Energetic Neutral Atom Observations Research Articles

The Effects of Turbulence on Heliosheath Ions and Implications for Energetic Neutral Atoms

The distribution of ions in the heliosheath—the region between the heliospheric termination shock and the heliopause—is important for understanding remote observations of energetic neutral atoms (ENAs). The ion distributions were estimated previously based on hybrid simulations of the heating and evolution of solar wind and interstellar pickup ions across the solar wind termination shock, but these estimates only provide the distributions near the shock. In this work, we use self-consistent hybrid kinetic simulations to investigate the effects of turbulence on ion distributions in the heliosheath. The simulations are compared against Voyager observations, constraining the feasible amplitude and compressibility of turbulence. We find that the heating due to turbulent dissipation can lead to a significant increase in the temperature of thermal solar wind ions. Both turbulent velocity fluctuations and the heating of solar wind ions increase the charge-exchange source for ENAs at low energies (around 100 eV), where current ENA models underpredict observations by more than an order of magnitude. However, the effects of turbulence are likely not strong enough to fully explain these discrepancies.

Inferring the Interstellar Magnetic Field Direction from Energetic Neutral Atom Observations of the Heliotail

Determining the magnitude and direction of the interstellar magnetic field (B ISM) is a long-standing problem. To date, some methods to infer the direction and magnitude have utilized best-fit models to the positions of the termination shock and heliopause measured by Voyager 1 and 2. Other models use the circularity of the Interstellar Boundary Explorer (IBEX) ribbon assuming a secondary energetic neutral atom (ENA) mechanism. Previous studies have revealed that the B ISM organizes the orientation of the heliotail with respect to the solar meridian. Here we propose a new way to infer the direction of the B ISM based on ENA observations of the heliotail. IBEX observations of the heliotail have revealed high-latitude lobes of enhanced ENA flux at energies >2 keV. Analyses showed that the high-latitude lobes are nearly aligned with the solar meridian, while also exhibiting a rotation with solar cycle. We show, using steady-state solar wind conditions, that the inclination of the lobes reproduced with commonly used values for the angle (α BV ) between B ISM and the interstellar flow in the hydrogen deflection plane (40° < α BV < 60°) is inconsistent with the IBEX ENA observations. We report that 0° < α BV < 20° best replicates the heliotail lobe inclinations observed by IBEX. Additionally, our model results indicate that the variation of the solar magnetic field magnitude with solar cycle causes the longitudinal rotation of the lobes observed by IBEX by affecting the inclination of the lobes.

Fourteen Years of Energetic Neutral Atom Observations from IBEX

The Interstellar Boundary Explorer (IBEX) has been observing the outer heliosphere and its interactions with the very local interstellar medium (VLISM) via measurements of energetic neutral atoms (ENAs) for over 14 yr. We discovered the IBEX Ribbon—a structure completely unanticipated by any prior theory or model—that almost certainly resides beyond the heliopause in the VLISM. We also characterized the other major source of heliospheric ENAs, the globally distributed flux (GDF), produced largely in the heliosheath between the termination shock and heliopause. In this study, we make three major new contributions. First, we validate, provide, and analyze the most recent 3 yr of IBEX-Hi (0.5–6 keV FWHM) data (2020–2022) for the first time. Second, we link these observations to the prior 11 yr of observations, exploring long-term variations. Finally, we provide the first IBEX team-validated Ribbon/GDF separation scheme and separated maps. Because of the uncertainty in separating different line-of-sight integrated sources, we provide not just best guess (median) maps, but also maps with upper and lower reasonable values of Ribbon and GDF fluxes, along with bounding fluxes that add the uncertainties to the upper and lower values. This allows theories and models to be compared with a range of possible values that the IBEX team believes are consistent with data. These observations, along with the reanalysis of the prior 11 yr of IBEX-Hi data, provide new insights and even further develop our detailed understanding of the heliosphere’s interaction with the local interstellar medium unlocked by IBEX.

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.

