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

Abstract

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.

Suprathermal Magnetospheric Atomic and Molecular Heavy Ions at and Near Earth, Jupiter, and Saturn: Observations and Identification

“…No atomic ions appear to be present in these 28 Ma + and 32 Ma + data collections either inside or outside Saturn's magnetosphere, only molecular ions. The labeling differences also highlight the different plasma regime identification procedures at the two planets (see section 2 and , Christon et al, 2017. While the species identification of O 2 + at 32 Ma + is much more certain than that of 28 Ma + .…”

“…The improved composition assessment of the long-term MI component of outflown ionospheric MI fluxes observed in Earth's magnetosphere presented herein is intended to help focus planning choices for future thermosphere-ionosphere-magnetosphere observations. Of note, Earth's ionospheric meteoric metal neutral and ion layers at~80-120 km altitude include Na, Mg, Al, Si, K, Ca, Ti, and Fe (Plane et al, 2018), many of the expected lunar PUIs (see also our discussions in Christon et al, 2015Christon et al, , 2017. A primary purpose of this paper is to clearly identify which MI ultimately flow out of Earth's ionosphere and are subsequently incorporated into Earth's magnetospheric ion population.…”

“…On Cassini's cruise to Saturn, cruise data were obtained as allowed by tracking schedules, so there was very limited data livetime, much less than when Cassini orbited Saturn. Saturn's Sphere data were obtained in the radial range from~4 Rs out to either (1)~20 Rs or (2) a magnetopause crossing if it was closer (see , Christon et al, 2017. At Saturn, Cassini, a three-axis stabilized S/C, occasionally rolled for limited intervals.…”

“…Geotail/STICS is described in detail most recently in Christon et al (2017) and Cassini/CHEMS in see also Williams et al, 1994;Krimigis et al, 2004, respectively). Although STICS was operational before CHEMS, the instruments are very similar in design and nearly identical functionally.…”

“…A PHA color spectrogram for the data from these orbits is shown in Figure S5 in the SI. The selected orbit segments include times when the Mass-30 ion counts near Fe +2 (M/Q~28 amu/e, M~50-70 amu) were consistent with the surrounding, extant, low background-count levels representative of relatively low-to-moderate geomagnetic and solar activity intervals near-Earth (Christon et al, 2017). Use of this observation interval selection scheme near Earth under low Mass-30 ion (e.g., Si + ) conditions is germane to arguments regarding lunar PUIs because various laboratory studies have suggested that Fe + is an expected product of lunar soil irradiation by typical solar wind energy ions through secondary ion mass spectrometry (e.g., Dukes & Baragiola, 2015;Elphic et al, 1991).…”