Medium-Scale Traveling Structure in the Ionosphere-Thermosphere System

In the literature, the term "Medium-Scale Traveling Ionospheric Disturbance (MSTID)" may refer to either the displacement of the ionosphere due to a passing gravity wave or to an internally-generated plasma instability structure. While structures induced by gravity wave passage may propagate in any direction, the plasma instability structure propagates preferentially to the southwest (northwest) in the northern (southern) hemisphere and is usually accompanied by sporadic-E layers. We call attention to these differences and explore the physical mechanisms that enable observation of both types of structure with oblique HF radar and airglow imaging systems.