Interstellar Boundary Explorer Observations Research Articles

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.

Constraints on the IBEX Ribbon’s Origin from Its Evolution over a Solar Cycle

In 2009, the Interstellar Boundary Explorer (IBEX) discovered a narrow “ribbon” of energetic neutral atom emissions across the sky with properties correlated with the solar wind latitudinal structure and the interstellar magnetic field draped around the heliosphere. It is widely believed that the ribbon is formed from the escape of heliospheric ENAs into the local interstellar medium and their eventual return as secondary ENAs. However, there is no consensus on the rate of pitch angle scattering of these PUIs before they become secondary ENAs. We test two opposing limits of scattering rates (“weak” versus “strong”) by solving a time-dependent model of the ribbon that evolves with the solar cycle, and we compare them to IBEX observations over 2009–2019. First, we find that both models qualitatively reproduce the evolution of IBEX fluxes for most of the data set, with a few exceptions, although the strong (or “spatial retention”) scattering model greatly underestimates the observed fluxes. Regardless, time dependence of fluxes cannot distinguish these models. Second, the ribbon’s geometric properties, i.e., its center and radius, are significantly different between the models. The spatial retention model reproduces the observed ribbon centers as a function of energy and time slightly better than the weak scattering model, and the spatial retention model reproduces the observed ribbon radius over energy and time almost perfectly, whereas the weak scattering model compares poorly. Our analysis favors the spatial retention mechanism as the source of the IBEX ribbon, but it requires modification to increase the flux of ENAs observed at 1 au.

Thermodynamics of the Inner Heliosheath

We derive annual sky maps of the proton temperature in the inner heliosheath (IHS), and track their temporal evolution over the years 2009–2016 of Interstellar Boundary Explorer observations. Other associated thermodynamic parameters also determined are the density, kappa (the parameter that characterizes kappa distributions), temperature rate, polytropic index, and entropy. We exploit the theory of kappa distributions and their connection with polytropes, to (i) express a new polytropic quantity Π that remains invariant along streamlines where temperature and density may vary, (ii) parameterize the proton flux in terms of the Π invariant and kappa, and (iii) derive the temperature and density, respectively, from the slope and intercept of the linear relationship between kappa and logarithm of Π. We find the following thermodynamic characteristics: (1) temperature sky maps and histograms shifted to their lowest values in 2012 and their highest in 2015; (2) temperature negatively correlated with density, reflecting the subisothermal polytropic behavior; (3) temperature positively correlated with kappa, revealing characteristics of the mechanism responsible for generating kappa distributions; (4) processes in IHS are subisothermal tending toward isobaric, consistent with previously published results; (5) linear relationship between kappa and polytropic indices, revealing characteristics of the particle potential energy; and (6) entropy positively correlated with polytropic index, aligned with the underlying theory that entropy increases toward the isothermal state where the kappa distribution reduces to the Maxwell–Boltzmann description.

Determining the Near‐Instantaneous Curvature of Earth's Bow Shock Using Simultaneous IBEX and MMS Observations

AbstractThe Interstellar Boundary Explorer (IBEX) mission provides a unique opportunity to study Earth's magnetosphere on a global scale via energetic neutral atom imaging. Due to the orbital configuration, IBEX is in a side‐viewing vantage point relative to the magnetosphere for a few months every year. This allows the Earth's magnetosphere to be imaged in the X‐Z geocentric solar ecliptic (GSE) plane. Using IBEX observations and near‐simultaneous in‐situ observations of Earth's bow shock (BS) from the Magnetospheric Multiscale mission, we determine the curvature of Earth's BS in the X‐Z GSE plane for 22 instances. We then compare the curvature to different upstream solar wind parameters in order to identify correlations with the contemporary physical conditions. We find that the solar wind dynamic pressure, as well as the interplanetary magnetic field (IMF) strength, X‐Y, and Y‐Z components are all linearly related to the curvature. We then investigate the time scales over which these correlations persist using variable time windows over which the upstream parameters are averaged. Lastly, we investigate the asymmetry of the BS in the X‐Z plane by comparing an asymmetry measure with different solar wind parameters. We find that the magnetic tension force from the IMF deflects the BS, and thus the underlying magnetopause obstacle. The results presented here provide the first near‐instantaneous observations of Earth's BS on a global scale, show the importance of IMF on generating asymmetries, and provide a basis for future modeling and observational studies of the BS.

