radio emission is smaller than the velocity of electrons responsible for this radio emission, thus the burst source radiation at lower frequency is recorded before that at higher frequency.