ENA Images Research Articles

Interstellar Boundary Explorer (IBEX) may have recorded a cross-tail current disruption event of the substorm

The spatial and temporal variations of satellite measurements correspond to different physical connotations. ENA imaging is an effective method to avoid the confusion of spatial and temporal variations of plasma space distribution in remote sensing region. However, for the neutral atom imager with scanning sampling, if the sampling time is too long (such as exceeding the evolution period of related physical events), it is still necessary to carefully analyze the time change factors of the space environment during the sampling period. Our re-certification of ENA scanning images of the IBEX-Hi’s “terrestrial plasma sheet disconnection” revealed that a magnetospheric substorm occurred during sampling, most likely due to the pitch angle diffusion of energetic ions in the ring current to create the so-called “plasma sheet disconnection” illusion. The observation of ENA imaging reflects the motion pattern of its parent ions, which have a certain distance from space plasma visualization. The dynamic evolution of ring current energetic ion diffusion has inspired us to create a new macroscopic model of substorms that can be visually monitored in the ecliptic plane using ENA imaging.

Global 3‐D Distributions of O+ and H+ Ions in the Inner Magnetosphere Reconstructed by Voxel Tomography From TWINS ENA Images During a Large Magnetic Storm

AbstractUsing improved voxel tomography method for energetic neutral atom (ENA) emission, global 3‐D distributions of O+ and H+ ions in the inner magnetosphere are reconstructed from ENA images remotely sensed by the Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission during 15 July 2012 magnetic storm. The compositions of ENA‐O and ‐H are separated by statistical fitting based on start‐pulse‐height distributions. The errors in iterative inversions range from 0.12 to 0.28 at 16 and 32 keV. The reconstructed ion flux intensities at different storm phases match well with independent satellite in situ measurements. It is found that: (a) Responding to a pulse jump of solar wind dynamic pressure along with a northward reversal of IMF Bz during the initial phase, H+ ion fluxes enhanced greatly on outer dipole L‐shell of 6.5–8.0 in pre‐midnight off equator. Ts‐05 model tracing and pitch angles distribution of line‐of‐sights for ENA imaging suggest these be likely attributed to nearly field‐aligned earthward plasma flow from plasma‐sheet boundary layer region. Meanwhile, the trapped ring current (RC) ion fluxes peak at L ∼ 5 in the pre‐midnight equatorial region, decreasing with increasing latitude. (b) During the storm maximum the equatorial O+ fluxes have two separated peaks. The main one appeared in dusk at L ∼ 4, while the minor one in the pre‐dawn appeared at very low L ∼ 2.75 where the flux ratio O+/(O++H+) reached 60%. (c) The O+ flux fractions over equator exhibit prominent dusk‐dawn asymmetry, reversing from dusk quadrant prevailing before the main phase to dawn ascendant during the storm maximum.

Simulation of ENA Imaging Measurements on a Geosynchronous Orbit

Simulation of ENA Imaging Measurements on a Geosynchronous Orbit

Моделирование визуализации мониторинга энергичных нейтральных атомов геомагнитосферы на лунной базе

Since the moon’s revolution cycle is exactly the same as its rotation cycle, we can only see the moon always facing Earth in the same direction. Based on the clean particle radiation environment of the moon, a neutral atomic telemetry base station could be established on the lunar surface facing Earth to realize long-term continuous geomagnetic activity monitoring. Using the 20°×20° field of view, the 0.5°×0.5° angle resolution, and the ~0.17 cm²sr geometric factor, a two-dimensional ENA imager is being designed. The magnetospheric ring current simulation at a 4–20 keV energy channel for a medium geomagnetic storm (Kp=5) shows the following: 1) at ~60 Rᴇ (Rᴇ is the Earth radius), the imager can collect 10⁴ ENA events for 3 min to meet the statistical requirements for 2D coded imaging data inversion, so as to meet requirements for the analysis of the substorm ring current evolution process of magnetic storms above medium; 2) the ENA radiation loss puzzles in the magnetopause and magnetotail plasma sheet regions have been deduced and revealed using the 2-D ENA emission model. High spatial-temporal resolution ENA imaging monitoring of these two important regions will provide the measurement basis for the solar wind energy input process and generation mechanism; 3) the average sampling interval of ENA particle events is about 16 ms at the moon’s orbit; the spectral time difference for the set energy range is on the order of minutes, which can provide location information to track the trigger of geomagnetic storm particle events.

