Hydrogen Energetic Neutral Atoms Research Articles

WawHelioIonMP: A Semiempirical Tool for the Determination of Latitudinal Variation in the Ionization Rate of Interstellar Hydrogen and the Solar Wind

The latitudinal structure of the solar wind varies during the cycle of solar activity. Analysis of this variation is important for understanding the solar activity and interpretation of observations of heliospheric energetic neutral hydrogen atoms and interstellar neutral (ISN) atoms inside the heliosphere, which yield information on the heliosphere and its interaction with the interstellar medium. Existing methods of retrieving this information from indirect remote-sensing measurements of phenomena, including the heliospheric backscatter glow and interplanetary scintillations of remote radio sources, are challenging to apply in real time. Here, we propose a method WawHelioIonMP of approximate retrieval of latitudinal profiles of the ionization rates of ISN H using a machine-learning-based interpretation of the helioglow. Assuming that we know their history during two past solar cycles and have observations of the helioglow for close-to-circumsolar circles with a radius close to 90°, we derive statistically an algebraic relation between the ionization profiles and lightcurves. With the relation reversed, we are then able to derive the ionization rate profiles based on observed light curves, such as those planned for the GLObal solar Wind Structure (GLOWS) experiment on the forthcoming NASA mission Interstellar Mapping and Acceleration Probe (IMAP). The application of this method is straightforward and rapid because complex simulations are no longer needed. We present the method of retrieval of the profiles of the ionization rates, leaving the discussion of details of the decomposition of the retrieved ionization rate profiles into profiles of the solar wind speed and density to a future paper.

Characterizing Precipitation Behaviors of H− in the Martian Atmosphere

AbstractSolar wind protons can charge exchange with the extensive hydrogen corona of Mars, resulting in a significant flux of energetic neutral atoms (ENAs). As these solar wind hydrogen ENAs precipitate into the upper atmosphere, they can experience electron attachment or detachment, resulting in populations of H− and H+, respectively, with upstream velocity. We seek to characterize the behavior of H− in the ionosphere of Mars through a combination of in situ data analysis and mathematical models. Observations indicate that measurable H− precipitation in the ionosphere of Mars is rare, occurring during only 1.8% of available observations. These events occur primarily during high energy solar wind conditions near perihelion. We also compare H− fluxes to those of H+ and find that H− fluxes are ∼4.5 times less than H+, indicating preferential conversion of hydrogen ENAs to H+. We develop a simple model describing the evolution of the charged and neutral fraction of ENAs and H− ions versus altitude. We find that 0.29%–0.78% of ENAs are converted to H− for solar wind energies 1–3 keV. We also predict that the effects of photodetachment on the H‐H− system are non‐negligible.

Effects of Airless Bodies’ Regolith Structures and of the Solar Wind’s Properties on the Backscattered Energetic Neutral Atoms Flux

The surfaces of airless planetary bodies, such as the Moon or Mercury, are covered with regoliths, which interact with the solar wind. The solar protons can either be absorbed by the surface or neutralized and reflected as hydrogen energetic neutral atoms (ENAs). The ENA flux is thought to depend mostly on the structure of the upper regolith layer. By using a model combining a Monte Carlo approach to describe a solar proton’s journey through the lunar surface with molecular dynamics to characterize its interactions with the regolith’s grains, we highlight the surface roughness as a key parameter that influences the backscattered H ENA flux. By considering spherical silica grains, the lunar regolith’s structure is described using the open-source code Large-scale Atomic/Molecular Massively Parallel Simulator (or LAMMPS), which allows a realistic description of grain-on-grain contacts. The roughness of the modeled regolith, characterized by the roughness ratio, is shown to be dictated by the surface energy and the grain-size distribution. This work shows that a rougher surface favors deeper penetration of the protons inside the regolith, which increases the number of collisions and thus decreases their reflected fraction. The angular distribution of the backscattered H ENAs is influenced by both the surface roughness and the solar zenith angle. We show that the angular distribution of the backscattered ENAs is anisotropic and is influenced by the regolith’s structure, which is consistent with Chandrayaan-1 measurements. This work aims for a better understanding of the interactions ongoing at this interface and intends to look into the possibility of deducing information on the surface structure solely from ENA flux measurements. Highlighting the key structural parameters influencing the ENA backscattering will also help the development of models of surface-bounded exospheres.