Variation of Hydrogen Energetic Neutral Atom Flux in the Subsolar Magnetosheath as a Function of Solar Cycle

AbstractWe analyzed observations of energetic neutral atoms (ENAs) of hydrogen collected from the subsolar magnetosheath by the Interstellar Boundary Explorer (IBEX) in Solar Cycle 24. We looked for long‐term solar activity‐related variation and correlation between the H ENA flux and the solar wind parameters. The H ENA flux in the energy range from 0.5 to 6 keV increased during the maximum and declining phases of the solar cycle. This H ENA flux increase happened during a period of enhanced solar wind dynamic pressure observed at 1 au. We study the relation between the ENA flux and solar wind pressure in the subsolar magnetosheath and conclude that the increase in the ENA flux is a consequence of enhanced ENA production. It is because of the increased solar wind pressure during the declining phase of the solar activity that moved the subsolar magnetosheath earthward into a region of denser hydrogen geocorona, thus increasing the H ENA production. Furthermore, we report a positive correlation between the H ENA flux in the subsolar magnetosheath and the solar wind pressure for the first four energy steps observed by IBEX‐Hi and a decrease for the highest energy acquired, which indicates the presence of a different parent ion population for those ENAs from that of the core solar wind. Our study also shows that the ENA flux correlates differently with the solar wind density and speed as a function of increasing energy of ENAs.

Energetic Neutral Atoms from Solar Energetic Particles due to Shocks: Inclusion of Upstream Particles

Many aspects of solar energetic particles are not well understood, including their acceleration mechanism. There has been recent interest in the potential of energetic neutral atoms (ENAs) as remote probes of solar energetic particles (SEPs) and their acceleration. The single accidental observation (in physical units) has been modeled as accelerated by a coronal mass ejection (CME)-driven shock by several authors, all of whom have assumed that the upstream component of the shock can be ignored. In this article, we relax this assumption and model the flux of ENAs at 1 au due to a CME-driven shock with an upstream component. We show the effect of varying parameters of the shock acceleration model, specifically α, the exponent of the power law in momentum of the mean free path, and η, a measure of the relative turbulence level. The main result is that including the upstream component significantly increases the flux at 1 au for typically assumed parameters in the energy range of the STEREO observation. We also derive the form of the ENA transport equation that we used in this study. These results enable a better understanding of potential observations of ENAs due to SEPs.

Probing the Length of the Heliospheric Tail with Energetic Neutral Atoms (ENAs) from 0.52 to 80 keV

The shape of the heliosphere is currently under active debate. Energetic neutral atoms (ENAs) offer the best method for investigating the global structure of the heliosphere. To date, the Interstellar Boundary Explorer (IBEX) and the Ion and Neutral Camera (INCA) that was on board Cassini provide the only global ENA observations of the heliosphere. While extensive modeling has been done at IBEX-Hi energies (0.52–6 keV), no global ENA modeling has been conducted for INCA energies (5.2–55 keV). Here, we use an ENA model of the heliosphere based on hybrid results that capture the heating and acceleration of pickup ions (PUIs) at the termination shock to compare modeled global ENA results with IBEX-Hi and INCA observations using both a long- and short-tail model of the heliosphere. We find that the modeled ENA results for the two heliotail configurations produce similar results from the IBEX-Hi through the INCA energies. We conclude from our modeled ENAs, which only include PUI acceleration at the termination shock, that ENA observations in currently available energy ranges are insufficient for probing the shape and length of the heliotail. However, as a prediction for the future IMAP-Ultra mission (3–300 keV) we present modeled ENA maps at 80 keV, where the cooling length (∼600 au) is greater than the distance where the long- and short-heliotail models differ (∼400 au), and find that IMAP-Ultra should be able to identify the shape of the heliotail, predicting differences in the north lobe to downwind flux ratio between the models at 48%.

An Anomalous Cosmic-Ray Mediated Termination Shock: Implications for Energetic Neutral Atoms

The Voyager 2 crossing of the termination shock indicated that most of the upstream energy from the thermal solar wind ions was transferred to pickup ions (PUIs) and other energetic particles downstream of the shock. We use hybrid simulations at the termination shock for the Voyager 2, flank, and tail directions to evaluate the distributions of different ion species downstream of the shock over the energy range of 0.52–55 keV. Here, we extend the work of Gkioulidou et al., which showed an energy-dependent discrepancy between modeled and energetic neutral atom (ENA) observations, and fit distributions to a hybrid model to show that a population of PUIs accelerated via diffusive shock acceleration (DSA) to become low-energy anomalous cosmic rays (ACRs) can bridge the gap between modeled and observed ENA fluxes. Our results with the inclusion of DSA via hybrid fitting give entirely new and novel evidence that DSA at the termination shock is likely to be an important physical process. These ACRs carry a significant fraction of the energy density at the termination shock (22%, 13%, and 19% in the Voyager 2, flank, and tail directions, respectively). Using these ACRs in global ENA modeling of the heliosphere from 0.52 to 55 keV, we find that scaling factors as large as 1.8–2.5 are no longer required to match ENA observations at energies of ∼1–4 keV. Large discrepancies between modeled and observed ENAs only remain over energies of 4–20 keV, indicating that there may be a further acceleration mechanism in the heliosheath at these energies.

