Energetic Neutral Atom Imaging Research Articles

The NUADU experiment on TC-2 and the first Energetic Neutral Atom (ENA) images recorded by this instrument

Abstract. The Earth’s ring current and how it responds to varying interplanetary conditions is described and an account provided of the production of Energetic Neutral Atoms (ENAs) in the geo-corona. Also, the potential to remotely monitor, on a global scale, the temporal and spatial evolution of magnetospheric plasma populations through analysing ENA images recorded during magnetic storms/substorms is indicated. A technical account of the Energetic NeUtral Atom Detector Unit NUADU aboard China’s TC-2 mission (measurement range 45–>158 keV) follows, together with an account of the scientific objectives of NUADU, both in stand-alone mode and in the context of multi-point imaging. Low altitude ENA emissions recorded by NUADU during south polar passages of TC-2 at the time of a moderate magnetic storm in September 2004, as well as bright ring current emissions recorded in November 2004 during a major geomagnetic storm, are presented and discussed in the context of various, accompanying, terrestrial disturbances. Also, ENA observations of the November 2004 ring current imaged simultaneously by TC-2/NUADU and by IMAGE/ HENA (viewing, respectively, from the Northern and Southern Hemispheres), are compared.

Energetic neutral atom imaging mass spectroscopy of the Moon and Mercury environments

Low-energy neutral atom (LENA) imaging is an important technique for doing planetary sciences at magnetized and unmagnetized planets. In the case of the Moon, the precipitating solar-wind causes sputtering, which releases surface atoms as LENAs into space. Moreover, the solar-wind ions may be back-scattered from the surface into space as neutral atoms. At Mercury, in addition to the above processes, LENAs are also generated by the charge-exchange of energetic ions with the exospheric gasses. Global LENA mass spectroscopic imagery at the Moon and at Mercury provides us information on their surfaces and the interaction processes between energetic particles and the surfaces via remote-sensing using LENAs. We are developing a state-of-the-art LENA instrument for the Indian lunar exploration mission Chandrayaan-1 and the Mercury exploration mission BepiColombo. The instrument is light-weight and capable of mass discrimination, including heavy components such as iron, and has high sensitivity to fulfill various scientific objectives in the area of planetary sciences.

Planetary ENA imaging: Effects of different interaction models for Mars

We present and compare energetic neutral atom (ENA) images that are calculated from plasma parameters given by three different simulation models of the interaction between the solar wind and Mars. The images are calculated by combining a model for the ion flow with a model of the neutral atmosphere using the cross-sections for the charge exchange collisions. The three ion models are: an empirical model that is based on Phobos 2 measurements; a three-dimensional hybrid simulation; and a three-dimensional MHD simulation. For the empirical and MHD models the images are obtained by integration of the ENA emission along lines of sight to a virtual ENA instrument. In the case of the hybrid model images are obtained by summing the contributions from all ions, whose positions, velocities, and weights are saved in files at regular intervals. Differences between the models can be detected in the images, for example the hybrid model produces ENA emissions from a larger region than the MHD model does. An asymmetry in the oxygen ion density develops in the hybrid model and can be seen in the oxygen ENA images. The images are influenced by finite gyro radius effects, which are included in the hybrid model but not in the other two. The total production rates of hydrogen ENAs are 2.7 × 10 25 s - 1 , 5.8 × 10 25 s - 1 , and 2.4 × 10 25 s - 1 for the empirical, hybrid and MHD models respectively. This study shows the importance of considering both the type of simulation model used and the proper inclusion of relevant physical phenomena and boundary conditions, when modelling the interaction between planets and the solar wind. Although the different models agree fairly well in terms of macroscopic plasma parameters they produce ENA images that differ substantially.

