Stephen P. Christon

Observations and present knowledge of heavy ions with mass ≥ 27 in the magnetosphere are reviewed. There are four ultimate sources of these heavy ions: the solar wind (mainly high charge-state atomic ions), the ionosphere (mainly molecular ions), the atmospheric metal layers that originate ultimately from ablation of meteoroids and possibly space debris (low charge-state metallic ions and metal-rich molecular ions), and lunar surface and exosphere (low charge-state metallic and molecular ions). The upstream heavy ions (solar wind origin and lunar origin) give independent information on the ion entry routes to the magnetosphere from proton (H⁺) and alpha particles (He⁺⁺): with similar mass-per-charge (m/q) values, or gyroradius, for the solar wind origin, and much larger gyroradius for the lunar origin. The lunar origin ions also give independent insights from laboratory observations on the sputtering processes. The atmospheric origin molecular and metallic ions are essential in understanding energization, ionization altitudes, and upward transport in the ionosphere during various ionospheric and magnetospheric conditions. These ions are also important when considering the evolution of the Earth’s atmosphere on the geological timescale. Only a few terrestrial missions have been equipped with instrumentation dedicated to separate these molecular and metallic ions, within only a limited energy range (cold ions of < 50 eV and energetic ions of ∼ 100 keV or more) and a limited mass range (mainly ≤ 40 amu). This is far too limited to make any quantitative discussion on the very heavy ions in the magnetosphere. For example, the existing data are far from sufficient for determining the dominant contributor from the four possible sources, or even to rule out any of the possible sources as a substantial contributor. Under this circumstance, it is worth to re-examine, using available tools, the existing data from the past and on-going missions, including those not designed for the required mass separation, to search for these ions. The purpose of this review is to summarize the availability of these datasets and tools. This review also shows some examples of combinations of different datasets that provide important indications of the sources of these heavy ions and their amounts that have been overlooked to date. Finally, we note the possible future contamination of specific masses (mainly aluminum (Al), but also lithium (Li), iron (Fe), nickel (Ni), copper (Cu), titanium (Ti) and germanium (Ge)) by the ablation of re-entering human-made objects in space (debris and alive satellites) in the coming decades. This possibility argues the need for dedicated observations of magnetospheric and ionospheric metallic ions before these metallic ions of space debris origin start to dominate over the natural contribution. The required observations can be performed with the available designs of space instrumentation and available ground-based instruments.

We examine long-term energetic heavy ion measurements including three planets’ magnetospheres, focusing on Fe ions (specifically, but not exclusively, Fe+) in and near Earth’s magnetosphere. We compare Fe data to that of other energetic ion species with masses greater than C (carbon) and consider the relationship(s) of energetic Fe ion measurements at the three planets to internal (ionospheres, exospheres, moons, rings, and trapped radiation) and external (solar wind and interplanetary dust) source candidates. Fe+ has been observed at Earth and Saturn, but not yet at Jupiter, as our observations there were brief. The measurements are from two functionally identical charge-energy-mass ion spectrometers: one on Geotail (~87-212 keV/e), orbiting Earth at ~9-30 Re; and the other on Cassini (~83-167 keV/e), in interplanetary space, during Jupiter flyby, and at ~4-20 Rs on its constantly varying orbits around Saturn. These ion spectrometers efficiently separate energetic light and heavy ions by mass, as well as lower charge state ions from higher charge state ions by mass-per-charge. Energetic low charge state ions often derive from magnetospheric sources, while energetic high charge state ions most often derive from the solar wind. We also enlist heavy ion measurements closer to the Earth from AMPTE/CCE which are used for C and Fe radiation-belt- modeling content, consideration, and estimation.

The plumes of Enceladus produce a cloud of neutral H2O molecules and, via dissociation, OH and O. These neutrals are ionized by charge exchange, solar UV, and electron impacts, producing the thermal water group ions W+ (O+, OH+, H2O+, and H3O+) which become energized in Saturn’s magnetosphere. We first separate the components of energetic (~96 keV) W+ using Cassini Charge-Energy-Mass Spectrometer (CHEMS) data from 78 near equatorial main ring current passes (dipole L = 7-16, ±10° in latitude) in 2004-2010. We find ~53% O+, ~22% OH+, ~22% H2O+, and ~3% H3O+ when averaged over L = 7-16, resulting in a mean water group mass of 16.7 amu. At 7 < L < 21, we find abundance ratios for O+/W+, OH+/W+, and H2O+/W+ that vary little with L. However, while H3O+/W+ is nearly constant at L > 13, H3O+/W+ tends to increase persistently at L < ~10. The large O+ abundance qualitatively agrees with the broad atomic O cloud observed by Cassini and predicted by some models. Our observation of H2O+/W+ > ~20% out to L ~ 21 suggests that neutral H2O spreads throughout the magnetosphere rather than being confined to a narrow H2O torus centered on Enceladus’ orbit.