Interstellar Neutral He Parameters from Crossing Parameter Tubes with the Interstellar Mapping and Acceleration Probe Informed by 10 yr of Interstellar Boundary Explorer Observations

Abstract The Sun's motion through the interstellar medium leads to an interstellar neutral (ISN) wind through the heliosphere. Several ISN species, including He, moderately depleted by ionization are observed with pickup ions and directly imaged. Since 2009, analyzed Interstellar Boundary Explorer (IBEX) observations returned a precise 4D parameter tube associated with the bulk velocity vector and the temperature of ISN flow distribution. This 4D parameter tube is typically expressed in terms of the ISN speed, the inflow latitudinal direction, and the temperature as a function of the inflow longitudinal direction and the local flow Mach number. We have used IBEX observations and those from other spacecraft to reduce statistical parameter uncertainties: V ISN ∞ = 25.99 ± 0.18 km s−1, λ ISN ∞ = 75 .° 28 ± 0 .° 13 , β ISN ∞ = −5 .° 200 ± 0 .° 075 , and T ISN ∞ = 7496 ± 172 K. IBEX ISN viewing is restricted almost perpendicular to the Earth–Sun line, which limits observations in ecliptic longitude to ∼130° ± 30° and results in relatively small uncertainties across the IBEX parameter tube but large uncertainties along it. Operations over the last three years enabled the IBEX spin axis to drift to the maximum operational offset (7°) west of the Sun, helping to break the ISN parameter degeneracy by weakly crossing the IBEX parameter tubes: the range of possible inflow longitudes extends over the range λ ISN ∞ = 75 .° 28 − 2.21 + 2.27 and the corresponding range of other ISN parameters is V ISN ∞ = 25.99 − 1.76 + 1.86 km s−1, β ISN ∞ = −5 .° 200 − 0.085 + 0.093 , and T ISN ∞ = 7496 − 1528 + 1274 K. This enhances the full χ 2 analysis of ISN parameters through comparison with detailed models. The next-generation IBEX-Lo sensor on IMAP will be mounted on a pivot platform, enabling IMAP-Lo to follow the ISN flow over almost the entire spacecraft orbit around the Sun. A near-continuous set of 4D parameter tube orientations on IMAP will be observed for He and for O, Ne, and H that cross at varying angles to substantially reduce the ISN flow parameter uncertainties and mitigate systematic uncertainties (e.g., from ionization effects and the presence of secondary components) to derive the precise parameters of the primary and secondary local interstellar plasma flows.

Turbulent Acceleration of Interstellar Pickup Ions at the Heliospheric Termination Shock Forms the Global ENA Spectrum

Abstract The heliospheric energetic neutral atom spectrum observed by the Interstellar Boundary Explorer (IBEX) reveals that the heliosheath proton distribution is consistent with a power law. The origin of the spectrum is likely from interstellar pickup ions (PUIs) accelerated at the heliospheric termination shock (HTS). We present an explanation of the proton spectrum origin using a test particle simulation of PUIs accelerated at the HTS. PUIs experience preferential heating by the motional electric field in the shock foot, but do not develop a power-law tail without the presence of turbulence at wavenumbers (k) close to the PUI gyroradius scale (R g). Voyager 2 observations of the magnetic field downstream of the HTS indicate a moderate amount of turbulence at kR g ≅ 1, δ B / B 0 2 ≅ 0.01 , which we find to be sufficient for producing a downstream suprathermal PUI tail but not at intensities observed by IBEX. Within the shock ramp, however, Voyager observed the turbulence power at much smaller scales to be nearly 100 times stronger, suggesting the possibility of strong turbulence at the PUI gyroradius scale. We show that a proton distribution can develop a power law downstream of the HTS consistent with IBEX observations if δ B / B 0 2 ≳ 0.1 at kR g ≅ 1 in the shock foot. Shock drift acceleration of PUIs by the motional electric field is aided by interactions with turbulence upstream of the shock overshoot. Steepening of the IBEX proton spectrum in directions farther from the heliospheric nose suggests the HTS compression ratio and/or turbulence power weakens near the heliotail.

