Energetic Neutral Atom Emissions Research Articles

Close up observation and inversion of low-altitude ENA emissions during a substorm event

During a series of substorm events on November 12 2004, the Neutral Atom Detector Unit (NUADU) flying onboard the TC-2 spacecraft observed, close to perigee, bright low-altitude Energetic Neutral Atom (ENA) emissions from both north and south poles. Through utilizing high temporal and spatial resolution data inversion techniques we present here a global view the associated processes of energetic ion deposition and injection at the uniquely high temporal resolution of a few minutes. It is demonstrated that the ENA emissions observed came mainly from the, low-altitude, high magnetic latitude polar region. Since precipitating ions follow magnetic field lines and have more chance to produce ENA emissions through charge exchange there. Therefore, the employment of a low-orbit satellite to record at high temporal and spatial resolution ENA image data is inferred to be the best way to monitor those dynamic ring current variations that occur during geomagnetic activity.

Enhanced Energetic Neutral Atom Imaging

Over the past two decades, instruments designed to image plasmas in energetic neutral atom (ENA) emission have flown in space. In contrast to typical satellite-based in situ instruments, ENA imagers provide a global view of the magnetosphere because they remotely measure ion distributions via neutrals that are not tied to the magnetic field. An intrinsic challenge that arises during analysis of magnetospheric ENA images is that the ENA fluxes are integrated along the line-of-sight of the instrument. We propose a method of enhancing ENA emission from a localized region in space, thereby enabling spatially resolved measurements of ENA emission in a remotely obtained ENA image. Here we show that releases of modest volumes (~ 1.4 m3) of liquid hydrogen in space are sufficient to accomplish the ENA localization.

Europa Neutral Torus Confirmation and Characterization Based on Observations and Modeling

Abstract The confirmation and constraints of a Europa-generated neutral torus are presented based on available observations and computational modeling of relevant source species. These results are applied to determine if the Energetic Neutral Atom (ENA) images reported by Mauk et al. were produced by interactions with a Europa neutral particle torus or with the local charged particle population. Neutral tori provide important insights into the processes operating in large planet magnetospheres. Such features emerge when particles escape an embedded satellite and form a population that co-orbits with the moon. Torus distributions and compositions are directly related to the satellite composition and source mechanisms. Significant findings are as follows. (1) The presence of a Europa neutral torus is confirmed through the development of 3D Io and Europa neutral tori models. (2) Density constraints for H, H2, O, and O2 in the Europa neutral torus are determined. (3) The torus is highly asymmetric: Near Europa, ∼99% of the ENA image is produced by the neutral torus. Furthest from Europa, ∼15%–69% of the ENA emissions are caused by charge exchange interaction within the ambient plasma. In addition, we showed that (4) 7%–34% of the ENAs near Europa would be produced by charge exchange with neutrals, even with a three order of magnitude neutral source rate reduction (<2 × 1024). (5) Satellite source rate variations take longer to impact the Europa neutral torus than the Io neutral torus. Dominant processes at Europa’s orbit have lifetimes >2–8 days, while at Io, the neutral particles lifetimes are as short as 8–13 hr.

Time Dependence of the IBEX Ribbon and the Globally Distributed Energetic Neutral Atom Flux Using the First 9 Years of Observations

Abstract Observations from the Interstellar Boundary Explorer (IBEX) of energetic neutral atoms (ENAs) reveal two populations, those emitted from a narrow (∼20°–40°) ribbon that is centered on the local interstellar magnetic field, and a globally distributed flux (GDF) that is controlled by processes in the heliosheath. This is a third study utilizing a previously developed technique to separate ENA emissions in the ribbon from the GDF. In the first ribbon separation study, we analyzed the first year of IBEX data at the energies of 0.7 keV and above; the second study analyzed data down to 0.2 keV using the first five years of IBEX data. Here, we utilize the separation analysis from 0.7 keV and above to study time evolution in 3 year intervals over the first nine years of IBEX data. This study is the first to reveal the global time evolution of the GDF distinct from that of the IBEX ribbon. We show that the time evolution of the GDF within 40° of the upwind pressure maximum is driven by changes in the solar wind ram pressure through compression and rarefaction in the heliosheath. In contrast, the GDF is relatively stable in the region centered on the heliotail downwind with respect to the interstellar flow. The evolution of the IBEX ribbon is observed to have a time lag with respect to the upwind GDF evolution, likely due to the secondary (neutral) solar wind source. The time lag observed in the ribbon evolution is consistent with the generation of ions retained for several years beyond the heliopause. These observations lend further support to secondary solar wind models of the IBEX ribbon, but also require that there is a significant several year time lag for reneutralization of ions that form the IBEX ribbon. We use this study of the 9 year separation of the IBEX ribbon from the globally distributed flux to prepare for a formal IBEX data release of ribbon and globally distributed flux maps to the heliophysics community.