Cassini UVIS observations of Europa's oxygen atmosphere and torus

Observations of the Europa environment using the Cassini UltraViolet Imaging Spectrograph (UVIS) show the presence of an extended atomic oxygen atmosphere in addition to the bound molecular oxygen atmosphere first detected by Hubble Space Telescope in 1994 [D.T. Hall, D.F. Strobel, P.D. Feldman, M.A. McGrath, H.A. Weaver, 1995, Detection of an oxygen atmosphere on Jupiter's moon Europa, Nature 373, 677–679]. The atomic oxygen measurement provides a direct constraint on the sputtering and loss of Europa's water ice surface and the interaction of Europa's atmosphere with Jupiter's magnetosphere. We derive a loss rate for O 2 based on the emission rate of the OI 1356 Å multiplet. UVIS detected substantial variability in the oxygen emission from Europa's oxygen atmosphere that we attribute to the viewing geometry. B.H. Mauk, D.G. Mitchell, S.M. Krimigis, E.C. Roelof, C.P. Paranicas [2003, Energetic neutral atoms from a trans-Europa gas torus at Jupiter, Nature 421, 920–922] inferred the presence of a torus of neutral gas at Europa's orbit based on Cassini's energetic neutral atom (ENA) image of the Jupiter system acquired with the Magnetospheric Imaging Instrument (MIMI), with the most likely torus constituents being hydrogen and oxygen species sputtered from Europa. Cassini UVIS data rule out O and O 2 as the possible torus species reported by Mauk et al. however, unless the torus density is so low that it is undetectable by UVIS (less than 8 atoms / cm 3). The UVIS observations indicate the presence of atomic hydrogen and possibly other species, but a full analysis is deferred to a following paper. The hydrogen in the present observations shows a local-time asymmetry and complex spatial distribution.

Dynamics of Saturn's Magnetosphere from MIMI During Cassini's Orbital Insertion

The Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft observed the saturnian magnetosphere from January 2004 until Saturn orbit insertion (SOI) on 1 July 2004. The MIMI sensors observed frequent energetic particle activity in interplanetary space for several months before SOI. When the imaging sensor was switched to its energetic neutral atom (ENA) operating mode on 20 February 2004, at approximately 10(3) times Saturn's radius RS (0.43 astronomical units), a weak but persistent signal was observed from the magnetosphere. About 10 days before SOI, the magnetosphere exhibited a day-night asymmetry that varied with an approximately 11-hour periodicity. Once Cassini entered the magnetosphere, in situ measurements showed high concentrations of H+, H2+, O+, OH+, and H2O+ and low concentrations of N+. The radial dependence of ion intensity profiles implies neutral gas densities sufficient to produce high loss rates of trapped ions from the middle and inner magnetosphere. ENA imaging has revealed a radiation belt that resides inward of the D ring and is probably the result of double charge exchange between the main radiation belt and the upper layers of Saturn's exosphere.

Proton temperatures in the ring current from ENA images and in situ measurements

[1] Ion equatorial flux distributions, inverted from energetic neutral atom (ENA) images in different energy channels obtained by the HENA imager on the IMAGE satellite, are used to deduce the plasma ion temperature in the ring current. During the early period of the recovery phase of the strong storm on 12 August 2000, the deduced ion temperatures of the ring current are consistent with the values inferred from in situ LANL MPA measurements. The ion temperatures show a peak in the post-midnight/pre-dawn sector coincident with the peak in the equatorial ion distributions. The temperature, however, is more symmetrical than the ion flux implying that, at least in this part of the energy spectrum, the density variations make the major contribution to pressure gradients.

Planetary ENA Imaging: Venus and a comparison with Mars

We present simulated images of energetic neutral atoms (ENAs) produced in charge exchange collisions between solar wind protons and neutral atoms in the exosphere of Venus, and make a comparison with earlier results for Mars. The images are found to be dominated by two local maxima. One produced by charge exchange collisions in the solar wind, upstream of the bow shock, and the other close to the dayside ionopause. The simulated ENA fluxes at Venus are lower than those obtained in similar simulations of ENA images at Mars at solar minimum conditions, and close to the fluxes at Mars at solar maximum. Our numerical study shows that the ENA flux decreases with an increasing ionopause altitude. The influence of the Venus nighttime hydrogen bulge on the ENA emission is small.