KINETIC MODEL OF THE STELLAR WIND FORCING ON THE EXTENDED HYDROGEN ATMOSPHERE OF THE EXOPLANEt π Men c

In this paper, an extension of the kinetic model of the aeronomy of the upper atmosphere of an exoplanet is performed by including the processes of the effect of stellar wind plasma on the extended hydrogen corona of a hot sub-neptune. For this purpose, previously developed kinetic Monte Carlo models were used to study the precipitation of protons and hydrogen atoms with high energies into planetary atmospheres. The kinetic model is adapted to the upper atmospheres of hot sub-neptunes, which made it possible to calculate the rate of absorption of stellar wind plasma energy in the planetary corona and to refine estimates of the non-thermal loss rate of the atmosphere due to the influence of the stellar wind. The calculations carried out for the hot sub-neptune π Men c showed that the energy of a flux of energetic neutral hydrogen atoms (ENA H) penetrating the atmosphere, formed during the charge exchange of stellar wind protons with thermal hydrogen corona atoms, mainly goes to heating the hydrogen corona of a hot exoplanet.

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.

Inversion of Upstream Solar Wind Parameters from ENA Observations at Mars

An algorithm has been developed to invert the solar wind parameters from the hydrogen energetic neutral atom (H-ENA) measured in near-Mars space. Supposing the H-ENA is produced by change exchange collision between protons that originated in the solar wind and neutrals in the exosphere, an H-ENA model is established based on the magnetohydrodynamic (MHD) simulation of the solar wind interaction with Mars, to study the H-ENA characteristics. It is revealed that the solar wind H-ENAs are high-speed, low-temperature beams, just like the solar wind, while the magnetosheath H-ENAs are slower and hotter, with broader energy distribution. Assuming Maxwellian velocity distribution, the solar wind H-ENA flux is best fitted by a Gaussian function, from which the solar wind velocity, density, and temperature can be retrieved. Further investigation, based on the ENA flux simulated by the H-ENA model, reveals that the accuracy of inversed solar wind parameters is related to the angular and energy resolutions of the ENA detector. Finally, the algorithm is verified using the H-ENA observations from the Tianwen-1 mission. The upstream solar wind velocity when inversed is close to that of the in situ plasma measurement. Our result suggests the solar wind parameters inversed from H-ENA observation could be an important supplement to the dataset supporting studies on the Martian space environment, where long-term continuous monitoring of the upstream SW condition is lacking.

One Solar Cycle of Heliosphere Observations with the Interstellar Boundary Explorer: Energetic Neutral Hydrogen Atoms Observed with IBEX-Lo from 10 eV to 2 keV

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.

MAVEN Observations of H− Ions in the Martian Atmosphere

AbstractAt Mars, charge exchange between solar wind protons and neutral exospheric hydrogen produces energetic neutral atoms (ENAs) that can penetrate into the collisional atmosphere, where they can be converted through collisions into H+ and H−. The Mars Atmosphere and Volatile EvolutioN mission observed a population of negatively charged particles at low altitudes, whose energies, angular distribution, and dependence on the upstream solar wind were consistent with H− produced by solar wind hydrogen ENAs. The highest fluxes of H− were observed near perihelion and the southern summer solstice. We calculated an average ratio of ∼4% between H− density and H+ density, implying a slightly smaller relative abundance than reported previously (∼10%). We found that the fraction of H ENAs converted to H− increases with the solar wind energy, in agreement with laboratory measurements of the H–CO2 electron capture cross section.