Energetic Neutral and Charged Particle Measurements in the Inner Saturnian Magnetosphere During the Grand Finale Orbits of Cassini 2016/2017

AbstractWe give an overview of the charged and neutral particle measurements during the Grand Finale of Cassini performed by the Magnetosphere Imaging Instrument, MIMI, on board. The first in situ measurements of MeV particles between the D‐ring and the atmosphere of the planet showed protons with energies above 300 MeV with higher count rates when magnetically connected to the D‐ring which was less dense than expected, while more material was present at the inner edge. The variability in the count rates was a combination of sensor look direction, pitch angle coverage, changes in the L‐range, and latitude. Enhanced emissions of energetic neutral atoms during ring current activity was detected and confirmed the existence of a low‐energy ion radiation belt discovered by Cassini in 2004. No detection of low‐energy ions was indicative of a source population from a region closer than 1,800 km above the 1 bar cloud level.

Recurrent Magnetic Dipolarization at Saturn: Revealed by Cassini

AbstractPlanetary magnetospheres receive plasma and energy from the Sun or moons of planets and consequently stretch magnetic field lines. The process may last for varied timescales at different planets. From time to time, energy is rapidly released in the magnetosphere and subsequently precipitated into the ionosphere and upper atmosphere. Usually, this energy dissipation is associated with magnetic dipolarization in the magnetosphere.This process is accompanied by plasma acceleration and field‐aligned current formation, and subsequently auroral emissions are often significantly enhanced. Using measurements from multiple instruments on board the Cassini spacecraft, we reveal that magnetic dipolarization events at Saturn could reoccur after one planetary rotation and name them as recurrent dipolarizations. Three events are presented, including one from the dayside magnetosphere, which has no known precedent with terrestrial magnetospheric observations. During these events, recurrent energizations of plasma (electrons or ions) were also detected, which clearly demonstrate that these processes shall not be simply attributed to modulation of planetary periodic oscillation, although we do not exclude the possibility that the planetary periodic oscillation may modulate other processes (e.g., magnetic reconnection) which energizes particles. We discuss the potential physical mechanisms for generating the recurrent dipolarization process in a comprehensive view, including aurora and energetic neutral atom emissions.

Simulation of the Solar Wind Dynamic Pressure Increase in 2014 and Its Effect on Energetic Neutral Atom Fluxes from the Heliosphere

Abstract In late 2014, the solar wind dynamic pressure increased by ∼50% over a relatively short time (∼6 months). In early 2017, the Interstellar Boundary Explorer (IBEX) observed an increase in heliospheric energetic neutral atom (ENA) fluxes from directions near the front of the heliosphere. These enhanced ENA emissions resulted from the increase in SW pressure propagating through the inner heliosheath (IHS), affecting the IHS plasma pressure and emission of ∼keV ENA fluxes. We expand on the analysis by McComas et al. on the effects of this pressure change on ENA fluxes observed at 1 au using a three-dimensional, time-dependent simulation of the heliosphere. The pressure front has likely already crossed the termination shock (TS) in all directions, but ENA fluxes observed at 1 au will change over the coming years, as the TS, heliopause, and IHS plasma pressure continue to change in response to the SW pressure increase. Taken in isolation, the pressure front creates a “ring” of increasing ENA fluxes projected in the sky that expands in angular radius over time, as a function of the distances to the heliosphere boundaries and the ENA propagation speed. By tracking the position of this ring over time in our simulation, we demonstrate a method for estimating the distances to the TS, heliopause, and ENA source region that can be applied to IBEX data. This will require IBEX observations at 4.3 keV up through ∼2020, and longer times at lower ENA energies, in order to observe significant changes from the heliotail.