Cassini/Huygens flyby of the Jovian system

This is an introduction to the series of 15 papers that follow. These report scientific results from the fields‐and‐particles instruments on Cassini/Huygens. For these works, their data acquisition started as early as 16 months before the passage by Jupiter and, for some, continued up to the approach to Saturn. The flyby of Jupiter provided a gravitational assist, which was necessary for the spacecraft to reach its destination, the Saturnian system. In addition to the scientific results, the Cassini/Huygens team gained much because Jupiter provided a dress rehearsal for skills needed at Saturn. The flyby also fostered international cooperation between the Cassini/Huygens teams, the Galileo teams, and the scientific teams using instruments in orbit about the Earth and on the ground. A number of important discoveries and insights came from these cooperative efforts. The boundary physics and compression of the Jovian magnetosphere was observed by both fields and particle instruments. The propagation of the solar wind between Earth and Jupiter was studied. The source of Jovian radio emission was further characterized. The first images of energetic neutral atoms at Jupiter were obtained.

First comparisons of local ion measurements in the inner magnetosphere with energetic neutral atom magnetospheric image inversions: Cluster‐CIS and IMAGE‐HENA observations

Data provided by the CIS (Cluster Ion Spectrometry) instruments on board the Cluster spacecraft are used to survey recent crossings of the inner magnetosphere and ring current. CIS is capable of obtaining full three‐dimensional ion distributions (about 0 to 40 keV/q) with one spacecraft spin time resolution and with mass‐per‐charge composition determination. Events are selected for which the Cluster spacecraft are within the field of view of the HENA (high‐energy neutral atom) imager on board IMAGE. HENA provides energetic neutral atom images with a high geometric factor and with a 120° × 360° field of view over the spin. The H+ ion distribution functions obtained in situ by CIS are then compared to the ones deduced by inverting the HENA hydrogen neutral atom images for the overlapping energy range of the two instruments (27–39 keV). This analysis concerns events obtained both during well‐developed ring current conditions (e.g., 18 April 2002 event) and during quiet magnetospheric conditions (e.g., 9 August 2001 event). The results show the consistency between the ion fluxes deduced from energetic neutral atom (ENA) image inversions and the fluxes measured locally. They thus show the complementarity of the two approaches. The locally measured fluxes provide the “ground truth,” and they give the detailed ion distributions. ENA images allow to situate local measurements into a global context and to position them with respect to the ring current large‐scale structure. Our results also show the limitations of the ion fluxes deduced from the ENA image inversions for images taken from a single vantage point, with a substantial scatter of the inversion fluxes with respect to the in situ measured ones and a more limited dynamic range.

Energetic neutral atom imaging at Mercury

The paper reviews recent developments in the simulation of energetic neutral atom (ENA) imaging near Mercury. ENAs are produced in a wide energy range, from a few eV and up to 50 keV by: (1) charge-exchange of ions injected from the tail to the inner magnetosphere; (2) charge-exchange of freshly ionized exospheric ions accelerated by the magnetospheric electric field; (3) sputtering from the planetary surface by precipitating energetic ions. Using a scaled Tsyganenko 89 magnetic field model and assuming a convection electric field of 10 mV/m, we follow the trajectories of ions injected from the tail and Na + photoions from the exosphere to obtain their global distribution in the magnetosphere and precipitation maps. That allows us to estimate the ENA fluxes in each energy range and understand the general morphology of ENA images. The high variability of the Hermean magnetosphere gives rise to pulsating ENA emissions (ENA “flashes”), that can be used to study the global dynamics of the magnetosphere. Sputtered ENAs display the particle fluxes impinging on the surface in a very similar way as Earth's upper atmosphere visualizes precipitating electrons displaying auroras. One can thus speak in terms of an “ENA aurora” on Mercury. Sputtering results in the production of very low energy neutral atoms (VLENA) with large masses (K, Na, Ca) and energy less than 10 eV. The VLENA trajectories experience gravitation bending making it possible to image regions behind the limb of the planet. We conclude the review with a summary of the Mercury ENA environment characteristics and implications for ENA instrumentation.