Time dependent hydrogen ENAs fluxes and structure of the heliosphere in the solar magneto-gravitational field

Energetic neutral atoms (ENAs) consisting mainly of hydrogen with energies of several keV and coming from the outer heliosphere have been detected by various spacecraft missions. The characteristics of the energy spectrum of ENAs include a dependence on the proton distribution at the heliospheric boundaries where the solar wind interaction with the local interstellar medium (LISM) occurs. We determine here the time-dependent differential flux for the neutral hydrogen atoms resulting from the kinetic exchange of energy in the outer heliosphere under the solar magnetic field. With the proton flow along the magnetic field lines, the mean kinetic energy of the particles is shown to increases due to the magnetic field line compression at the outer heliosphere, causing the acceleration of the solar wind protons to the observed range of speeds. Also, the toroidal field driven compression and expansion of the magnetic field lines corresponds to the temperature variations observed in the INCA line-of-sight observations of the ENAs in the inner heliosheath, where as the pickup ions retain their accelerated speed inside the heliosphere in the subsequent motion up to ∼1au.

Evidence for Energetic Neutral Hydrogen Emission from Solar Particle Events

Abstract We report the probable detection of energetic neutral hydrogen atoms (ENAs) at >0.8 MeV in several large solar energetic particle events observed between 1997 and 2004. The low Earth orbiting SAMPEX satellite detected transient increases of quasi-trapped equatorial protons beginning typically ∼3 hr after the X-ray flare and lasting for up to several hours. Since the magnetic cutoff rigidity is >10 GV at the magnetic latitude where the particles were observed, we interpret the signal as due to ENAs that penetrate Earth’s magnetic field and charge exchange in the upper atmosphere, whereupon the charged particles may become trapped. One event outside our survey period (2006 December 5) had previously reported solar flare ENAs, the only example of this phenomenon of which we are aware. Although the statistics are limited, the events we report suggest that the ENAs are produced as the flare-associated coronal mass wjection moves through the corona, as concluded previously for the 2006 December 5 event. The finding of ENAs emitted in conjunction with large solar flares opens a new avenue to understanding these events.

Breathing of the Heliosphere

Abstract In late 2016, the Interstellar Boundary Explorer (IBEX) observed an enhancement of hydrogen energetic neutral atom (ENA) flux in ∼20° south from the nose direction. This enhancement was a consequence of an abrupt increase of the solar wind (SW) dynamic pressure observed at 1 au in late 2014. In subsequent years, the increased flux of 4.3 keV ENAs was observed at higher latitudes filling in the heliosheath, in ENAs at lower energies, and the Ribbon flux. We observe that the rapid increase of SW pressure occurs every solar cycle (SC) from the beginning of the regular in situ SW measurements in the ecliptic plane. The SW pressure pulse happens about 4.7 yr from the beginning of each SC, it is during the maximum phase of solar activity, and repeats with a period of ∼10.2 yr. These repeating pulses of the SW pressure can cause periodic SC variations of the ENA production in the heliosheath. We follow McComas et al. results for the relation between SW pressure increase and ENA flux enhancement to investigate the periodic SW pressure increases and their consequences for the heliosphere. Our study of time delay between the cause (pressure pulse at 1 au) and the consequence (ENA enhancement) show that IBEX observed in 2009–2011 remnants of the SW pressure pulse that happened during the maximum of SC 23.

The Statistical Morphology of Saturn’s Equatorial Energetic Neutral Atom Emission

AbstractSaturn’s magnetosphere is an efficient emitter of energetic neutral atoms (ENAs), created through charge exchange of energetic ions with the extended neutral cloud originating from the icy moon Enceladus. We present an analysis using the complete image set captured by Cassini’s Ion Neutral Camera to characterize Saturn’s average ENA morphology. Concentric tori are formed around the planet by oxygen and hydrogen ENAs, with intensity peaks between 7 and 10 RS radial distance, with a ∼1–2 RS dayside offset. Nightside intensity is brighter than the dayside, likely the result of enhancements following large‐scale plasma injections from the magnetotail, and influence of the noon‐midnight electric field. Global intensity is clearly modulated with the near‐planetary rotation period. This Cassini‐era profile of Saturn’s ENA emission advances our understanding of how volcanic moons can influence plasma dynamics in giant magnetospheres and is timely ahead of the planned JUICE mission, which carries the first dedicated ENA detector to Jupiter.