Energetic Neutral Atoms From Jupiter's Polar Regions

AbstractEnergetic Neutral Atom (ENA) cameras on orbiting spacecraft at Earth and Saturn helped greatly to diagnose these complex magnetospheres. Within this decade, the European Space Agency's Jupiter Icy Moons Explorer will make ENA imaging a major thrust in understanding Jupiter's complex magnetosphere. The present polar‐orbiting Juno mission at Jupiter carries no ENA camera. But, the Jupiter Energetic‐particle Detector Instrument is sensitive to >50 keV ENAs, provided there are no local charged particles to mask their presence. Juno offers great service to interpreting past serendipitous and future dedicated ENA imaging with its orbit providing unique viewing perspectives. Here we report Juno observations of ENAs from Jupiter's polar regions. These ENAs likely arise from energetic ions that nearly precipitate in the main auroral regions and mirror magnetically within, and charge exchange with, Jupiter's upper atmosphere. Jupiter proves itself different from Saturn, as ENAs generated from precipitating ions were not identified there.

Dynamics of a geomagnetic storm on 7–10 September 2015 as observed by TWINS and simulated by CIMI

Abstract. For the first time, direct comparisons of the equatorial ion partial pressure and pitch angle anisotropy observed by TWINS and simulated by CIMI are presented. The TWINS ENA images are from a 4-day period, 7–10 September 2015. The simulations use both the empirical Weimer 2K and the self-consistent RCM electric potentials. There are two moderate storms in succession during this period. In most cases, we find that the general features of the ring current in the inner magnetosphere obtained from the observations and the simulations are similar. Nevertheless, we do also see consistent contrasts between the simulations and observations. The simulated partial pressure peaks are often inside the observed peaks and more toward dusk than the measured values. There are also cases in which the measured equatorial ion partial pressure shows multiple peaks that are not seen in the simulations. This occurs during a period of intense AE index. The CIMI simulations consistently show regions of parallel anisotropy spanning the night side between approximately 6 and 8 RE, whereas the parallel anisotropy is seen in the observations only during the main phase of the first storm. The evidence from the unique global view provided by the TWINS observations strongly suggests that there are features in the ring current partial pressure distributions that can be best explained by enhanced electric shielding and/or spatially localized, short-duration injections.

CRCM + BATS‒R‒US two‒way coupling

We present the coupling methodology and validation of a fully coupled inner and global magnetosphere code using the infrastructure provided by the Space Weather Modeling Framework (SWMF). In this model, the Comprehensive Ring Current Model (CRCM) represents the inner magnetosphere, while the Block‒Adaptive‒Tree Solar‒Wind Roe‒Type Upwind Scheme (BATS‒R‒US) represents the global magnetosphere. The combined model is a global magnetospheric code with a realistic ring current and consistent electric and magnetic fields. The computational performance of the model was improved to surpass real‒time execution by the use of the Message Passing Interface (MPI) to parallelize the CRCM. Initial simulations under steady driving found that the coupled model resulted in a higher pressure in the inner magnetosphere and an inflated closed field‒line region as compared to simulations without inner‒magnetosphere coupling. Our validation effort was split into two studies. The first study examined the ability of the model to reproduce Dst for a range of events from the Geospace Environment Modeling (GEM) Dst Challenge. It also investigated the possibility of a baseline shift and compared two approaches to calculating Dst from the model. We found that the model did a reasonable job predicting Dst and Sym‒H according to our two metrics of prediction efficiency and predicted yield. The second study focused on the specific case of the 22 July 2009 moderate geomagnetic storm. In this study, we directly compare model predictions and observations for Dst, THEMIS energy spectragrams, TWINS ENA images, and GOES 11 and 12 magnetometer data. The model did an adequate job reproducing trends in the data. Moreover, we found that composition can have a large effect on the result.