Tracking the Rapid Opening and Closing of Polar Coronal Holes through IBEX ENA Observations

Fast solar wind (SW) flows outward from polar coronal holes (PCHs). The latitudinal extent of the fast SW varies during different phases of the solar cycle. The fast SW in the inner heliosheath produces a flatter proton spectrum than the slow SW that can be observed through energetic neutral atoms (ENAs) by the Interstellar Boundary Explorer (IBEX). In this study, we investigate the evolution of PCHs as reflected in the high-time resolution ENA flux measurements from IBEX-Hi, where the PCHs are identified by ENA spectral indices <1.8. The ENA spectral index over the poles shows a periodic evolution over the solar cycle 24. The surface area with flatter ENA spectra (<1.8) around the ecliptic south pole increases slightly from 2009–2011 and then decreased gradually from 2012–2014. The PCH completely disappears in 2016 and then starts to appear again starting in 2017, gradually growing until 2019. This evolution shows a clear correlation with the change in the PCH area observed at the Sun once the delay in the ENA observation time is included. In addition, the higher-cadence ENA data at the highest latitudes show a rapid evolution of the ENA spectrum near the south pole in 2014 and 2017. The rapid evolution in 2014 is related to a rapid closing of PCHs in 2012 and that in 2017 is related to a rapid opening of PCHs in late 2014. These results also agree qualitatively with the evolution of the ENA spectral index from simulations using a simple time-dependent heliospheric flow model.

Explanation of Heliospheric Energetic Neutral Atom Fluxes Observed by the Interstellar Boundary Explorer

Interstellar neutral atoms propagating into the heliosphere experience charge exchange with the supersonic solar wind (SW) plasma, generating ions that are picked up by the SW. These pickup ions (PUIs) constitute ∼25% of the proton number density by the time they reach the heliospheric termination shock (HTS). Preferential acceleration of PUIs at the HTS leads to a suprathermal, kappa-like PUI distribution in the heliosheath, which may be further heated in the heliosheath by traveling shocks or pressure waves. In this study, we utilize a dynamic, 3D magnetohydrodynamic model of the heliosphere to show that dynamic heating of PUIs at the HTS and in the inner heliosheath (IHS), as well as a background source of energetic neutral atoms (ENAs) from outside the heliopause, can explain the heliospheric ENA signal observed by the Interstellar Boundary Explorer (IBEX) in the Voyager 2 direction. We show that the PUI heating process at the HTS is characterized by a polytropic index larger than 5/3, likely ranging between γ ∼ 2.3 and 2.7, depending on the time in solar cycle 24 and SW conditions. The ENA fluxes at energies >1.5 keV show large-scale behavior in time with the solar cycle and SW dynamic pressure, whereas ENAs < 1.5 keV primarily exhibit random-like fluctuations associated with SW transients affecting the IHS. We find that ≲20% of the ENAs observed at ∼0.5–6 keV come from other sources, likely from outside the heliopause as secondary ENAs. This study offers the first model replication of the intensity and evolution of IBEX-Hi ENA observations from the outer heliosphere.

Energetic Neutral Atoms near Mars: Predicted Distributions Based on MAVEN Measurements

Abstract Through phase-space mapping 6 yr of H+, O+, and O 2 + distributions measured in situ by the Mars Atmosphere and Volatiles EvolutioN (MAVEN) orbiter, we derive semiempirical average energetic neutral atom (ENA) distributions near Mars. Differential fluxes of H-ENAs and O-ENAs are estimated by line-integrating ENA production and loss rates in the average phase-space ion distributions. By repeating the procedure for a systematic series of vantage points, we produce synthetic ENA observations for virtual circular orbits with radii twice that of Mars, revealing the angular dependence of the observer’s position. Accounting for known ENA production and loss terms, we find that H+ in the upstream solar wind yields total antisunward H-ENA fluxes of ∼3 × 105 cm−2 s−1. The dayside magnetosheath produces H-ENA angular-differential fluxes of up to 3 × 105 cm−2 s−1 sr−1, while the magnetosheath flanks populate the planet’s nightside with H-ENA fluxes in the range of 104–105 cm−2 s−1 sr−1. The O-ENA environment is dominated by a near-isotropic ∼105 cm−2 s−1 sr−1 relatively low-energy (tens of eV) population originating in the top-side ionosphere, particularly on the dayside. Lower fluxes (103–104 cm−2 s−1 sr−1) of energetic O-ENAs (≳100 eV) are mainly found on the nightside and above the electric polar regions of the induced magnetosphere. Asymmetries in the ENA flow are largely limited to the energetic O-ENA populations, while the H-ENA distribution is mostly symmetric around the Sun‒Mars line. We discuss how synthetic ENA observations can provide insight into the near-Mars space environment, including the planet’s plasma environment and exosphere.

