Energetic Magnetospheric Ions Research Articles

A Novel Backtracing Model to Study the Emission of Energetic Neutral Atoms at Titan

AbstractTo study the emission of energetic neutral atoms (ENAs) at Titan, we have developed a novel model that takes into account a spacecraft detector's limited field of view and traces energetic magnetospheric particles backward in time. ENAs are generated by charge exchange between Titan's atmospheric neutrals and energetic magnetospheric ions. By tracing these ions through the draped electromagnetic fields in Titan's environment, we generate synthetic ENA images and compare them to Cassini observations from the TA flyby. Our model can reproduce the intensity and morphology of the observed images only when field line draping is included. Using a realistic detector geometry is necessary to determine the influence of this draping on the ENA images: the non‐uniform fields eliminate a localized feature of increased ENA flux, which is a different effect than in models utilizing an infinitely extended detector. We demonstrate that ENA observations from TA contain signatures of the time‐varying Saturnian magnetospheric environment at Titan: the modeled ENA emission morphology and the effect of field line draping are different for the background field vectors measured during the inbound and outbound legs of TA. The visibility and qualitative effect of the draping on observed ENA images vary strongly between different detector locations and pointings. Depending on the viewing geometry, field line draping may add segments of elevated flux to the synthetic ENA images, remove such segments, or have no qualitative effect at all. Our study emphasizes the challenges and the potential for remote sensing of Titan's interaction region using ENA imaging.

Low Frequency ULF Waves in the Earth's Inner Magnetosphere: Power Spectra During High Speed Streams and Quiet Solar Wind and Seeding of EMIC Waves.

Here, we extend the scope of the Gamayunov and Engebretson (2021, hereinafter Paper 1), https://doi.org/10.1029/2021JA029247 work by analyzing the low frequency ultra-low-frequency (ULF) wave power spectra in the Earth's inner magnetosphere during high speed stream (HSS) and quiet solar wind (QSW) driving conditions in the upstream solar wind (SW) and comparing our results to the results of Paper 1, where the statistics of ULF wave power spectra during coronal mass ejections (CMEs) are presented. The most important results of our statistical and comparative analyses are as follows. (a) During CMEs, HSSs, and QSW, the magnetic field power spectra of the transverse and compressional fluctuations are well approximated by power laws in the ∼mHz-Hz frequency range, where on average the parameters of power law fits during CMEs and HSSs are close, and those during QSW differ considerably from the respective parameters during CMEs and HSSs. (b) The dominance of the average compressional power over the average transverse power for the low frequency ULF waves during the 0<SYM/H≲25nT geomagnetic conditions may serve as a proxy of HSSs in the upstream SW, whereas the opposite relation between the average powers is an indication of CMEs. (c) Independently of the SW driving conditions, a turbulent energy cascade from low frequencies in the ULF wave frequency range into the higher frequency range exists in the Earth's inner magnetosphere, supplying the nonthermal electromagnetic seed fluctuations needed for the growth of electromagnetic ion cyclotron waves (∼Hz) due to relaxation of unstable distributions of energetic magnetospheric ions.

Influence of Titan's Variable Electromagnetic Environment on the Global Distribution of Energetic Neutral Atoms

AbstractWe combine the electromagnetic fields from a hybrid plasma model with a particle tracing tool to study the spatial distribution of energetic neutral atoms (ENAs) emitted from Titan's atmosphere when the moon is exposed to different magnetospheric upstream regimes. These ENAs are generated when energetic magnetospheric ions undergo charge exchange within Titan's atmosphere. The spatial distribution of the emitted ENA flux is largely determined by the parent ions' trajectories through the draped fields in Titan's interaction region. Since images from the ENA detector aboard Cassini captured only a fraction of the ENA population, we provide context for such observations by calculating maps of the ENA flux through a spherical detector concentric with Titan. We determine the global distribution of ENA emissions and constrain deviations between the locations of ENA production and detection. We find that the ENA flux is highest in a band that encircles Titan perpendicular to the ambient magnetospheric field, which was strictly perpendicular to the moon's orbital plane during only one Cassini flyby. The field line draping strongly attenuates the emitted ENA flux, but does not alter the overall morphology of the detectable flux pattern. The majority of detectable ENAs leave Titan's atmosphere far from where they are produced, that is, even a spacecraft located directly above the moon's atmosphere would detect ENAs generated beyond its immediate environment. Some energetic parent ions produce ENAs only after they are mirrored by the field perturbations in Titan's wake and return to the moon, demonstrating the complex histories of detectable ENAs.

