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The term “Medium-Scale Traveling Ionospheric Disturbances” is used to describe a number of different propagating phenomena in ionospheric plasma density with a scale size of hundreds of km. This includes multiple generation mechanisms, including ion-neutral collisions, plasma instabilities, and electromagnetic forcing. Observational limitations can impede characterization and identification of MSTID generation mechanisms. We discuss inconsistencies in the current terminology used to describe these and provide a set of recommendations for description and discussion. 

The impact of regional-scale neutral atmospheric waves has been demonstrated to have profound effects on the ionosphere, but the circumstances under which they generate ionospheric disturbances and seed plasma instabilities are not well understood. Neutral atmospheric waves vary from infrasonic waves of <20 Hz to gravity waves with periods on the order of 10 min, for simplicity, hereafter they are combined under the common term Acoustic and Gravity Waves (AGWs). There are other longer period waves like planetary waves from the lower and middle atmosphere, whose effects are important globally, but they are not considered here. The most ubiquitous and frequently observed impact of AGWs on the ionosphere are Traveling Ionospheric Disturbances (TIDs), but AGWs also affect the global ionosphere/thermosphere circulation and can trigger ionospheric instabilities (e.g., Perkins, Equatorial Spread F). The purpose of this white paper is to outline additional studies and observations that are required in the coming decade to improve our understanding of the impact of AGWs on the ionosphere. 

Abstract Reported observations of picket fence signatures associated with subauroral Strong Thermal Emission Velocity Enhancement (STEVE) emission events look strikingly similar to rayed auroral curtains. Rayed auroral curtains are often the visible signatures of tearing‐mode‐unstable current sheets with precipitating auroral electron current carriers. Picket fence signatures are not located where auroral precipitation explanations apply, and are closely collocated with STEVE emission events. A similar tearing‐mode‐instability explanation can be invoked with a different source for the originating field‐aligned current (FAC) sheet. In this explanation, the FAC sheet is sourced by ionospheric conductance gradients adjacent to the localized flows of the STEVE event. Geospace Environment Model of Ion‐Neutral Interactions (GEMINI) models of the 3D ionosphere near counterstreaming STEVE‐associated flow structures show the development of sufficiently strong current sheets for tearing mode instabilities to take hold. These instabilities can locally accelerate ambient ionospheric thermal electrons to the few eV needed for the reported observed green picket fence signatures.