A New 3D Solar Wind Speed and Density Model Based on Interplanetary Scintillation

Abstract The solar wind (SW) is an outflow of the solar coronal plasma, which expands supersonically throughout the heliosphere. SW particles interact by charge exchange with interstellar neutral atoms; on the one hand, they modify the distribution of this gas in interplanetary space, and, on the other hand, they are the seed populations for heliospheric pickup ions and energetic neutral atoms (ENAs). The heliolatitudinal profiles of the SW’s speed and density evolve during the solar activity cycle. A model of the evolution of the SW’s speed and density is needed to interpret observations of ENAs, pickup ions, the heliospheric backscatter glow, etc. We derive the Warsaw Heliospheric Ionization Model 3DSW—WawHelIon 3DSW—based on interplanetary scintillation (IPS) tomography maps of the SW speed. We use the IPS tomography data from 1985 to 2020, compiled by Tokumaru et al. We derive a novel statistical method of filtering these data against outliers; we present a flexible analytic formula for the latitudinal profiles of the SW speed, based on Legendre polynomials of varying order with additional restraining conditions at the poles; fit this formula to the yearly filtered data; and calculate yearly SW density profiles using the latitudinally invariant SW energy flux observed in the ecliptic plane. Despite the application of a refined IPS data set, a more sophisticated data filtering method, and a more flexible analytic model, the present results mostly agree with those obtained previously, demonstrating the robustness of IPS studies of the SW’s structure.

IBEX Ribbon Separation Using Spherical Harmonic Decomposition of the Globally Distributed Flux

Abstract Remote imaging of plasmas in the heliosphere and very local interstellar medium is possible with energetic neutral atoms (ENAs), created through the charge exchange of protons with interstellar neutral atoms. ENA observations collected by the Interstellar Boundary Explorer (IBEX) revealed two distinctive sources. One source is the globally distributed flux (GDF), which extends over the entire sky and varies over large spatial scales. The other source encompasses only a narrow circular band in the sky and is called the IBEX ribbon. Here, we utilize the observed difference in spatial scales of these two ENA sources to separate them. We find that linear combinations of spherical harmonics up to degree ℓ max = 3 can reproduce most of the ENA fluxes observed outside the ribbon region. We use these combinations to model the GDF and the difference between the observed fluxes and the GDF yields estimation of the ribbon emission. The separated ribbon responds with a longer time delay to the solar wind changes than the GDF, suggesting a more distant source of the ribbon ENAs. Moreover, we locate the direction of the maximum plasma pressure based on the GDF. This direction is 17°.2 ± 0°.5 away from the upwind direction within the plane containing the interstellar flow and interstellar magnetic field vectors. This deflection is consistent with the expected position of the maximum external pressure at the heliopause. The maps with separated ribbon and GDF are posted concurrently with this paper and can be used to further study these two sources.

Variations in the Pickup Ion Density Structure in Response to the Growth of the Kelvin–Helmholtz Instability along the Heliopause

Abstract Features of the response of pickup ions (PUIs) to the Kelvin–Helmholtz instability (KHI) on the heliopause (HP) are examined by means of two-dimensional hybrid simulations. We assume the supersonic neutral solar wind as the source of PUIs gyrating about the magnetic field in the outer heliosheath. These PUIs become energetic neutral atoms (ENAs) via charge exchange with interstellar hydrogen, and a portion of these ENAs are detected by spacecraft such as the Interstellar Boundary Explorer (IBEX). To evaluate the possibility of identifying the KHI on the HP from ENA observations, we assume that an imprint of the KHI may be displayed in spatial and temporal variations in the observed ENA profile. As an alternative to ENA, the column density of PUIs integrated across the HP is calculated. The KH-inducing vortex forces not only background protons but also PUIs to roll up deep in the inner heliosheath. The KH vortex also results in the emission of magnetosonic pulses that sweep PUIs in the outer heliosheath and lead to their local confinement. These effects elongate the PUIs’ spatial distribution in the direction normal to the HP. The appearance of the local confined structure in the PUIs’ column density is consequently confirmed, and this feature can be confirmed as the KHI evolution. Although the simulation cannot be quantitatively compared with the observations currently available because its resolution is too low, we expect that the derived properties will be useful for diagnosing the nature of HP fluctuation in future missions.