Low Frequency ULF Waves in the Earth's Inner Magnetosphere: Statistics During Coronal Mass Ejections and Seeding of EMIC Waves

AbstractHere we perform a statistical analysis of low frequency ultra‐low‐frequency (ULF) waves ( mHz ‐ Hz) in the Earth's inner magnetosphere excluding electromagnetic ion cyclotron waves concurrently observed. We use the magnetic field data from the two Van Allen Probes during their first magnetic local time (MLT) revolution that cover the periods of coronal mass ejections. The major results of our analysis are as follows. (a) Spectra of both the transverse and compressional ULF waves are well approximated by the power‐laws in the mHz ‐ Hz frequency range. (b) There are two sources of the low frequency ULF waves: an internal magnetospheric source and an external source outside of the magnetosphere. (c) The average transverse power in the 6–24 h MLT sector dominates that in the 0–6 h sector, whereas the compressional power in the 12–24 h sector dominates that in the 0–12 h sector. (d) The average powers of transverse and compressional ULF waves in the plasmasphere dominate the average powers in the high L shell region of , and there is a deep power minimum in the intermediary region of . (e) The compressional ULF wave power has a maximum in the near equatorial region, whereas the transverse power has a minimum there. (f) A wave energy cascade from low frequency ULF waves into the higher frequency range of EMIC waves ( Hz) supplies the nonthermal seed fluctuations from which EMIC waves can then grow due to instabilities of the energetic magnetospheric ions.

Energetic ion dynamics near Callisto

We examine the dynamics of energetic magnetospheric ions in the highly perturbed and asymmetric electromagnetic environment of the Jovian moon Callisto. The Alfvénic interaction of the (nearly) corotating magnetospheric plasma with Callisto's ionosphere and induced dipole generates electromagnetic field perturbations near the moon, the structure of which vary as a function of Callisto's orbital position. For this study, these perturbations are obtained from the AIKEF hybrid model (kinetic ions, fluid electrons) which has already been successfully applied to Callisto's local plasma environment (Liuzzo et al., 2015, 2016, 2017). To isolate the influence of Callisto's ionosphere and induced dipole field on energetic ion dynamics, we analyze the trajectories of energetic hydrogen, oxygen, and sulfur ions exposed to various configurations of the locally perturbed electromagnetic fields. We present spatially resolved surface maps that display accessibility of Callisto to these ion populations for select energies from 1 keV to 5000 keV. The Alfvénic plasma interaction with (i) Callisto's induced magnetic field, (ii) Callisto's ionosphere, and (iii) the combination of both, all leave distinct imprints in these accessibility patterns. Draping of the magnetospheric field around Callisto's ionosphere partially shields the moon's trailing (ramside) hemisphere from energetic ion impacts, and the induced field tends to focus energetic ion impacts near Callisto's Jupiter-facing and Jupiter-averted apices. Depending on the nature of Callisto's Alfvénic plasma interaction, the accessibility of its surface to energetic protons may evolve non-monotonically with increasing energy. We also present maps of energetic ion accessibility and the resulting energy deposition onto Callisto at the time of the Galileo C3, C9, and C10 flybys. Our findings show that the shielding of Callisto's surface from energetic ion impacts is most effective during flybys that took place while the moon was located at an intermediate distance to the center of Jupiter's magnetospheric current sheet. In this case, the ionosphere and induced dipole field both make substantial contributions to the electromagnetic field perturbations near Callisto.

Observing planets and small bodies in sputtered high-energy atom fluxes

[1] The evolution of the surfaces of bodies unprotected by either strong magnetic fields or thick atmospheres in the solar system is caused by various processes, induced by photons, energetic ions, and micrometeoroids. Among these processes, the continuous bombardment of the solar wind or energetic magnetospheric ions onto the bodies may significantly affect their surfaces, with implications for their evolution. Ion precipitation produces neutral atom releases into the exosphere through ion sputtering, with velocity distribution extending well above the particle escape limits. We refer to this component of the surface ejecta as sputtered high-energy atoms (SHEA). The use of ion sputtering emission for studying the interaction of exposed bodies (EB) with ion environments is described here. Remote sensing in SHEA in the vicinity of EB can provide mapping of the bodies exposed to ion sputtering action with temporal and mass resolution. This paper speculates on the possibility of performing remote sensing of exposed bodies using SHEA and suggests the need for quantitative results from laboratory simulations and molecular physic modeling in order to understand SHEA data from planetary missions. In Appendix A, referenced computer simulations using existing sputtering data are reviewed.

Comparison of TWINS images of low‐altitude emission of energetic neutral atoms with DMSP precipitating ion fluxes

The brightest energetic neutral atom (ENA) intensity viewed by spacecraft in high‐inclination Earth orbit is from low‐altitude emission (LAE). It is a prominent feature in the stereo ENA images obtained by cameras on the NASA TWINS 1/2 Mission of Opportunity. This emission is produced by energetic magnetospheric ions precipitating into the atomic oxygen exosphere at latitudes near the auroral zones at altitudes ∼300 km. The ions undergo multiple atomic collisions including charge exchange of ions and stripping of the resulting neutrals. Consequently, this is a “thick target” process. We introduce a “thick‐target” approximation that allows us to extract the shape of the spatially averaged spectra of the precipitating ions from the ENA spectra in the TWINS 1/2 pixels viewing the LAE from near their orbital apogees. These ENA‐extracted ion spectra are compared with in situ precipitating ion spectra measured concurrently by DMSP satellites (F15 and F16) at ∼825 km altitude, while they are passing directly over the LAE regions in the TWINS 1/2 images. We obtain good agreement between the shape of the ENA‐extracted and in situ ion spectra from three distinct precipitation regions over energies from 2 to 32 keV (assuming precipitating protons). The absolute normalization of the ENA‐extracted and in situ spectra depends upon the TWINS viewing geometry because the ENA LAE source is not resolved by the imager. None of the spectral shapes obtained is consistent with a simple thermal intensity spectrum with kT = 5 keV.

