Discovery of Suprathermal Ionospheric Origin Fe⁺ in and Near Earth's Magnetosphere

Suprathermal (87–212 keV/e) singly charged iron, Fe⁺, has been discovered in and near Earth's ~9–30 RE equatorial magnetosphere using ~21 years of Geotail STICS (suprathermal ion composition spectrometer) data. Its detection is enhanced during higher geomagnetic and solar activity levels. Fe⁺, rare compared to dominant suprathermal solar wind and ionospheric origin heavy ions, might derive from one or all three candidate lower‐energy sources: (a) ionospheric outflow of Fe⁺ escaped from ion layers near ~100 km altitude, (b) charge exchange of nominal solar wind iron, Fe⁺≥⁷, in Earth's exosphere, or (c) inner source pickup Fe⁺ carried by the solar wind, likely formed by solar wind Fe interaction with near‐Sun interplanetary dust particles. Earth's semipermanent ionospheric Fe⁺ layers derive from tons of interplanetary dust particles entering Earth's atmosphere daily, and Fe⁺ scattered from these layers is observed up to ~1000 km altitude, likely escaping in strong ionospheric outflows. Using ~26% of STICS's magnetosphere‐dominated data when possible Fe⁺² ions are not masked by other ions, we demonstrate that solar wind Fe charge exchange secondaries are not an obvious Fe⁺ source. Contemporaneous Earth flyby and cruise data from charge‐energy‐mass spectrometer on the Cassini spacecraft, a functionally identical instrument, show that inner source pickup Fe⁺ is likely not important at suprathermal energies. Consequently, we suggest that ionospheric Fe⁺ constitutes at least a significant portion of Earth's suprathermal Fe⁺, comparable to the situation at Saturn where suprathermal Fe⁺ is also likely of ionospheric origin.