Ionopause

Part of speech: noun

Definitions

1. The boundary between Earth’s outer atmosphere and the ionosphere where ionized particles and neutral gases meet and atmospheric density changes abruptly
2. The transition layer in a planet’s upper atmosphere marking the limit of significant ionization and separating the ionized region from neutral atmospheric layers
3. A distinct atmospheric boundary where charged particles in the ionosphere give way to the less ionized upper atmosphere, affecting atmospheric composition and electromagnetic interactions

Etymology: The ionopause is a scientific coinage, built from two modern parts: "iono-", the combining form of "ion" (itself from Greek "ion", "going", the present participle of "ienai", "to go"), and "pause", from Latin "pausa" and Greek "pausis", meaning a stopping or ceasing. Together they name the place where the ionized plasma of the upper atmosphere gives way to the thinner, slower-moving gas beyond it. The word is a compound rather than an older term that has changed meaning. Its first element points to the ions, electrically charged particles created when sunlight strips electrons from atmospheric gases, and its second points to a sudden stop in their behaviour. Planetary scientists applied it to boundaries around planets and comets, where the charged particles and the surrounding neutral gas meet, and the density of the atmosphere shifts abruptly.