Composition of 1–128 keV Magnetospheric ENAs

AbstractThe Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission has observed the inner magnetosphere since 2008. TWINS flies energetic neutral atom (ENA) cameras aboard two spacecraft in separate Molniya orbits. TWINS images the ENA emissions from the inner magnetosphere across a broad range of energies (1 to 128 keV for H, 16 to 128 keV for O, and higher energies for total ENAs). This allows TWINS to observe the evolution, on minute timescales, of the trapped and precipitating particles that dominate storm time dynamics. Presented here are ENA observations over this broad energy range of the large storm of 17 March 2015 (Dst < −223)—the St. Patrick's Day storm. The ENA observations are presented with a 15‐min cadence for improved counting statistics. During the St. Patrick's Day storm the flux of ENAs and the concentration of O+/H+ increased significantly during the main phase. The concentration increased to a value of 1.0 for the 16 keV ions, and the temperature in the inner magnetosphere dropped during the time between the prestorm phase and the main phase. Comparing the results from this storm to a moderate storm on 22 July 2009, and a collection of nine other large storms, we find that the most energetic O+ ions in the ring current occur before the peak of the storm.

Heliosphere Responds to a Large Solar Wind Intensification: Decisive Observations from IBEX

Abstract Our heliosphere—the bubble in the local interstellar medium produced by the Sun’s outflowing solar wind—has finally responded to a large increase in solar wind output and pressure in the second half of 2014. NASA’s Interstellar Boundary Explorer (IBEX) mission remotely monitors the outer heliosphere by observing energetic neutral atoms (ENAs) returning from the heliosheath, the region between the termination shock and heliopause. IBEX observed a significant enhancement in higher energy ENAs starting in late 2016. While IBEX observations over the previous decade reflected a general reduction of ENA intensities, indicative of a deflating heliosphere, new observations show that the large (∼50%), persistent increase in the solar wind dynamic pressure has modified the heliosheath, producing enhanced ENA emissions. The combination of these new observations with simulation results indicate that this pressure is re-expanding our heliosphere, with the termination shock and heliopause already driven outward in the locations closest to the Sun. The timing between the IBEX observations, a large transient pressure enhancement seen by Voyager 2, and the simulations indicates that the pressure increase propagated through the heliosheath, reflected off the heliopause, and the enhanced density of the solar wind filled the heliosheath behind it before generating significantly enhanced ENA emissions. The coming years should see significant changes in anomalous cosmic rays, galactic cosmic radiation, and the filtration of interstellar neutral atoms into the inner heliosphere.

Influence of asymmetries in the magnetic draping pattern at Titan on the emission of energetic neutral atoms

We model the emission of energetic neutral atoms (ENAs) that are generated by the interaction between energetic ions from Saturn's magnetosphere and neutrals from the upper atmosphere of the giant planet's largest moon Titan. The trajectories of the parent ions and the resulting ENA emission morphology are highly sensitive to the electromagnetic field configuration near the moon. We therefore compare the ENA emission pattern for spatially homogeneous fields to the emission obtained from a magnetohydrodynamic (MHD) and a hybrid (kinetic ions, fluid electrons) model of Titan's magnetospheric interaction, by computing the trajectories of several billion energetic test particles. While the MHD model takes into account the draping of the magnetic field lines around Titan, the hybrid approach also considers the significant asymmetries in the electromagnetic fields due to the large gyroradii of pick-up ions from Titan's ionosphere. In all three models, the upstream parameters correspond to the conditions during Cassini's TA flyby of Titan. The shape, magnitude, and location of the ENA emission maxima vary considerably between these three field configurations. The magnetic pile-up region at Titan's ramside deflects a large number of the energetic parent ions, thereby reducing the ENA flux. However, the draped magnetic field lines in Titan's lobes rotate the gyration planes of the incident energetic ions, thereby facilitating the observable ENA production. Overall, the ENA flux calculated for the MHD model is weaker than the emission obtained for the electromagnetic fields from the hybrid code. In addition, we systematically investigate the dependency of the ENA emission morphology on the energy of the parent ions and on the upstream magnetic field strength.

