10^{10}$cm s$^{-1}$.[]{data-label="fig3"}](ms1121f3b.eps "fig:"){width="90mm"}

The movement of the particles leads to the growth of the plateau height at the front of the structure for $1.9\mbox{ s}<t<2.7\mbox{
s}$. Due to the fact that at the front of the structure electrons come with a positive derivative, $\partial f/\partial v
>0$, the level of plasma waves also increases. When the peak of the plateau height is reached at $t\approx 2.7$s the reverse process takes place. The plateau height decreases and the arriving electrons have a negative derivative $\partial
f/\partial v <0$ that leads to absorption of waves. The growth and decrease of the plateau height and the level of plasma waves is typical for a homogeneous plasma ([@Melnik00a]). However, while the structure passes a given point the spectrum of Langmuir waves experiences the change. This change depends on the sign of the plasma density gradient. In the region with decreasing density ($x=15.47d$) the Langmuir waves have a negative shift in velocity space while the growing plasma density ($x=15.2d$) supplies a positive shift in phase velocity of the plasma waves. At the point with the positive gradient, the Langmuir waves shifted in phase velocity space are effectively absorbed by the electrons while the negative plasma 