LAUSR

An ordinary high frequency radio wave radiated at the Spitze angle after reflection from the ionosphere does not return to the ground. It is converted into the Z-mode, and in the vicinity of the critical height, this mode becomes quasi-electrostatic and it is absorbed by electrons. This well-known result obtained in the cold plasma approximation is revised in our paper. It is shown that the Z-mode twice changes the direction of propagation in the vicinity of the critical height. When the Z-mode becomes quasi-electrostatic due to the increase in the vertical wave number, thermal corrections come into play. It is argued that this wave changes the direction of its vertical component of the group velocity at a particular altitude slightly smaller than the critical height. Various nonlinear effects (modulation instability, formation of solitons, and acceleration of electrons) possibly take place in the localized region near the reflection height.