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The Advanced Small Analyzer for Neutrals (ASAN) is a compact mass resolving energetic neutral atom analyzer flown on the Yutu-2 rover of the Chinese Chang’E-4 mission to the Moon. ASAN measures energetic neutral atoms in the energy range from 10 eV to 10 keV with typically 30% energy resolution. The field-of-view is a single angular pixel of 37° × 30° pointing at lunar surface. Energetic neutral atoms enter the instrument through a charge particle deflection system and are then ionized on a diamond-like carbon conversion surface. Positive ions from the conversion surface are energy analyzed using an electrostatic analyzer and their velocity is subsequently determined in a time-of-flight section using ceramic channel electron multipliers as detectors. The obtained mass resolution m/ Δ m is about 2, comfortably separating neutral hydrogen, helium, and the oxygen-group (≥16 amu). A full energy spectrum for energetic neutral atoms is obtained in 3 seconds. Switching the charged particle deflection system off, ASAN also directly measures positive ions with similar performance as for energetic neutral atoms. Including electronics the instrument weighs 970 g and consumes 3.4 W.

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.

Energetic neutral atom (ENA) observations provide valuable insights into the plasma conditions in the heliosphere and the surrounding interstellar medium. Unlike plasma detectors, which measure charged particles tied to the magnetic fields at their location, ENA detectors capture former ions that were neutralized in distant regions and traverse the heliosphere in straight trajectories. ENA fluxes near the Sun represent line-of-sight integrals of parent ion fluxes multiplied by neutralization (production) rates and reduced by the probability of ENA reionization (loss) processes. So far, most ENA analyses have focused on charge exchange between hydrogen atoms and protons as the primary source of ENAs. Here, we examine various ENA production and loss processes throughout the heliosphere in the broad energy range (5 eV to 500 keV) encompassing the next-generation ENA instruments aboard the Interstellar Mapping and Acceleration Probe mission. Our study considers binary collisions involving the most abundant species: protons, electrons, α-particles, He+ ions, photons, as well as hydrogen and helium atoms. Our findings indicate that, in addition to ENAs produced by charge exchange of energetic protons with hydrogen atoms, a significant portion of high-energy ENAs originate from the charge exchange with helium atoms. Below 10 keV, the dominant ENA loss processes are charge exchange collisions with protons and photoionization. However, stripping ionization processes, e.g., from collisions with ambient interstellar neutral hydrogen, become the main loss mechanism for higher energies because the charge exchange rate rapidly decreases.

In this study, we analyze an important property of the very local interstellar medium, i.e., turbulence that affects the intensity and shape of the Interstellar Boundary Explorer (IBEX) ribbon. Specifically, we simulate the propagation of the ribbon’s parent ions before they become secondary energetic neutral atoms that can be observed at 1 au by IBEX. We then test how different slab/2D turbulence fractions affect the intensity and shape of the modeled ribbon and compare to the latest IBEX ribbon-separated data. We compare 1D cuts across the modeled and observed ribbon as a function of angle away from the ribbon center. We find that the intensities of the modeled ribbon are larger than most of the data, except near the ecliptic plane. However, the model intensities are sensitive to how we model the neutral solar wind, which forms the source ion population for the ribbon. We then compare the model and data by normalizing the fluxes to their respective peak intensities and find the peaks’ angular distances from the ribbon center. We find that most of the model peaks lie within 3° of the data peaks, which we consider our 1σ uncertainty (half the size of an IBEX pixel). By averaging the instances where the model peaks match the data peaks (within 3°), we find that the mean is slab-50%, with a standard deviation of ±28% and standard error of ±5%.

Heliospheric energetic neutral atoms (ENAs) originate from energetic ions that are neutralized by charge exchange with neutral atoms in the heliosheath and very local interstellar medium (VLISM). Since neutral atoms are unaffected by electromagnetic fields, they propagate ballistically with the same speeds as parent particles. Consequently, measurements of ENA distributions allow one to remotely image the energetic ion distributions in the heliosheath and VLISM. The origin of the energetic ions that spawn ENAs is still debated, particularly at energies higher than ∼keV. In this work, we summarize five possible sources of energetic ions in the heliosheath that cover the ENA energy from a few keV to hundreds of keV. Three sources of the energetic ions are related to pickup ions (PUIs): those PUIs transmitted across the heliospheric termination shock (HTS), those reflected once or multiple times at the HTS, i.e., reflected PUIs, and those PUIs multiply reflected and further accelerated by the HTS. Two other kinds of ions that can be considered are ions transmitted from the suprathermal tail of the PUI distribution and other particles accelerated at the HTS. By way of illustration, we use these energetic particle distributions, taking account of their evolution in the heliosheath, to calculate the ENA intensities and to analyze the characteristics of ENA spectra observed at 1 au.

