Dominance of high‐energy (>150 keV) heavy ion intensities in Earth's middle to outer magnetosphere

Abstract

Previous observations have driven the prevailing assumption in the field that energetic ions measured by an instrument using a bare solid state detector (SSD) are predominantly protons. However, new near‐equatorial energetic particle observations obtained between 7 and 12 RE during Phase 1 of the Magnetospheric Multiscale mission challenge the validity of this assumption. In particular, measurements by the Energetic Ion Spectrometer (EIS) instruments have revealed that the intensities of heavy ion species (specifically oxygen and helium) dominate those of protons at energies ≳150–220 keV in the middle to outer (>7 RE) magnetosphere. Given that relative composition measurements can drift as sensors degrade in gain, quality cross‐calibration agreement between EIS observations and those from the SSD‐based Fly's Eye Energetic Particle Spectrometer (FEEPS) sensors provides critical support to the veracity of the measurement. Similar observations from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instruments aboard the Van Allen Probes spacecraft extend the ion composition measurements into the middle magnetosphere and reveal a strongly proton‐dominated environment at L≲6 but decreasing proton intensities at L≳6. It is concluded that the intensity dominance of the heavy ions at higher energies (>150 keV) arises from the existence of significant populations of multiply‐charged heavy ions, presumably of solar wind origin.

Internal Versus External Sources of Plasma at Saturn: Overview From Magnetospheric Imaging Investigation/Charge‐Energy‐Mass Spectrometer Data

“…While He ++ only reaches a peak relative abundance near 10% (Figures e, f, and h, red line), He ++ does reach higher abundances than other minor species within the magnetosphere (i.e., He + and H 2 + ; Figures e, f, and h, green and blue lines, respectively). This is similar to studies at Earth which found the abundance of solar wind‐originating O 6+ surpassing that of ionospheric‐originating O + for high energies in the outer magnetosphere (Allen et al, ; Cohen et al, ). The distribution of He + follows closely with He ++ , consistent with He + originating largely from charge exchange with He ++ as well as from interplanetary pickup ions in the solar wind (Hamilton et al, ; Möbius et al, ).…”

“…Similar to the magnetosphere of Earth (Allen et al, ; Allen, Livi, & Goldstein, ; Allen, Livi, Vines, et al, ), the spatial distribution of He ++ is consistent with entry through both Dungey‐cycle related reconnection and Kelvin‐Helmholtz Instabilities. The outer magnetotail (>30 R S ) has similar intensities of H 2 + and He ++ when integrated over all (32–220 keV) energy channels (Figures c and e). This is reminiscent of results at Earth; for example, Cohen et al () found the abundances of solar wind O 6+ exceed that of ionospheric‐originating O + in the outer magnetotail (>7 R E ) for energies above 150 keV. This suggests that a heightened abundance of solar wind ions in planetary magnetotails may be a common occurrence—however, more comparisons to other magnetospheres, such as at Jupiter, should still be made. That the densities of W + , H + , H 2 + , He + and He ++ in the magnetotail are observed to minimize near midnight in the outer magnetosphere (>30 R S ) is likely an orbital effect due to the plasma sheet being displaced from Cassini's location in the geographic equatorial plane during the years 2005–2007, when much of the distant magnetotail observations were made. Larger W + density at the dusk magnetotail flank indicates that W + experiences more loss processes than generation processes (for the energy range of 32 to 220 keV) as it rotates across the magnetotail.…”

“…The primary solar wind entry processes at Earth were found to be Dungey‐cycle reconnection and Kelvin‐Helmholtz instabilities ([KHI]; Allen et al, ; Allen, Livi, & Goldstein, ; Allen, Livi, Vines, et al, ). The combination of these processes leads to the magnetotail plasma sheet to be comprised of both internally generated and externally generated ion populations, with the externally generated ion populations having higher observed flux values at high energies (>150 keV) for large L shells ([>9 Earth Radii]; Allen et al, ; Cohen et al, ). However, the inner terrestrial magnetosphere was observed to be dominated by ionospheric‐originating ion species (Allen et al, ; Allen, Livi, & Goldstein, ; Allen, Livi, Vines, et al, ; Christon et al, ; Cohen et al, ; Fritz et al, ; Grande et al, ; Kremser et al, , ; Welling et al, ; Wing et al, ).…”

Ionospheric Outflow During the Substorm Growth Phase: THEMIS Observations of Oxygen Ions at the Plasma Sheet Boundary

“…Therefore, examining characteristics of ionosphere oxygen outflows is critical for modeling magnetotail dynamics. Most oxygen ion observations in the magnetotail are focused on the central plasma sheet, where these ions have already been energized (Cohen et al, 2017;Kistler & Mouikis, 2016;Kronberg et al, 2015;Slapak et al, 2017) by various acceleration mechanisms operating in this region (Bosqued et al, 2009;Keika et al, 2013;Kronberg et al, 2014). Much less is known about oxygen outflows Our study of oxygen outflows around the plasma sheet boundary supplements previous investigations on oxygen ions in the plasma sheet (Kistler & Mouikis, 2016;Kronberg et al, 2015;Maggiolo & Kistler, 2014).…”

The He++/H+ Density Ratio Across Earth's Subsolar Magnetopause and Its Implications for the Presence of a Mass‐Dependent Reflection Coefficient

“…material originating from solar wind entering on the dayside). Solar wind ions are also a measurable constituent of the ring current (e.g., Borovsky et al, 1998;Gloeckler et al, 1985) and the middle to outer magnetosphere (Cohen et al, 2017), and so our results are relevant for understanding the composition of these regions as well.…”