Oblique Langmuir solitons and their selfcompression under the ''free path'' conditions

Slow oblique Langmuir solutons with HF filling and approximately fixed relative to the plasma caused by the modulational instability of the wave branch ω=ωsub(p) cos THETA have been first experimentally observed in a magnetized collisionless plasma of the ''beam afterglow''. The longitudinal size of the solitons is less than the initial oscillation wave length (excited by the electron beam in a plasma), and therefore they can not be called as the envelope solitons. It is shown that the solitons exhibit substantial evolution (self-compression) during their time of flight along the magnetic field (together with the plasma). The observed solitons are in principle different from the well known fast soliton existing in the same dispersion wave branch: the fast soliton moves several orders of magnitude faster, representing a potential hump without HF filling, it is formed due to a quite different mechanism - as a Korteveg-de Vries ion sound soliton or as a shallow water soliton. The observed solitons having frequencies ω=(1/3-1/2)ωsub(p) are still not described in theory