Energetic Neutral Atom Imager on the Swedish Microsatellite Astrid

The Swedish microsatellite ASTRID was launched by a Russian Cosmos rocket on January 24, 1995 into a 1000 km circular orbit with 83 deg inclination. Besides the main objective of technological demonstration, imaging of energetic neutral atoms (ENAS) was attempted. The imager detected ENA in the energy range 0.1 - 140 keV utilizing two different techniques. Neutrals of the energy 13 - 140 keV were recorded by 14 solid state detectors with the total field of view 5 deg x 322 deg. For half a spin (approx. 1.5 s) of the ASTRID spacecraft, almost all of space was covered with an angular resolution 2.5 deg x 25 deg. Less energetic neutrals of approx. 0.1 - 70 keV were converted on a graphite target into secondary particles which then were detected by a microchannel plate with 32 anodes. A fraction of primary neutrals was directly reflected towards the sensor. This technique provided the total ENA flux with an angular resolution 4.6 deg x 11.5 deg. The instrument weight is 3.13 kg. Successful operation of the instrument during the first 5 weeks of the mission provided the first ENA images of the ring current at low altitudes.

Evolution of CIR storm on 22 July 2009

Global images of ion intensities are deconvolved from TWINS ENA images during the main and recovery phase of a CIR storm on 22 July 2009. The global spatial ion images taken at different times, along with solar wind data, geomagnetic activity indices, geosynchronous orbit observations, and in situ measurements from the THEMIS mission provide a picture of the evolution of the ring current during both the main phase and early recovery phase of the storm. Major features of the evolution are consistent with expectations based upon time dependent geomagnetic indices, e.g., SYM/H and ASY/H, and geosynchronous orbit detection of dipolarizations from GOES 11 and 12. Direct comparisons are made with in situ THEMIS ESA and SST spectral measurements. The peak energy of the ion spectrum in the ring current is seen to decrease in the recovery phase. The time evolution of the ion energy spectra over the range from 2.5 to 97.5 keV at the spatial peaks of the ring current in the inner magnetosphere obtained from ENA images is presented for the first time.

Evolvement of the Ring Current During a Magnetic Storm: TC‐2 Neutral Atom Imager Observations

AbstractA series of ENA images were obtained by the NUADU instrument aboard TC‐2 during a strong, long duration magnetic storm that occurred in November 2004. This paper follows the evolution of the ring current during this magnetic storm. It is shown that the southward IMF Bz component plays a key role in facilitating ion injection from the magnetotail into the inner magnetosphere with consequent enhancement of the ring current. Ion injection stopped soon after the IMF turned from southward to northward. Also during ion injection episodes, the ion drift paths opened and ring current ions were lost at the noon to dusk magnetopause soon after they were injected at the dusk side region. As a result, the ring current intensified after the occurrence of an open‐to‐closed transformation of the ion drift paths, rather than at the stage when ion injection was strong. In the main phase of the storm, ion injection took place between local times about 17:00~22:00, leading to an extremely asymmetric ring current. In contrast in the recovery phase of the storm, the ion injection region reached the post‐midnight sector. We attributed this phenomenon to a significant positive IMF By component. A symmetrical ring current was formed after the drift paths changed back to a closed configuration due to a decrease in tail convection. When the storm entered its late recovery phase, the ring current faded away while the Dst still indicated a strong magnetic disturbance. This suggested an important contribution to Dst from the cross tail current.