We examine long-term energetic heavy ion measurements including three planets’ magnetospheres, focusing on Fe ions (specifically, but not exclusively, Fe+) in and near Earth’s magnetosphere. We compare Fe data to that of other energetic ion species with masses greater than C (carbon) and consider the relationship(s) of energetic Fe ion measurements at the three planets to internal (ionospheres, exospheres, moons, rings, and trapped radiation) and external (solar wind and interplanetary dust) source candidates. Fe+ has been observed at Earth and Saturn, but not yet at Jupiter, as our observations there were brief. The measurements are from two functionally identical charge-energy-mass ion spectrometers: one on Geotail (~87-212 keV/e), orbiting Earth at ~9-30 Re; and the other on Cassini (~83-167 keV/e), in interplanetary space, during Jupiter flyby, and at ~4-20 Rs on its constantly varying orbits around Saturn. These ion spectrometers efficiently separate energetic light and heavy ions by mass, as well as lower charge state ions from higher charge state ions by mass-per-charge. Energetic low charge state ions often derive from magnetospheric sources, while energetic high charge state ions most often derive from the solar wind. We also enlist heavy ion measurements closer to the Earth from AMPTE/CCE which are used for C and Fe radiation-belt- modeling content, consideration, and estimation.

We examine long-term energetic heavy ion measurements including three planets’ magnetospheres, focusing on Fe ions (specifically, but not exclusively, Fe+) in and near Earth’s magnetosphere. We compare Fe data to that of other energetic ion species with masses greater than C (carbon) and consider the relationship(s) of energetic Fe ion measurements at the three planets to internal (ionospheres, exospheres, moons, rings, and trapped radiation) and external (solar wind and interplanetary dust) source candidates. Fe+ has been observed at Earth and Saturn, but not yet at Jupiter, as our observations there were brief. The measurements are from two functionally identical charge-energy-mass ion spectrometers: one on Geotail (~87-212 keV/e), orbiting Earth at ~9-30 Re; and the other on Cassini (~83-167 keV/e), in interplanetary space, during Jupiter flyby, and at ~4-20 Rs on its constantly varying orbits around Saturn. These ion spectrometers efficiently separate energetic light and heavy ions by mass, as well as lower charge state ions from higher charge state ions by mass-per-charge. Energetic low charge state ions often derive from magnetospheric sources, while energetic high charge state ions most often derive from the solar wind. We also enlist heavy ion measurements closer to the Earth from AMPTE/CCE which are used for C and Fe radiation-belt- modeling content, consideration, and estimation.

Enceladus produces plumes of water vapor and ice particles whose gaseous H₂O molecules dissociate into OH and O. The gas ejecta form a large toroidal planetary cloud colocated with and partially sourced by a similarly large torus of ice grain ejecta. Gas torus neutrals are ionized by charge exchange, solar ultraviolet, and electron impacts, producing the thermal water group ions, W⁺ (O⁺, OH⁺, H₂O⁺, and H₃O⁺), which become energized in Saturn's magnetosphere. We study the components of energetic (~96 keV) W⁺ using Cassini Charge‐Energy‐Mass Spectrometer (CHEMS) data from 78 near‐equatorial main ring current passes (dipole L = 7–16, ±10° in latitude) in 2004–2010. W⁺ fractional abundances are ~53% O⁺, ~22% OH⁺, ~22% H₂O⁺, and ~3% H₃O⁺, when averaged over L = 7–16, resulting in a mean water group mass of 16.7 amu. This large energetic O⁺ fractional abundance, over twice that observed for thermal O⁺ at L < 10, qualitatively agrees with the broad atomic O cloud observed by Cassini and predicted by some models. Fractional abundance ratios for O⁺/W⁺, OH⁺/W⁺, and H₂O⁺/W⁺ vary little over L = 7–21. Our observation of energetic H₂O⁺/W⁺ ≈ 22% out to L ~ 21 suggests that neutral H₂O spreads throughout the magnetosphere rather than being confined to a narrow torus centered on Enceladus' orbit. Although nearly constant at 13, H₃O⁺/W⁺ tends to increase significantly to ~4–5% at L ~ 7–8.