Energetic neutral atom imaging at Mercury

The paper reviews recent developments in the simulation of energetic neutral atom (ENA) imaging near Mercury. ENAs are produced in a wide energy range, from a few eV and up to 50 keV by: (1) charge-exchange of ions injected from the tail to the inner magnetosphere; (2) charge-exchange of freshly ionized exospheric ions accelerated by the magnetospheric electric field; (3) sputtering from the planetary surface by precipitating energetic ions. Using a scaled Tsyganenko 89 magnetic field model and assuming a convection electric field of 10 mV/m, we follow the trajectories of ions injected from the tail and Na + photoions from the exosphere to obtain their global distribution in the magnetosphere and precipitation maps. That allows us to estimate the ENA fluxes in each energy range and understand the general morphology of ENA images. The high variability of the Hermean magnetosphere gives rise to pulsating ENA emissions (ENA “flashes”), that can be used to study the global dynamics of the magnetosphere. Sputtered ENAs display the particle fluxes impinging on the surface in a very similar way as Earth's upper atmosphere visualizes precipitating electrons displaying auroras. One can thus speak in terms of an “ENA aurora” on Mercury. Sputtering results in the production of very low energy neutral atoms (VLENA) with large masses (K, Na, Ca) and energy less than 10 eV. The VLENA trajectories experience gravitation bending making it possible to image regions behind the limb of the planet. We conclude the review with a summary of the Mercury ENA environment characteristics and implications for ENA instrumentation.

Neutral atom imaging of solar wind interaction with the Earth and Venus

Observations from the Low‐Energy Neutral Atom (LENA) imager on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) mission have emerged as a promising new tool for studying the solar wind interaction with the terrestrial magnetosphere. Strong LENA emissions are seen from the magnetosheath during magnetic storms, especially during high solar wind dynamic pressure when the magnetopause is strongly compressed and the magnetosheath penetrates deeply into the exosphere. Venus, unlike the Earth, has no intrinsic magnetic field, so the solar wind penetrates deeply and interacts directly with its upper atmosphere. Energy transfer processes enhance atomic escape and thus play a potentially important role in the evolution of the atmosphere. We have performed simulations of the solar wind interaction with both Earth and Venus and compared the results to LENA observations at the Earth. Low‐energy neutral atom emissions from Venus are calculated based on the global MHD model of Tanaka. We found the simulated energetic neutral atom (ENA) emissions from Venus magnetosheath are comparable or greater than for the Earth. The Venus ionopause is clearly seen in the modeled oxygen ENA images. This simulation work demonstrates the feasibility of remotely sensing the Venusian solar wind interaction and resultant atmospheric escape using fast neutral atom imaging.

IMAGE/HENA: pressure and current distributions during the 1 October 2002 storm

Global energetic neutral atom images (ENA) of the Earth’s ring current can be used to derive the global ion and pressure distribution by applying a constrained linear inversion technique. The ENA images are obtained in the 10–60 keV range by the high energy neutral atom (HENA) onboard the IMAGE spacecraft. We present ion and pressure distributions from the 1 October 2002 storm main phase. The typical storm main phase displays a pressure peak around midnight at L=3−4. We use this pressure distribution to compute the three-dimensional electrical current system. The computed current system is then compared to the region 2 current system obtained by the magnetometers on board the Indium spacecraft.

