[en] Self-modulation of the high-frequency electric field is observed when its frequency is near the electron plasma frequency. The localized field due to the self-modulation is found to make depressions in the plasma density and to be trapped in the density dip which moves with the ion-acoustic speed. The depth of the dip amounts to 20% of the unperturbed density. The maximum amplitudes of the high-frequency field and the density dip are proportional to each other. All these features are quantitatively explained by the results of the theory of coupled electron-plasma and ion-acoustic solitary waves. (author)