A parametric instability at the lower hybrid frequency which is known as the ion quasi-mode instability may be excited in the ionosphere. The instability considered is a three-wave interaction in which an incident whistler mode wave near the lower hybrid frequency dcays into a lower hybrid wave and an ion acoustic type of oscillation. Threshold calculations are made at intervals along the L = 4 magnetic field line starting at the earth's surface (the geomagnetic position of Siple Station, Antarctica) and proceeding to the equatorial plane. A model is developed for the ionospheric plasma parameters needed to evaluate the expression for threshold field strength. The field directly radiated by the Siple transmitter are not strong enough to meet the requirement for threshold. However, it is possible that the threshold can be met for Siple transmissions which are amplified by natural processes and also for natural emissions. |