Contributions of Mirror and Ion Bernstein Instabilities to the Scattering of Pickup Ions in the Outer Heliosheath

Abstract Maintaining the stability of pickup ions in the outer heliosheath is a critical element for the secondary energetic neutral atom (ENA) mechanism, a theory put forth to explain the nearly annular band of ENA emission observed by the Interstellar Boundary EXplorer. A recent study showed that a pickup ion ring can remain stable to the Alfvén/ion cyclotron (AC) instability at propagation parallel to the background magnetic field when the parallel thermal spread of the ring is comparable to that of a background population. This study investigates the potential role that the mirror or ion Bernstein (IB) instabilities can play in the stability of pickup ions when conditions are such that the AC instability is suppressed. Linear Vlasov theory predicts relatively fast mirror and IB instability growth even though AC instability growth is suppressed. For a few such cases, two-dimensional hybrid and macroscopic quasi-linear simulations are carried out to examine how the unstable mirror and IB modes evolve and affect the pickup ion ring beyond the linear theory picture. For the parameters used, the mirror mode dominates initially and leads to a rapid parallel heating of the pickup ions in excess of the parallel temperature of the background protons. The heated pickup ions subsequently trigger onset of the AC mode, which grows sufficiently large to be the dominant pitch angle scattering agent after the mirror mode has decayed away. The present results indicate that the pickup ion stability needed may not be guaranteed once the mirror and IB instabilities are taken into account.

Effects of Solar Activity on the Local Interstellar Magnetic Field Observed by Voyager 1 and IBEX

Abstract Interstellar Boundary Explorer (IBEX) observations have shown enhanced energetic neutral atom emission from a narrow, circular ribbon likely centered on the direction of the magnetic field in the local interstellar medium (LISM). IBEX observations are compared to Voyager 1's direct observations of the magnetic field in the LISM, which reveals structured changes that are shown here to be driven by pressure fluctuations transmitted across the heliopause boundary separating the LISM from the shocked solar wind beyond the termination shock. The changes in the local interstellar magnetic field observed by Voyager 1 occurred in 2015 and 2016, when the most powerful coronal mass ejections released from the Sun over the last 11-year solar cycle arrived in the outer heliosphere. These events propagated over more than two years through the solar system, ultimately pounding the heliopause and causing transient deviations of the magnetic fields detected by Voyager 1 in the LISM. Thus, we identify the response in pressure variations at Voyager 1 in the LISM with the activity of major solar events several years earlier, during the most active period of the last solar maximum.

Low‐Altitude Emission of Energetic Neutral Atoms: Multiple Interactions and Energy Loss

AbstractLow‐altitude emissions (LAEs) are the energetic neutral atom (ENA) signature of ring current ions precipitating along the magnetic field to an altitude of 200–800 km. This altitude region is considered to be “optically thick” because ring current ions undergo multiple charge changing interactions (MCCIs) with Earth's dense oxygen exosphere. While each interaction involves an energy loss of ~36 eV, no prior study has determined the accumulated energy lost by 1–100 keV H+ emerging as LAEs. We have developed a 2‐D model with a geomagnetic dipole that captures the net effects in energy loss and pitch angle evolution as a result of MCCIs without the computational requirements of a full Monte Carlo simulation. Dependent on the amount of latitudinal migration, the energy loss is greater than 20% for ions below 60 keV for equatorward moving particles (30 keV for poleward). Since the ENA travels ballistically across a geomagnetic dipole, upon reionization, ion velocity along the local field increases (antiparallel in the northern hemisphere). Redirecting the particle upward through MCCIs is most effective during poleward ENA motion. The net effect is to redirect precipitating ions (below 2,500 km) to eventually emerge from the optically thick region either as an ion or ENA. Precipitation is a joint ion‐neutral process, affecting both the energy and pitch angle distribution through the transverse motion of ENA segments in a converging field. For particles that enter the MCCI regime, the energy loss and evolution of the pitch angle distribution must be considered within a realistic magnetic field.

Modeling Emission of Heavy Energetic Neutral Atoms from the Heliosphere

Abstract Observations of energetic neutral atoms (ENAs) are a fruitful tool for remote diagnosis of the plasma in the heliosphere and its vicinity. So far, instruments detecting ENAs from the heliosphere were configured for observations of hydrogen atoms. Here, we estimate emissions of ENAs of the heavy chemical elements helium, oxygen, nitrogen, and neon. A large portion of the heliospheric ENAs is created in the inner heliosheath from neutralized interstellar pick-up ions (PUIs). We modeled this process and calculated full-sky intensities of ENAs for energies 0.2–130 keV/nuc. We found that the largest fluxes among considered species are expected for helium, smaller for oxygen and nitrogen, and smallest for neon. The obtained intensities are 50–106 times smaller than the hydrogen ENA intensities observed by IBEX. The detection of heavy ENAs will be possible if a future ENA detector is equipped with the capability to measure the masses of observed atoms. Because of different reaction cross-sections among the different species, observations of heavy ENAs can allow for a better understanding of global structure of the heliosphere as well as the transport and energization of PUIs in the heliosphere.