Heliospheric energetic neutral atoms: Non-stationary modelling and comparison with IBEX-Hi data

ABSTRACT The interstellar boundary explorer (IBEX) has been measuring fluxes of the energetic neutral atoms (ENAs) using the IBEX-Hi (0.3–6 keV) instrument since 2008. We have developed a numerical time-depended code to calculate globally distributed flux (GDF) of hydrogen ENAs employing both (1) 3D kinetic-MHD model of the global heliosphere and (2) reconstruction of atom trajectories from 1 au, where they are observed by IBEX, to the point of their origin in the inner heliosheath (IHS). The key factor in the simulation is a detailed kinetic consideration of the pickup ions (PUIs), the supra-thermal component of protons in the heliosphere, which is ‘parental’ to the ENAs and originates in the region of the supersonic solar wind being picked by the heliospheric magnetic field. As a result of our study, we have concluded that (1) the developed model is able to reproduce the geometry of the multilobe structure seen in the IBEX-Hi GDF maps, (2) the GDF is extremely sensitive to the form of the velocity distribution function of PUIs in the IHS, and the accounting for the existence of an additional energetic population of PUIs is essential to explain the data, (3) despite a relatively good agreement, there are some quantitative differences between the model calculations and IBEX-Hi data. Possible reasons for these differences are discussed.

Emission of energetic neutral atoms measured on the lunar surface by Chang’E-4

The Advanced Small Analyzer for Neutrals (ASAN) is an international payload onboard the Chinese Chang’E-4 rover. It performed for the first time measurements of energetic neutral atoms (ENAs) on the lunar surface. We show a typical mass separated ENA energy spectrum measured by the ASAN. After normalizing to the impinging solar wind proton energy, good agreement to previous measurements from Chandrayaan-1 and IBEX is found. The hydrogen ENA albedo is estimated to be 32% for energy above 30 ​eV, comparable to the estimates obtained from Chandrayaan-1 and IBEX. Particle fluxes at lower energies are generally higher than those observed by Chandrayaan-1 and IBEX, and possible reasons are discussed.

Energy Spectral Properties of Hydrogen Energetic Neutral Atoms Emitted From the Dayside Atmosphere of Mars

AbstractThe energy spectrum of hydrogen energetic neutral atoms (HENAs) emitted from the dayside atmosphere of Mars reveals the physical processes of their origin and provides means for remote sensing on the Mars plasma environment. We first compile all available energy spectra of sunward flowing HENA using high‐energy resolution data from the neutral particle detectors on Mars Express from February to June 2004. The median value of the HENA directional flux is 1.12 × 105 cm−2·s−1·sr−1 from 0.3 to 5 keV, making Mars a bright source of HENAs. The energy spectra of the measured HENAs can be approximated by a two‐segment power law spectra. Then we compared the measured statistical energy spectra with those from a kinetic particle transport code. The comparison indicates that the measured HENA energy spectrum is harder (more high‐energy components) than the modeled backscattered HENAs. We propose that a population of sunward‐flowing, charge exchange HENAs produced in the dayside magnetosheath is the most probable reason for this difference. This HENA population with unexpected energy spectrum highlights the kinetic nature of the dayside magnetosheath of Mars.

Frustrated Coulomb explosion of small helium clusters

Almost ten years ago, energetic neutral hydrogen atoms were detected after a strong-field double ionization of H$_2$. This process, called 'frustrated tunneling ionization', occurs when an ionized electron is recaptured after being driven back to its parent ion by the electric field of a femtosecond laser. In the present study we demonstrate that a related process naturally occurs in clusters without the need of an external field: we observe a charge hopping that occurs during a Coulomb explosion of a small helium cluster, which leads to an energetic neutral helium atom. This claim is supported by theoretical evidence. As an analog to 'frustrated tunneling ionization', we term this process 'frustrated Coulomb explosion'.

