Plasma-electron-echo experiments are described in which the relation between the echo wavelength and frequency is given by $\frac{{\omega }_3}{k_3}=v^{\prime }$, where $v^{\prime }$ is a constant independent of the primary frequencies used to generate the echo. The velocity $v^{\prime }$ is approximately equal to the velocity at which the electron distribution has maximum slope. It is concluded that the relation between echo frequency and wavelength is determined by the ballistic properties of the electrons.

• Received 6 February 1970

DOI:https://doi.org/10.1103/PhysRevLett.24.1330