Ozone Synthesis on the Icy Satellites

Condensed O2 and ozone on the surfaces of some icy satellites are thought to originate from the radiolytic decomposition of surface water ice by the impact of energetic magnetospheric ions, but decades of laboratory studies have produced no evidence for ozone from the radiolysis of pure water ice. Here we report for the first time the production of ozone in ice by 100 keV ions. Using a method that departs drastically from those used in all previous experiments, we have simulated more closely conditions on the icy satellites by performing ion irradiation while depositing water concurrently, which takes into account the effects of gravity and surface porosity. This codeposition causes the burial of a high concentration of radiolytic O2 from which ozone is formed. Our results demonstrate that the enhanced trapping of oxygen in surface ices depends on temperature and should vary locally, depending on the rates of irradiation and recondensation. The burial of radiolytic products by redeposition will likely occur in many varied astronomical environments, besides icy satellites.

First energetic neutral atom images from Polar

Energetic neutral atoms are created when energetic magnetospheric ions undergo charge exchange with cold neutral atoms in the Earth's tenuous extended atmosphere (the geocorona). Since they are unaffected by the Earth's magnetic field, these energetic neutrals travel away in straight line trajectories from the points of charge exchange. The remote detection of these particles provides a powerful means through which the global distribution and properties of the geocorona and ring current can be inferred. Due to its 2 × 9 RE polar orbit, the Polar spacecraft provides an excellent platform from which to observe ENAs because it spends much of its time in the polar caps which are usually free from the contaminating energetic charged particles that make observations of ENAs more difficult. In this brief report, we present the first ENA imaging results from Polar. Storm‐time ENA images are presented for a northern polar cap apogee pass on August 29, 1996 and for a southern polar cap perigee pass on October 23, 1996. As well, we show with a third event (July 31, 1996) that ENA emissions can also be detected in association with individual substorms.

Energetic magnetospheric ions at the dayside magnetopause: Leakage or merging?

We elaborate upon the leakage model for the escape of energetic magnetospheric particles into the magnetosheath. Unlike the merging model, no interconnection (or merging) of magnetospheric and magnetosheath magnetic field lines is required. Because outer magnetospheric energetic particle drift paths intersect the magnetopause, the leakage model requires the continual escape of ions at postnoon local times and electrons at prenoon local times, regardless of solar wind conditions. It also predicts a division between dawnward and duskward streaming ions at the point where most magnetosheath magnetic field lines make their closest approach to the magnetopause, typically near 1500 LT. Like the merging model, the leakage model predicts equatorward streaming just inside the magnetopause. We study the motion of an escaping energetic ion at a planar magnetopause to show that, without scattering, ions must move dawnward and northward in a duskward magnetosheath magnetic field and dawnward and southward in a dawnward magnetosheath magnetic field. Scattering permits some ions to move duskward. We present new observations of streaming ions outside the dayside magnetopause made by the Charge Composition Explorer satellite, a part of the Active Magnetospheric Particle Tracer Explorers program. To place these observations in context, we have performed a statistical study of previous particle observations both inside and outside the dayside magnetopause. The ensemble of observations indicates that energetic magnetospheric ions of all species continually escape from the dayside magnetosphere and stream along magnetosheath magnetic field lines, even when no merging is expected. The magnetosheath magnetic field controls the direction in which the ions stream: they move away from the magnetosphere. The results of this work indicate that energetic particle observations at the dayside magnetopause need not be taken as evidence for merging of magnetosheath and magnetospheric magnetic field lines.

A determination of the charge state of energetic magnetospheric ions by the observation of drift echoes

The mean charge states of heavy ions trapped in the earth's magnetosphere are important parameters for distinguishing among magnetospheric theories. No direct measurements have been made at energies greater than ∼ 50 keV. A technique, which uses the E/q dependence of ion drift velocity, is described in this letter. A drift‐echo event, which followed a substorm that occurred at 2100‐2200 UT on 25 February 1979, is discussed to illustrate the application of this technique. In this event the charge state of helium ions was +2 and CNO ions was &gt; +2 and probably ≥ +5. This result implies that the drift‐echo particles were from freshly injected solar‐wind plasma.

Solar system sputtering

The solar wind and energetic magnetospheric ions are important agents for modification of the planetary surfaces and atmospheres with which they interact. Knowledge of the details of the sputtering mechanisms involved in these interactions can help in determining the interrelation of a number of planetary processes. In this paper I shall discuss several sites in the solar system where sputtering phenomena are significant. Our quantitative understanding of these examples could be extended if we had a more detailed picture of the sputtering of insulators. Recent experimental results that bear on these problems as well as the gaps in our knowledge that prevent further progress will be presented.