For propagation in the magnetic meridian plane at two particular angles of incidence near the vertical, the ordinary (O) electromagnetic mode couples into what Budden <1961> calls the extraordinary (X) mode and what more recently has been called the Z mode. One of the angles is northward of zenith and one is southward. These angles form the transition between O mode reflections at a ''Spitze'' where the wave frequency ω matches the plasma frequency ωp and reflection below the height where ω=ωp. If the O mode propagates southward from Arecibo, the coupled Z mode is reflected up to a few kilometers above the height of the Spitze; it then propagates downward through the Spitze height to a height where the group path is horizontal. Assuming a horizontal, plane-stratified uniformly varying ionosphere and no nonlinear effects, the refractive index and wavenormal direction approach constant finite values at his height and the wave remains electromagnetic in character; this is a new result which has not been previously reported. The group velocity is small, and the wave becomes heavily damped due to electron-ion collisions after several milliseconds. If the O wave propagates northward, the coupled Z mode is reflected a couple of hundred meters or less above the Spitze height; it then propagates downward through the Spitze height and approaches horizontally the height at which the Appleton-Hartree refractive index becomes infinite.

For the same above mentioned assumptions, the Z mode converts gradually from electromagnetic to electrostatic in nature. The electrostatic or Langmuir mode attains a small downward component of group velocity and propagates downward for a few tens of meters while being heavily damped by electron-ion collisions. Neglecting damping and nonlinear effects, the intensity of the Z mode varies approximately inversely with group velocity. If the HF array near the Arecibo Observatory were modified to place the main or secondary beams in the coupling directions, the electric field due to the coupled Z wave could be enormous. Even weak transmissions from the array should produce a nonlinear region. It is thus difficult to say how much of the linear mode coupling description is valid before nonlinear effects dominate. The nonlinear effects might generate Langmuir waves of a similar frequency to the HF which could be detected by the Arecibo 430-MHz radar. Exciting new plasma line observations should thus be possible. ¿ American Geophysical Union 1993