The solar wind interaction with Venus and Mars: energetic neutral atom and X-ray imaging

At the non-magnetized planets Venus and Mars, the solar wind comes in direct contact with the planetary neutrals. This interaction produces energetic neutral atoms (ENAs) and X-rays. In this paper we review some of the different production mechanisms and present examples of simulated ENA and X-ray images. To extract as much information as possible about the ion and neutral distributions in the observed region one has to further process the images. We discuss different approaches to model the production of ENAs and X-rays, and then how to solve the resulting mathematical inverse problems.

Neutral hydrogen density profiles derived from geocoronal imaging

Measurements of the Lyman α column brightness by the Geocoronal Imager (GEO), part of the FUV imaging system on board the IMAGE satellite, have been used to derive an empirical model of the neutral hydrogen density distribution at high altitudes (>3.5 RE geocentric distance) on the night‐side of the Earth. The model presented is an effort to provide the density profiles needed to analyze the energetic neutral atom imaging data at ring current altitudes and above. The variable solar Lyman α flux is obtained from the UARS/SOLSTICE measurements and the scattered solar Lyman α emissions from interplanetary hydrogen are obtained from a model. Assuming that the exosphere at high altitudes (>3.5 RE geocentric distance) can be considered as an optical thin medium and that the hydrogen density profile can be expressed as a double exponential we show that the Lyman α column brightness can be converted to hydrogen density profiles. The hydrogen density above 5 RE is found to be slightly higher for large solar zenith angles than for 90° solar zenith angle. The hydrogen density shows temporal variations which are not controlled by any solar quantity or geomagnetic parameter alone. Our Lyman α profiles and derived hydrogen density profiles are close to what was observed by Dynamics Explorer 1 [Rairden et al., 1986]. Above 8 RE we find higher densities than they did for all solar zenith angles >90°. We do not find any evidence of depletion due to charge exchange with solar wind protons outside the magnetopause. Our results are only valid above 3.5 RE.

Tail‐dominated storm main phase: 31 March 2001

On 31 March 2001 a fast solar wind transient with strong southward interplanetary magnetic field Bz produced a large geomagnetic storm at Earth, with a drop in the Dst index to −350 nT between 0400 and 0900 UT. The Earth's magnetosphere was very compressed during this interval, with the bow shock crossing geosynchronous orbit on at least two occasions. Here we present space‐based and ground‐based observations demonstrating that tail currents, rather than ring currents, were the dominant contributor to the Dst index during the main phase of this storm. The plasma sheet during this interval was exceptionally dense and penetrated very deeply towards the Earth, leading to extremely strong tail currents flowing quite close to the Earth. These tail currents produced a very distorted magnetosphere, with strong stretching of the magnetic field lines in the nightside plasma sheet. Energetic neutral atom (ENA) images from the MENA and HENA instruments on IMAGE show a very narrow spatial distribution, with ENAs confined to the nightside until a magnetic field dipolarization at ∼0630 UT when Dst was −250 nT. Ground magnetometer measurements confirm that the disturbance was localized on the nightside and dominated by tail currents up until the field dipolarization. Following the dipolarization, higher energy ENAs began to drift toward dusk, forming a partial ring current. Even at that time, low‐energy ENAs were not observed on the dayside, either due to inhibited access or to strong charge exchange losses.

Energetic neutral atoms from a trans-Europa gas torus at Jupiter.

The space environments--or magnetospheres--of magnetized planets emit copious quantities of energetic neutral atoms (ENAs) at energies between tens of electron volts to hundreds of kiloelectron volts (keV). These energetic atoms result from charge exchange between magnetically trapped energetic ions and cold neutral atoms, and they carry significant amounts of energy and mass from the magnetospheres. Imaging their distribution allows us to investigate the structure of planetary magnetospheres. Here we report the analysis of 50-80 keV ENA images of Jupiter's magnetosphere, where two distinct emission regions dominate: the upper atmosphere of Jupiter itself, and a torus of emission residing just outside the orbit of Jupiter's satellite Europa. The trans-Europa component shows that, unexpectedly, Europa generates a gas cloud comparable in gas content to that associated with the volcanic moon Io. The quantity of gas found indicates that Europa has a much greater impact than hitherto believed on the structure of, and the energy flow within, Jupiter's magnetosphere.