The plumes of Enceladus produce a cloud of neutral H2O molecules and, via dissociation, OH and O. These neutrals are ionized by charge exchange, solar UV, and electron impacts, producing the thermal water group ions W+ (O+, OH+, H2O+, and H3O+) which become energized in Saturn’s magnetosphere. We first separate the components of energetic (~96 keV) W+ using Cassini Charge-Energy-Mass Spectrometer (CHEMS) data from 78 near equatorial main ring current passes (dipole L = 7-16, ±10° in latitude) in 2004-2010. We find ~53% O+, ~22% OH+, ~22% H2O+, and ~3% H3O+ when averaged over L = 7-16, resulting in a mean water group mass of 16.7 amu. At 7 < L < 21, we find abundance ratios for O+/W+, OH+/W+, and H2O+/W+ that vary little with L. However, while H3O+/W+ is nearly constant at L > 13, H3O+/W+ tends to increase persistently at L < ~10. The large O+ abundance qualitatively agrees with the broad atomic O cloud observed by Cassini and predicted by some models. Our observation of H2O+/W+ > ~20% out to L ~ 21 suggests that neutral H2O spreads throughout the magnetosphere rather than being confined to a narrow H2O torus centered on Enceladus’ orbit.

We examine long‐term suprathermal, singly charged heavy ion composition measured at three planets using functionally identical charge‐energy‐mass ion spectrometers, one on Geotail, orbiting Earth at ~9–30 Re, the other on Cassini, in interplanetary space, during Jupiter flyby, and then in orbit around Saturn. O⁺, a principal suprathermal (~80–220 keV/e) heavy ion in each magnetosphere, derives primarily from outflowing ionospheric O⁺ at Earth, but mostly from satellites and rings at Jupiter and Saturn. Comparable amounts of Iogenic O⁺ and S⁺ are present at Jupiter. Ions escaping the magnetospheres: O⁺ and S⁺ at Jupiter; C⁺, N⁺, O⁺, H₂O⁺, ²⁸M⁺ (possibly an aggregate of the molecular ions, MI, CO⁺, N₂⁺, HCNH⁺, and/or C₂H₄⁺), and O₂⁺ at Saturn; and N⁺, O⁺, N₂⁺, NO⁺, O₂⁺, and Fe⁺ at Earth. Generally, escaped atomic ions (MI) at Earth and Saturn have similar (higher) ratios to O⁺ compared to their magnetospheric ratios; Saturn's H₂O⁺ and Fe⁺ ratios are lower. At Earth, after O⁺ and N⁺, ionospheric origin N₂⁺, NO⁺, and O₂⁺ (with proportions ~0.9:1.0:0.2) dominate magnetospheric heavy ions, consistent with recent high‐altitude/latitude ionospheric measurements and models; average ion count rates correlate positively with geomagnetic and solar activity. At ~27–33 amu/e, Earth's MIs dominate over lunar pickup ions (PUIs) in the magnetosphere; MIs are roughly comparable to lunar PUIs in the magnetosheath, and lunar PUIs dominate over MIs beyond Earth's bow shock. Lunar PUIs are detected at ~39–48 amu/e in the lobe and possibly in the plasma sheet at very low levels.

We compare the long-term suprathermal heavy ion composition measured at three planets using functionally identical charge-energy-mass ion spectrometers, one on Geotail, orbiting Earth at ~9-30 Re, the other on Cassini, in interplanetary space, during Jupiter flyby, and then in orbit around Saturn. O+, a principal suprathermal (~80-220 keV/e) heavy ion in each magnetosphere, derives primarily from outflowing ionospheric O+ at Earth, but mostly from satellites and rings at Jupiter and Saturn. Comparable amounts of Iogenic O+ and S+ are present at Jupiter. Ions escaping the magnetospheres are: O+ and S+ at Jupiter; C+, N+, O+, H2O+, CO+(N2+), and O2+ at Saturn; and N+, O+, N2+, NO+, O2+, and Fe+ at Earth. Generally, escaped atomic ions (molecular ions, MI) at Earth and Saturn have similar (higher) ratios to O+ compared to their magnetospheric ratios; Saturn’s H2O+ and Fe+ ratios are lower. At Earth: after O+ and N+, ionospheric origin N2+, NO+, and O2+ (with proportions ~0.9:1.0:0.2) dominate magnetospheric heavy ions, consistent with recent high-altitude/latitude ionospheric measurements and models; average ion count rates correlate positively with geomagnetic and solar activity. At ~27-33 amu/e: Earth’s MIs dominate over lunar pickup ions (PUIs) in the magnetosphere; MIs are roughly comparable to lunar PUIs in the magnetosheath; and lunar PUIs dominate over MIs beyond Earth’s bow shock. Lunar PUIs are detected at ~39-48 amu/e in the lobe and possibly in the plasma sheet at very low levels.