The ionosphere is that region of the upper atmosphere of a planet where charged particles (electrons and ions) of thermal energy are present, which are the result of ionization of the neutral atmospheric constituents by electromagnetic and corpuscular radiation. The lower boundary of the ionosphere (which is by no means sharp) coincides with the region where the most penetrating radiation (generally, cosmic rays) produce free electron and ion pairs in numbers sufficient to affect the propagation of radio waves (D-region). The upper boundary of the ionosphere is directly or indirectly the result of the interaction of the solar wind with the planet. For weakly or essentially non-magnetic planets (e.g.; Venus), the interaction region between the solar wind and the ionospheric plasma represents the termination of the ionosphere on the sunward side; it may be called the ionopause. On the night-side the ionosphere can extend to greater distances in a tail-like formation, representing the solar wind shadow. In the tail the extent of the ionosphere is limited by the condition for ion escape.