This review summarizes the current state of research aiming at a description of the global heliosphere using both analytical and numerical modeling efforts, particularly in view of the overall plasma/neutral flow and magnetic field structure, and its relation to energetic neutral atoms. Being part of a larger volume on current heliospheric research, it also lays out a number of key concepts and describes several classic, though still relevant early works on the topic. Regarding numerical simulations, emphasis is put on magnetohydrodynamic (MHD), multi-fluid, kinetic-MHD, and hybrid modeling frameworks. Finally, open issues relating to the physical relevance of so-called “croissant” models of the heliosphere, as well as the general (dis)agreement of model predictions with observations are highlighted and critically discussed.

The Interstellar Boundary Explorer (IBEX) is a NASA satellite in Earth orbit, dedicated to observing both interstellar neutral atoms entering the heliosphere and energetic neutral atoms (ENAs) from the interstellar boundaries from roughly 10 eV to 6 keV. This work presents the averaged maps, energy spectra, and temporal variability of heliospheric ENA intensities measured with the IBEX-Lo instrument at 1 au at energies between 10 eV and 2 keV, covering one entire solar cycle from 2009 through 2019. These results expand the range in time and energy for studying the globally distributed ENA flux and the IBEX Ribbon. The observed ENA intensities exceed model predictions, in particular below 500 eV. Moreover, the ENA intensities between 50–200 eV energy show an unexpected rise and fall around the year 2015 in most sky regions.

We revisit previous hybrid simulations of the heating and acceleration of interstellar pickup ions (PUIs) at the solar wind termination shock. In previous simulations, a relatively cold initial distribution of PUIs was assumed; and while the resulting shock-heated distribution was consistent with Voyager 2 LECP measurements at about 30 keV, the intensity of the distribution downstream of the shock in the ~1–10 keV energy range was lower than predictions based on analysis of energetic neutral atoms (ENAs) from the Interstellar Boundary Explorer-Hi and Cassini's Ion and Neutral Camera. Here we perform new simulations with more realistic initial PUI distributions. We assume the distribution is a partially filled spherical shell in velocity space with a radius that varies from 320 to 640 km s−1. We then use the distributions downstream of the shock from these new simulations to estimate the ENA flux spectrum and compare with observations. We find that the predicted ENA spectrum from the new simulations much better matches the observations over a broad range of energies. We conclude that the hybrid simulations provide reasonable predictions for the distribution of charged particles in the energy range from ~0.5 to 50 keV.

Our understanding of the solar–interstellar interaction greatly improved with measurements of energetic neutral atoms (ENAs) from the outer heliosphere by NASA’s Interstellar Boundary Explorer (IBEX). The IBEX mission measurements showed us the solar–interstellar interaction was far more complex than we originally thought. One such measurement was the Ribbon: a narrow, enhanced band of ENA emissions wrapped across the sky. We now generally understand its origin being outside the heliopause. This was confirmed, among other methods, using the astronomical method of parallax and temporal cross correlations with the solar wind output from the Sun. Here, we perform cross correlations of Ribbon ENAs with the solar wind to derive the distance to the Ribbon, and associated uncertainties, in two different sections of the sky (centered ∼30° below and ∼30° above the ecliptic plane). Parameters that introduce uncertainties include the neutral H density in the very local interstellar medium, the speed of the parent neutral particles that travel across the heliopause, and the cross correlation. We derive the distance to the Ribbon in these two spatially averaged sections, yielding distances of dR = 90 ± 56 and dR = 192 ± 42 au for the southern and northern sections of the Ribbon, respectively, or an average of 155 ± 34 au—a rough value one may use for the Ribbon source distance in the forward hemisphere of the heliosphere. Quantifying uncertainties is important for understanding the accuracy of these calculations as they can be a significant fraction of the calculated distance to the Ribbon itself.

We show theoretically that it is possible to coherently transfer vibrational excitation between trapped neutral atoms over a micrometer apart. To this end we consider three atoms, where two are in the electronic ground state and one is excited to a Rydberg state whose electronic orbital overlaps with the positional wave functions of the two ground-state atoms. The resulting scattering of the Rydberg electron with the ground-state atoms provides the interaction required to transfer vibrational excitation from one trapped atom to the other. By numerically investigating the dependence of the transfer dynamics on the distance between traps and their relative frequencies we find that there is a ‘sweet spot’ where the transfer of a vibrational excitation is nearly perfect and fast compared to the Rydberg lifetime. We investigate the robustness of this scenario with respect to changes of the parameters. In addition, we derive a intuitive effective Hamiltonian which explains the observed dynamics.