The Low-Energy Neutral Imager (LENI).

To achieve breakthroughs in the areas of heliospheric and magnetospheric energetic neutral atom (ENA) imaging, a new class of instruments is required. We present a high angular resolution ENA imager concept aimed at the suprathermal plasma populations with energies between 0.5 and 20 keV. This instrument is intended for understanding the spatial and temporal structure of the heliospheric boundary recently revealed by Interstellar Boundary Explorer instrumentation and the Cassini Ion and Neutral Camera. The instrument is also well suited to characterize magnetospheric ENA emissions from low‐altitude ENA emissions produced by precipitation of magnetospheric ions into the terrestrial upper atmosphere, or from the magnetosheath where solar wind protons are neutralized by charge exchange, or from portions of the ring current region. We present a new technique utilizing ultrathin carbon foils, 2‐D collimation, and a novel electron optical design to produce high angular resolution (≤2°) and high‐sensitivity (≥10−3 cm2 sr/pixel) ENA imaging in the 0.5–20 keV energy range.

THE ENERGY-DEPENDENT POSITION OF THE IBEX RIBBON DUE TO THE SOLAR WIND STRUCTURE

ABSTRACT Observations of energetic neutral atoms (ENAs) allow for remote studies of the condition of plasma in the heliosphere and the neighboring local interstellar medium. The first results from the Interstellar Boundary Explorer (IBEX) revealed an arc-like enhancement of the ENA intensity in the sky, known as the ribbon. The ribbon was not expected from the heliospheric models prior to the launch of IBEX. One proposed explanation for the ribbon is the mechanism of secondary ENA emission. The ribbon reveals energy-dependent structure in the relative intensity along its circumference and in its position. That is, the geometric center of the ribbon varies systematically by about 10° in the energy range 0.7–4.3 keV. Here, we show by analytical modeling that this effect is a consequence of the helio-latitudinal structure of the solar wind reflected in the secondary ENAs. Along with a recently measured distance to the ribbon’s source just beyond the heliopause, our findings support the connection of the ribbon with the local interstellar magnetic field by the mechanism of secondary ENA emission. However, the magnitude of the center shift in the highest IBEX energy channel is much larger in the observations than expected from the modeling. This may be due to another, not currently recognized, process of ENA generation.

THE IBEX RIBBON AND THE PICKUP ION RING STABILITY IN THE OUTER HELIOSHEATH. I. THEORY AND HYBRID SIMULATIONS

ABSTRACT The nearly circular band of energetic neutral atom emission dominating the field of view of the Interplanetary Boundary Explorer (IBEX ) satellite, is most commonly attributed to the effect of charge exchange of secondary pickup ions (PUIs) gyrating about the magnetic field in the outer heliosheath and the interstellar space beyond. Several models for the PUI dynamics of this mechanism have been proposed, each requiring either strong or weak scattering of the initial pitch angle. Conventional wisdom states that ring distributions tend to generate waves and scatter onto a shell on timescales too short for charge exchange to occur. We performed a careful study of ring and thin shell proton distribution stability using theoretical tools and hybrid plasma simulations. We show that the kinetic behavior of a freshly injected proton ring is a far more complicated process than previously thought. In the presence of a warm Maxwellian core, narrower rings could be more stable than broader toroidal distributions. The scattered rings possess a fine structure that can only be revealed using very large numbers of macroparticles in a simulation. It is demonstrated that a “stability gap” in ring temperature exists where the protons could retain large gyrating anisotropies for years, and the wave activity could remain below the level of the ambient magnetic fluctuations in interstellar space. In the directions away from the ribbon, however, a partial shell distribution is more likely to be unstable, leading to significant scattering into one hemisphere in velocity space. The process is accompanied by turbulence production, which is puzzling given the very low level of magnetic fluctuations measured in the outer heliosheath by Voyager 1.

