The linear and nonlinear ion acoustic waves with electric field parallel to the external magnetic field have been investigated in a two-ion component plasma. The theoretical model is applied to the upper ionosphere to explain the solitary structures with predominantly parallel polarization and density fluctuations observed by Freja. The hot ion precipitation to the ionospheric region has been taken into account. The estimates made by the present theoretical model for electric field magnitude (~100 mV/m) and density humps (15--20)% within the nonlinear solitary structures are in good agreement with the observations. The vertical scale size of the structure is predicted to be of the order of a few meters depending upon the background density and electron temperature.