Precipitation of Hydrogen Energetic Neutral Atoms at the Upper Atmosphere of Mars

AbstractWe study the properties of neutral hydrogen atoms precipitating onto the upper atmosphere of Mars. Energetic neutral atoms (ENAs) are produced by the charge exchange process between protons of solar wind (both upstream and shocked) as well as planetary origins and the Martian exospheric neutrals. Using a global hybrid plasma model for Mars‐solar wind interaction combined with an up‐to‐date exosphere model of Mars, we calculate the fluxes, spatial distributions, energy spectra, and direction distributions of hydrogen ENAs (H‐ENAs) at the Martian exobase for each source proton population. H‐ENAs originating from the upstream solar wind region and the magnetosheath dominate the precipitation. Two percent of the solar wind flux penetrates through the magnetic barrier as H‐ENAs under solar minimum conditions. The precipitating solar wind H‐ENA flux is axially symmetric about Sun‐Mars line, while the magnetosheath and planetary H‐ENAs have higher fluxes and a more‐extended precipitation area in the hemisphere where the convection electric field is pointing away from the planet, causing a significant precipitation beyond the terminator. The observed asymmetry is consistently explained by an asymmetric plasma flow in the dayside magnetosheath. The solar wind dynamic pressure increases the solar wind H‐ENA precipitation normalized by the upstream proton flux, due to a closer bow shock position and thus a higher exospheric column density for charge exchange. The spatial distribution of the magnetosheath solar wind and planetary H‐ENAs becomes more axially symmetric with increased dynamic pressure. The solar wind interaction with Mars exhibits more gas‐dynamic‐like signatures for higher dynamic pressure.

ARTEMIS Observations of Solar Wind Proton Scattering off the Lunar Surface.

We study the scattering of solar wind protons off the lunar surface, using ion observations collected over 6 years by the ARTEMIS satellites at the Moon. We show the average scattered proton energy spectra, directional scattering distributions, and scattering efficiency, for different solar wind incidence angles and impact speeds. We find that the protons have a scattering distribution that is similar to existing empirical models for scattered hydrogen energetic neutral atoms, with a peak in the backward direction (toward the Sun). We provide a revised model for the scattered proton energy spectrum. We evaluate the positive to neutral charge state ratio by comparing the proton spectrum with existing models for scattered hydrogen. The positive to neutral ratio increases with increasing exit speed from the surface but decreases with increasing impact speed. Combined, these counteracting effects result in a scattering efficiency that decreases from ~0.5% at 300 km/s solar wind speed to ~0.3% at 600 km/s solar wind speed.

Stochastic Acceleration of ∼0.1–5 keV Pickup Ions in the Heliotail

Abstract We seek to understand the quantitative role of the dominant physical processes (charge-exchange, adiabatic heating, stochastic acceleration) governing the proton distribution in the heliotail using observations of hydrogen energetic neutral atoms (ENAs) from the Interstellar Boundary Explorer (IBEX ). We solve the Parker transport equation for solar wind protons and pickup ions (PUIs) as they propagate from the termination shock (TS) down the heliotail, including charge-exchange between protons and neutral hydrogen atoms as source terms derived from an MHD-fluid and kinetic-neutral simulation of the heliosphere. We compute ENA fluxes at 1 au from the results of the proton transport model and compare them with IBEX observations. We find that, under the assumptions of our model, a stochastic acceleration process is needed to counteract the energy-dependent losses of ∼0.1–5 keV PUIs from charge-exchange to reproduce IBEX data. The power-law velocity dependence of the diffusion coefficient (spectral index γ) is limited to the range 0.67 < γ < 2, and the best fit to IBEX data appears close to γ ∼ 1.25. The diffusion rate ∼1.1 × 10−8 km2 s−3 (v/v 0)1.25 nearly balances the loss of ∼0.1–5 keV PUIs by charge-exchange. Our analysis suggests that cyclotron resonance with two widely known incompressible MHD turbulence: namely, isotropic Kolmogorov and anisotropic Goldreich–Sridhar turbulence, as well as stochastic particle interactions with compressive waves are not by themselves the dominant diffusion mechanisms. However, some intermediate processes may be occurring due to the presence of PUIs.

Modeling Emission of Heavy Energetic Neutral Atoms from the Heliosphere

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