Statistical analysis of the energetic ion and ENA data for the Titan environment

The MIMI experiment (Magnetosphere Imaging Instrument) onboard Cassini is dedicated to the study of energetic particles, with in particular LEMMS analyzing charged particles, or the INCA detector which can image the Energetic Neutral Atoms produced by charge exchange collisions between cold neutrals and energetic ions. The MIMI experiment is thus well adapted to the study of the interaction between the Titan nitrogen rich atmosphere and the energetic Saturnian magnetospheric plasma. We analyze here the energetic protons at the Titan orbit crossings before January 2008 (MIMI-LEMMS data; 27–255 keV), which are very dynamic, with tri-modal flux spectra and probably quasi-isotropic pitch angle distributions. We provide statistical parameters for the proton fluxes, leading to estimates of the average energy deposition into Titan's atmosphere, before we discuss the possible influence of Titan on the magnetopause. We then analyze the H ENA images (24–55 keV) during the Titan flybys before June 2006 to obtain a better diagnostic of the Titan interaction: the ENAs variability is mostly related to the magnetospheric variability (the exosphere being roughly stable) or the distance from the moon, the ENAs halo around Titan is very stable (corresponding to a lower limit for ENAs emission at the exobase), and strong asymmetries are observed, due to finite gyroradii effects for the parent ions.

Energetic, ∼5–90 keV neutral atom imaging of a weak substorm with STEREO/STE

We present imaging and high resolution energy spectra of energetic ∼5–90 keV neutral atoms (ENA) of a weak geomagnetic substorm (Dst > −8 nT and AE ≲ 200 nT), made by the Suprathermal Electron (STE) instrument on the STEREO B spacecraft. Enhanced ENA emissions were observed coming from around local midnight near the equator with different spatial distribution and/or temporal behavior at ∼5–20keV compared to ∼20–90 keV. By forward modeling using a parameterized ring‐current model, we show that the ENA images imply the parent equatorial protons have pitch‐angle distributions peaked at 90°, an energy spectrum consistent with in situ proton measurements at geosynchronous orbit, and a spatial asymmetry with the maximum flux at midnight for 5–20 keV and at 2240 MLT for 20–90 keV. These are the first ENA measurements at ∼5 to 26 keV from low altitude, and the first for such weak activity.

Direct observation of warping in the plasma sheet of Saturn

The ENA images from the Ion Neutral CAmera (INCA) on the Cassini spacecraft are projected onto the noon‐midnight plane of Sun‐Saturn orbital coordinates, and a composite “image” of Saturn's plasma sheet is constructed from dawn‐side observations of 20–50 keV hydrogens obtained from days 352 to 361 in 2004. The maxima in the intensity contours define the center of the plasma sheet in the noon‐midnight plane. This plasma sheet surface displays a distinct bending or “warping” above Saturn's equatorial plane at radial distances of beyond ∼15 RS on the nightside. On the dayside, the plasma sheet lies close to the equator all the way to the magnetopause. The observed warping agrees with the “bowl” model derived from measurements of Saturn's magnetic field, but fits more closely a simple third‐order polynomial.

Periodic intensity variations in global ENA images of Saturn

During select time periods in orbit about Saturn, Cassini's Ion and Neutral Camera (INCA) has detected a periodic global ENA signal. Images taken during such periods reveal the presence of a rotating source component in addition to a steady‐state component; both components have peak brightness in the midnight sector, when viewed from near dawn. We present a model that synthesizes some of the basic features of the data and gives good agreement with the integrated signal. Both components, as well as the Compton‐Getting effect whose importance was previously unrecognized in ENA imaging, are needed to explain some aspects of the data.

