Nonlinear propagation of electrostatic modes in ultrarelativistic dense
elelectron-positron gravito-plasma at the polar cap region of pulsar
magnetosphere is considered. A nonlinear Schr\"{o}dinger equation is obtained
from the reductive perturbation method which predicts the existence of Langmuir
dark solitons. Relevance of the propagating dark solitons to the pulsar radio
emission is discussed.Comment: 6 pages, 7 figures. Accepted for publication in Astrophysics and
Space Science. arXiv admin note: text overlap with arXiv:astro-ph/9808047 by
other authors without attributio