Statistical correlation of low‐altitude ENA emissions with geomagnetic activity from IMAGE/MENA observations

AbstractPlasma sheet particles transported Earthward during times of active magnetospheric convection can interact with exospheric/thermospheric neutrals through charge exchange. The resulting Energetic Neutral Atoms (ENAs) are free to leave the influence of the magnetosphere and can be remotely detected. ENAs associated with low‐altitude (300–800 km) ion precipitation in the high‐latitude atmosphere/ionosphere are termed low‐altitude emissions (LAEs). Remotely observed LAEs are highly nonisotropic in velocity space such that the pitch angle distribution at the time of charge exchange is near 90°. The Geomagnetic Emission Cone of LAEs can be mapped spatially, showing where proton energy is deposited during times of varying geomagnetic activity. In this study we present a statistical look at the correlation between LAE flux (intensity and location) and geomagnetic activity. The LAE data are from the MENA imager on the IMAGE satellite over the declining phase of solar cycle 23 (2000–2005). The SYM‐H, AE, and Kp indices are used to describe geomagnetic activity. The goal of the study is to evaluate properties of LAEs in ENA images and determine if those images can be used to infer properties of ion precipitation. Results indicate a general positive correlation to LAE flux for all three indices, with the SYM‐H showing the greatest sensitivity. The magnetic local time distribution of LAEs is centered about midnight and spreads with increasing activity. The invariant latitude for all indices has a slightly negative correlation. The combined results indicate LAE behavior similar to that of ion precipitation.

LOCAL INTERSTELLAR MAGNETIC FIELD DETERMINED FROM THE INTERSTELLAR BOUNDARY EXPLORER RIBBON

ABSTRACT The solar wind emanating from the Sun interacts with the local interstellar medium (LISM), forming the heliosphere. Hydrogen energetic neutral atoms (ENAs) produced by the solar-interstellar interaction carry important information about plasma properties from the boundaries of the heliosphere, and are currently being measured by NASA's Interstellar Boundary Explorer (IBEX). IBEX observations show the existence of a “ribbon” of intense ENA emission projecting a circle on the celestial sphere that is centered near the local interstellar magnetic field (ISMF) vector. Here we show that the source of the IBEX ribbon as a function of ENA energy outside the heliosphere, uniquely coupled to the draping of the ISMF around the heliopause, can be used to precisely determine the magnitude (2.93 ± 0.08 μG) and direction (227.°28 ± 0.°69, 34.°62 ± 0.°45 in ecliptic longitude and latitude) of the pristine ISMF far (∼1000 AU) from the Sun. We find that the ISMF vector is offset from the ribbon center by ∼8.°3 toward the direction of motion of the heliosphere through the LISM, and their vectors form a plane that is consistent with the direction of deflected interstellar neutral hydrogen, thought to be controlled by the ISMF. Our results yield draped ISMF properties close to that observed by Voyager 1, the only spacecraft to directly measure the ISMF close to the heliosphere, and give predictions of the pristine ISMF that Voyager 1 has yet to sample.

Emission of energetic neutral atoms from water ice under Ganymede surface-like conditions

The co-rotating plasma around Jupiter precipitates on the surfaces of the jovian moons, where it is not hindered by a local magnetic field. Precipitating ions lead to the emission of energetic neutral atoms, which are produced via backscattering and sputtering processes, from the surface. The European Space Agency’s JUICE mission to Jupiter carries as part of the Particle Environment Package experiment an imaging energetic neutral atom spectrometer called the jovian Neutrals Analyzer (JNA). When it is in orbit around Ganymede, JNA will measure the energetic neutral atom flux emitted from the surface of Ganymede in the energy range from 10eV to 3300eV. The surface of Ganymede consists of a large fraction of water ice. To characterize the expected energetic neutral atom fluxes from water ice due to precipitating jovian plasma, we impacted protons and singly charged oxygen ions with energies up to 33keV on a salty water ice target kept at Ganymede surface conditions. Emitted energetic atoms were measured energy- and mass-resolved using the JNA prototype instrument. The data show high yields for energetic neutral atoms per incident ion in the JNA energy range. For incident protons, energetic neutral atom yields between 0.28 at 1keV and ∼40 at 33keV were observed. For incident singly charged oxygen ions, the observed energetic neutral atom yield ranged from 0.8 for at 3keV to ∼170 at 23keV.