A Complete Data Set of Equatorial Projections of Saturn's Energetic Neutral Atom Emissions Observed by Cassini‐INCA

AbstractObservations of energetic neutral atoms (ENAs) are a useful tool for analyzing ion and neutral abundances in planetary magnetospheres. They are created when hot plasma, originating for example from magnetic reconnection sites, charge‐exchanges with the ambient neutral population surrounding the planet. The motion of ENAs is not governed by the magnetic field, allowing remote imaging. During the Cassini mission, the Ion and Neutral CAmera (INCA) of the Magnetosphere Imaging Instrument (MIMI) collected vast amounts of hydrogen and oxygen ENA observations of Saturn's magnetosphere from a variety of different viewing geometries. To enable investigations of the morphology and dynamics of Saturn's ring current, it is useful to re‐bin and re‐project the camera‐like views from the spacecraft‐based perspective into a common reference frame. We developed an algorithm projecting INCA's ENA observations into a regular grid in Saturn's equatorial plane. With most neutrals and ions being confined into an equatorial rotating disc, this projection is quite accurate in both spatial location and preservation of ENA intensity, provided the spacecraft is located at large enough elevations. Such projections were performed for all INCA ENA data from the Cassini Saturn tour; the data are available for download together with a Python routine flagging contaminated data and returning detailed spacecraft geometry information. The resulting data set is a good foundation for investigating for example the statistical properties of Saturn's ring current and its complicated dynamics in relation to other remote and in situ observations of, for example, auroral emissions and magnetotail reconnection events.

Neutral Atom Imaging of the Solar Wind-Magnetosphere-Exosphere Interaction Near the Subsolar Magnetopause.

Energetic neutral atoms (ENAs) created by charge‐exchange of ions with the Earth's hydrogen exosphere near the subsolar magnetopause yield information on the distribution of plasma in the outer magnetosphere and magnetosheath. ENA observations from the Interstellar Boundary Explorer (IBEX) are used to image magnetosheath plasma and, for the first time, low‐energy magnetospheric plasma near the magnetopause. These images show that magnetosheath plasma is distributed fairly evenly near the subsolar magnetopause; however, low‐energy magnetospheric plasma is not distributed evenly in the outer magnetosphere. Simultaneous images and in situ observations from the Magnetospheric Multiscale (MMS) spacecraft from November 2015 (during the solar cycle declining phase) are used to derive the exospheric density. The ~11–17 cm−3 density at 10 RE is similar to that obtained previously for solar minimum. Thus, these combined results indicate that the exospheric density 10 RE from the Earth may have a weak dependence on solar cycle.

Juno Energetic Neutral Atom (ENA) Remote Measurements of Magnetospheric Injection Dynamics in Jupiter's Io Torus Regions

AbstractIn planetary magnetospheres, singly charged energetic particles, trapped by the planet's magnetic field, can steal electrons from cold gas atoms and become neutralized. These now energetic neutral atoms (ENAs), no longer confined by the magnetic field, can travel out of the system similar to photons leaving a hot oven. ENAs have been used to image magnetospheric processes at Earth, Jupiter, and Saturn. At Jupiter, the opportunities to image the magnetosphere have been limited and always from the perspective of the near‐equatorial plane at distance &gt;139 RJ. The polar‐orbiting Juno mission carries the Jupiter Energetic particle Detector Instrument that is serendipitously sensitive to ENAs with energies &gt;50 keV, provided that there are no charged particles in the environment to mask their presence. Here we report on the first ENA observations of Jupiter's magnetosphere from a nonequatorial perspective. In this brief report we concentrate on emissions seen during Perijove 22 (PJ22) during very active conditions and compare them with emissions during the inactive Perijove 23 (PJ23). We observe, and discriminate between, distinct ENA signatures from the neutral gases occupying the orbit of Io (away from Io itself), the orbit of Europa (away from Europa), and from Jupiter itself. Strong ENA emissions from Io's orbit during PJ22 are associated with energetic particle injections observed near Io's orbit several hours earlier. Some injections occurred planetward of Io's L‐shell (magnetic position), somewhat of a surprise given that injections are thought to be driven by outward transport of plasmas generated by Io.