Dynamics of ring current ions as obtained from IMAGE HENA and MENA ENA images

Ion distributions from three storms in the year 2000 have been obtained from HENA and MENA ENA images at 27–39 keV and 5–12 keV, respectively. The observed motion of the ring current is compared with calculations of expected radial and angular drift speeds made using a combination of the Tsyganenko 89 magnetic field model and the Weimer 2001 electric field model. In one case between 1000 and 1100 UT on 10 June 2000 that had already been reported, the westward drift of the 33 keV ions is as expected. Comparisons with speeds inferred from the motion of peaks in the ion distributions, however, often did not agree with the model predictions. Examination of the predicted paths showed that the ions are expected to experience significant energy changes as they drift so that they quickly move out of the observed energy channel. Thus we are able to conclude that the observed motion is consistent with equatorial electric fields in the inner magnetosphere of a few tenths of a mV/m on 10 June 2000, several mV/m on 12 August 2000, and ≤1 mV/m on 4 October 2000 as predicted. Other interesting observations include (1) the suggestion of a two step injection process during the main phase of the 4 October 2000 storm, (2) the unexplained disappearance of the 33 keV protons when their drift motion brings them to noon in the recovery phase of the 10 June 2000 storm, (3) the apparent subtle changes in the electric potential pattern in the equator early in the 4 October 2000 storm, and (4) the energy dependence of the spatial source of the injected particles in the plasma sheet during the strong storm on 12 August 2000.

Derivation of currents and diamagnetic effects from global plasma pressure distributions obtained by IMAGE/HENA

The global ion distributions extracted from ENA images measured by IMAGE/HENA can be used to calculate the plasma pressure. In principle, knowing the plasma-pressure distribution and the magnetic field, one can also calculate the three-dimensional current system that is driven by the pressure gradient. The currents depend on the pressure gradient and the magnetic-field strength. Assuming current continuity ( ∇· J =0 ), we can use Euler potentials for J itself to calculate the current distribution. The method is applied to some pressure distributions measured by IMAGE/HENA and the resulting field-aligned currents (FACs) are compared to the global FAC maps derived from irridium magnetometer data.

Ring Current Energy Input and Decay

A new view of the ring current as an active element in the geospace system has emerged in which the ring current responds not only to changing convection electric fields imposed by solar wind interactions but to internal dynamics of the magnetosphere-ionosphere-atmosphere (geospace) system. Variations in the plasma sheet density, temperature and composition, saturation of the polar cap potential drop (and presumably the cross-tail potential drop), modifications to the imposed convection potential in the inner magnetosphere due to ring current shielding effects, the presence of a pre-existing ring current population, storm-substorm coupling, and strong convection with and without accompanying substorm activity all have an impact on the ring current strength, formation and loss. All of these internal processes imply that the geoeffectiveness of a solar wind driver cannot be predicted on the basis of the characteristics of the driver alone but must reflect key aspects of the dynamically changing geospace environment, itself. This review gives a summary of new information on ring current input and decay processes focusing on implications for the global geospace response to solar wind drivers during magnetic storms and on open questions that can be addressed with new ENA imaging techniques.

Remote ion temperature measurements of Earth's magnetosphere: Medium energy neutral atom (MENA) images

Energy spectra of ENA images obtained with the MENA imager on the IMAGE observatory are used to construct remote images of the plasma ion temperature. The remotely measured ion temperatures are consistent with in‐situ ion temperature measurements. During the storm of August 12, 2000, the ion temperatures on the dawn side of the magnetosphere are significantly lower than the dusk side ion temperatures. Similar observations of ion temperature asymmetry are presented for three other storm periods.

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.

Computer simulations of energetic neutral atom imaging from low and high altitude spacecraft

ENA cameras are being readied for flight on the Argentine SAC-B low-altitude mission and the NASA Cassini Saturn orbiter and an upcoming NASA MidEx mission (IMAGE) will be devoted completely to magnetospheric imaging. Computer simulation of ENA images has proved an indispensable tool for both scientific data analysis and instrument/mission design for a number of reasons. ENA emission is usually “optically thin”, so it is very difficult to imagine how magnetospheric ion distributions will image from different vantage points. ENA emission has a wide dynamic range in intensity, and contemporary cameras do not always have angular resolution sufficient to resolve the structures. Consequently, image simulation is an essential element in the extraction of the ion distributions from ENA images. Examples of simulations from past, present, and future missions that illustrate these effects are presented.