The Role and Contributions of Energetic Neutral Atom (ENA) Imaging in Magnetospheric Substorm Research

Energetic Neutral Atom (ENA) imaging has contributed substantially to substorm research. This technique has allowed significant advances in areas such as observation and quantification of injected particle drift as a function of energy, observation of dynamics in the tail that are directly related to the effects of imposed (growth phase) and induced (expansion phase) electric fields on the plasma, the prompt extraction of oxygen from the ionosphere during substorms, the relationship between storms and substorms, and the timing of substorm ENA signatures. We present discussion of the advantages and shortcomings of the ENA technique for studying space plasmas. Although the technique is in its infancy, it is yielding results that enrich our understanding of the substorm process and its effects.

Global imaging of O+from IMAGE/HENA

The magnetospheric O+ population in the 52–180 keV range during storms is investigated through the analysis of energetic neutral atom (ENA) images. The images are obtained from the high energy neutral atom (HENA) imager onboard the IMAGE satellite. At each substorm onset following the commencement of a geomagnetic storm the oxygen ENA display ~30 min intense bursts. Only very weak corresponding features in the 60–119 keV hydrogen ENA can be occasionally seen. The dominating fraction of the oxygen ENA emissions are produced when O+ ions mirror/precipitate at low altitudes, where the number density of the neutral atmosphere is high. During the storm we observed several bursts of oxygen ENA, but it is still not clear how much the O+ content of the ring current increases during the storm main phase. Our observations suggest that the responsible injection mechanism is mass-dependent and scatters the pitch angles. This leads us to favor a non-adiabatic mechanism proposed by (Delcourt, 2002).

Telescopic and Microscopic views of the magnetosphere: Multispacecraft observations

The magnetospheric research community has long sought the capability to view the Sun-Earth system in a global way and to probe concurrently the microphysical details of key physical regions. This objective has now been substantially realized with the combination of the CLUSTER and IMAGE missions. With the additional use of SOHO, ACE, FAST, SAMPEX, POLAR, and geostationary orbit spacecraft, there is a remarkable ability to apply both telescopic and microscopic principles. As an example, a bright active region on the Sun gave rise on 29 March 2001 to a fast halo coronal mass ejection (CME) event observed by SOHO instruments. Subsequently on 31 March, a strong interplanetary shock wave ahead of a magnetic cloud (probably arising from the earlier CME) passed the ACE spacecraft and hit the Earth’s magnetosphere. This driver compressed the subsolar magnetopause to ≤ 4 RE geocentric distance and initiated a powerful geomagnetic storm (minimum Dst ~ −360 nT). The CLUSTER set of four spacecraft were located in the midnight sector of the magnetosphere near perigee (r~4 RE) at ~0635 UT and observed a dispersionless injection of energetic (E ≥ 20 keV) electrons in association with a large magnetospheric substorm expansion phase onset (AE ~1200 nT). Concurrent to these in situ observations, the IMAGE spacecraft was returning a sequence of global Energetic Neutral Atom (ENA) images from the medium-energy (MENA) and high-energy (HENA) sensor systems. These data showed a very prominent injection of substorm ions in the premidnight (and postdusk) sector of the inner magnetosphere. This event is consistent with a substorm onset that pushed the substorm ‘injection boundary’ far inside of geostationary orbit and far toward the dusk sector. In another event on August 27, 2001, the IMAGE-CLUSTER combination provided evidence that magnetic reconnection began in the mid-tail plasma sheet some 7 min prior to auroral onset and brightening. Alternative interpretations may also be possible even with the multis-pacecraft data available. Notwithstanding, these data gave an unprecedented view both telescopically and microscopically of a magnetospheric substorm onset and help establish key process timing in the magnetosphere. Such available events reveal the power of multispacecraft observations.