Heliospheric Structure as Revealed by the 3–88 keV H ENA Spectra

Energetic neutral atoms (ENA) are an important tool for investigating the structure of the heliosphere. Recently, it was observed that fluxes of ENAs (with energy $\le$ 55 keV) coming from the upwind and downwind regions of the heliosphere are similar in strength. This led the authors of these observations to hypothesize that the heliosphere is bubble-like rather than comet-like, meaning that it has no extended tail. We investigate the directional distribution of the ENA flux for a wide energy range (3--88 keV) including the observations from IBEX (Interstellar Boundary Explorer), INCA (Ion and Neutral Camera, on board Cassini), and HSTOF (High energy Suprathermal Time Of Flight sensor, on board SOHO, Solar and Heliospheric Observatory). An essential element is the model of pickup ion acceleration at the termination shock (TS) proposed by Zank. We use state of the art models of the global heliosphere, interstellar neutral gas density, and pickup ion distributions. The results, based on the "comet-like" model of the heliosphere, are close in flux magnitude to ENA observations by IBEX, HSTOF and partly by INCA (except for the 5.2-13.5 keV energy channel). We find that the ENA flux from the tail dominates at high energy (in agreement with HSTOF, but not INCA). At low energy, our comet-like model produces the similar strengths of the ENA fluxes from the upwind and downwind directions, which, therefore, removes this as a compelling argument for a bubble-like heliosphere.

Terrestrial Energetic Neutral Atom Emissions and the Ground‐Based Geomagnetic Indices: Implications From IBEX Observations

AbstractWe report the first daylong continuous observations of bright terrestrial energetic neutral atom (ENA) emissions by the Interstellar Boundary Explorer (IBEX). The unique vantage point of IBEX, up to ∼48 Earth radii (Re) from the dawn‐dusk side, allowed it to monitor ENAs from 0.3 to 6.0 keV over an unprecedented length of time. We have taken advantage of this capability to discover correlations between auroral electrojet (AE) indices and ENA emissions. Although it was difficult to separate the source from low‐altitude emissions and ring current emissions using IBEX's one‐pixel observations, the characteristics of these correlations and e‐folding times hint at the processes behind the ENA emissions. By introducing exponential e‐folding times ranging from 2 to 4 hr in the AE index temporal profiles, the correlation coefficient was maximized to around 0.75−0.85. This indicates (1) that the building and trapping of parent ions, accompanied by injections, cause AE enhancements and (2) that the shorter e‐folding time (typically ∼7–8 hr for energetic ring current ions, assuming charge exchange reactions) supports a more efficient decay process by Coulomb collisions in this energy range. On the other hand, no correlation was found with Sym‐H, while Asy‐H correlated for one orbit. Long‐term continuous ENA observations can also be applied to future space weather monitoring outside the magnetosphere. Based on our results, we suggest that comparisons among multiple spacecraft observations with high‐angular‐resolution ENA sensors or ENA imagers will be required to determine magnetospheric activity more accurately due to the expected asymmetry of the ENA sources.

Inner Heliosheath Shocks and Their Effect on Energetic Neutral Atom Observations by IBEX

Abstract A collision between an interplanetary disturbance in the solar wind and the heliospheric termination shock leads to the generation and propagation of plasma structures in the inner heliosheath (IHS). This interaction can lead to one or more shocks propagating in the IHS until they collide with the heliopause (HP). IHS shocks are (1) partially reflected at the HP and propagate back into the IHS and (2) partially transmitted into the very local interstellar medium. The IHS is dominated by the pressure of energetic particles as was observed by the Low Energy Charged Particle instrument on Voyager 2 and by remote observations from Interstellar Boundary Explorer (IBEX), making the plasma beta, when the energetic particle pressure is included, much greater than one. We model IHS shocks using a pickup ion (PUI)-mediated plasma model and show that they are mediated by PUIs. The dissipation mechanism at perpendicular IHS shocks results primarily in PUIs being heated. Only a very small percentage of the upstream solar wind flow energy is converted to heating of lower energy solar wind ions at the shock. IHS shocks are broad because the diffusion coefficient associated with PUIs is large. The presence of IHS shocks results in greater heating of the PUI component in the IHS. The increased temperature enhances the production of energetic neutral atoms (ENAs) due to charge exchange between IHS PUIs and interstellar neutral gas. When IHS shocks are included in the model, we find that the predicted enhancement of the ENA flux leads to better consistency with corresponding IBEX observations.