Between Local Interstellar Magnetic and Dynamic Pressure Balance of Heliospheric Boundaries Measured with the IBEX Ribbon—A New Paradigm

Abstract The Interstellar Boundary Explorer (IBEX) mission discovered a global structure observed in energetic neutral atoms (ENA) emissions—the IBEX “ribbon.” We study the ribbon spatially retained by Alfvén waves. The IBEX ribbon width is used to derive Alfvén speeds in the modified local interstellar medium (LISM), which, averaged across the ribbon, is v A = 42.7 ± 3.0 km s−1. The increase of Alfvén speeds from polar regions to near the nose yields the compression ratio from the pristine LISM into the modified LISM as a function of angle from the ENA pressure maximum in the globally distributed flux. Averaged across the ribbon, the average compression ratio is r c = 1.63 ± 0.22. The compression ratios and Alfvén speeds across the ribbon provide for extrapolations into the pristine LISM, resulting in an average LISM Alfvén speed, v A−LISM = 31.7 ± 3.3 km s−1. With the LISM flow speed of u LISM = 25.82 ± 0.33 km s−1 derived from previous IBEX studies, we find that the ratio of the ram flow energy density to the magnetic energy density in the LISM is u LISM 2 / v A − LISM 2 = 0.66 ± 0.14 . This indicates that the heliosphere is between the extremes of dynamic and magnetic pressure balance. The complex and time-varying structure of the heliosphere is strongly influenced both by interactions with the interstellar magnetic field and the interstellar flow, representing a new paradigm that will be explored as we connect IBEX observations with those made in the future by the Interstellar Mapping and Acceleration Probe (IMAP).

Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions Beyond the Heliopause.

Direct sampling of interstellar neutral (ISN) atoms close to the Sun enables studies of the very local interstellar medium (VLISM) around the heliosphere. The primary population of ISN helium atoms has, until now, been assumed to reflect the pristine VLISM conditions at the heliopause. Consequently, the atoms observed at 1 au by the Interstellar Boundary Explorer (IBEX) were used to determine the VLISM temperature and velocity relative to the Sun, without accounting for elastic collisions with other species outside the heliopause. Here, we evaluate the effect of these collisions on the primary ISN helium population. We follow trajectories of helium atoms and track their collisions with slowed plasma and interstellar hydrogen atoms ahead of the heliopause. Atoms typically collide a few times in the outer heliosheath, and only ~1.5% of the atoms are not scattered at all. We use calculated differential cross sections to randomly choose scattering angles in these collisions. We estimate that the resulting primary ISN helium atoms at the heliopause are slowed down by ~0.45 km s-1 and heated by ~1100 K compared to the pristine VLISM. The resulting velocity distribution is asymmetric and shows an extended tail in the antisunward direction. Accounting for this change in the parameters derived from IBEX observations gives the Sun's relative speed of 25.85 km s-1 and temperature of 6400 K in the pristine VLISM. Finally, this paper serves as a source of the differential cross sections for elastic collisions with helium atoms.

Heliosheath Proton Distribution in the Plasma Reference Frame

Abstract Properties of the inner heliosheath (IHS) plasma are inferred from energetic neutral atom (ENA) observations by ∼1 au spacecraft. However, the Compton–Getting effect due to the plasma velocity relative to the spacecraft is rarely taken into account, even though the plasma speed is a significant fraction of the ENA speed. In this study, we transform Interstellar Boundary Explorer (IBEX) ENA spectra to the IHS plasma frame using flow profiles from a 3D heliosphere simulation. We find that proton spectra in the plasma frame are steeper by ∼30% to 5% at ∼0.5 to 6 keV, respectively, compared to ENAs in the spacecraft frame. While radial plasma flows contribute most to the Compton–Getting effect, transverse flows at mid/high latitudes and the heliosphere flanks account for up to ∼30% of the frame transformation for IBEX-Hi at ∼0.7 keV and up to ∼60% for IBEX-Lo at ∼0.1 keV. We determine that the majority of IHS proton fluxes derived from IBEX-Hi measurements in 2009–2016 are statistically consistent with power-law distributions, with mean proton index ∼2.1 and standard deviation ∼0.4. We find significantly fewer spectral breaks in IBEX observations compared to early analyses, which we determine were a product of the “ion gun” background prevalent in ∼2009–2012 before corrections made by McComas et al. in subsequent data releases. We recommend that future analyses of the IHS plasma utilizing ENA measurements take into account the Compton–Getting effect including radial and transverse flows, particularly IBEX and Interstellar Mapping and Acceleration Probe measurements below ∼10 keV.

