The second Z-propagation window

Reference is made to conditions in the ionosphere under which an upgoing ordinary radio wave is converted, through a coupling process, into an upgoing extraordinary wave in the Z mode. Propagation in the Z mode requires the presence of both an ionised gas and a magnetic field - hence such waves cannot be received in free space. Attention is drawn to a window that is relevant to the escape of Z mode radiation produced naturally in the topside ionosphere. The window involves coupling between Z mode and whistler-mode waves, the latter being free to propagate to the ground. (U.K.)