Distance to the Energetic Neutral Hydrogen Source from the Heliotail

Abstract The interaction of the solar wind (SW) with the partially ionized interstellar medium forms the heliosphere. As the supersonic SW flows away from the Sun and incorporates pickup ions (PUIs), they are slowed, compressed, and heated at a termination shock, creating an energetic ion population in the inner heliosheath. The neutralization of PUIs in the heliosheath creates energetic neutral atoms (ENAs) at ∼keV energies that travel ballistically and can be observed at 1 au by the Interstellar Boundary Explorer (IBEX). IBEX uses single-pixel cameras to map ENAs from the heliosphere. In this study, we analyze IBEX observations of &gt;1 keV ENAs from the heliotail during 2009–2017. The ENA spectral index maximizes near the ecliptic plane and decreases at higher latitudes, reflecting the latitudinal structure of the SW. We show that the angular spread of this structure can be used to derive the distance at which the observed ENAs originate, i.e., their cooling length. Using Ulysses observations of the SW we determine that the distance from the Sun to the source of ∼1–6 keV ENAs in the heliotail is ≥289 ± 35 au in 2009–2013 and ≥489 ± 56 au in 2014–2017, using the distance to the termination shock in the downwind direction as 160 au based on the analysis of McComas et al. The increase in ENA source distance over time suggests that IBEX is observing a fast/hotter plasma parcel propagating down the heliotail before being replaced by slow/cooler plasma as the solar cycle evolves.

Asymmetric Structure of the Solar Wind and Heliosphere from IBEX Observations

Abstract Our heliosphere is formed by the interaction between the dynamic solar wind (SW) and partially ionized, local interstellar medium. “Pickup ions” (PUIs) are produced by charge exchange between SW ions and interstellar neutral atoms and propagate with the SW into the outer heliosphere. Neutralization of these PUIs in the heliosheath creates energetic neutral atoms (ENAs) that are observed near Earth by the Interstellar Boundary Explorer (IBEX). In this study, we analyze the evolution of ENAs observed by IBEX between ∼0.5 and 6 keV from 2009 to 2018, presenting the evolution of ENA spectral indices as a function of latitude in different regions of the sky. Our results show that IBEX ENA spectra reflect unique differences in the northern and southern polar coronal holes (PCHs) as a function of latitude and time. ENA spectra reflect a delayed closure of the southern PCH compared to the north by ∼1–2 yr in solar cycle 24, but then a rapid opening of the southern PCH beginning ∼1–2 yr later, in agreement with evolving SW speeds emanating from the PCHs. While ENA spectra from the front and flanks of the heliosphere exhibit similar behavior, ENA spectra from the back behave differently, suggesting that the heliotail and ENA source structures are significantly different. The mean ENA spectral index observed by IBEX evolves gradually and quasi-periodically over the ∼11 yr solar cycle, reflecting large-scale changes in the PCHs and SW speed. The standard deviation of the spectral index exhibits bumps at times when the SW dynamic pressure changes abruptly.

Turbulence in the Local Interstellar Medium and the IBEX Ribbon.

The effects of turbulence in the very local interstellar medium (VLISM) have been proposed by Giacalone & Jokipii (2015) to be important in determining the structure of the Interstellar Boundary Explorer (IBEX) ribbon via particle trapping by magnetic mirroring. We further explore this effect by simulating the motion of charged particles in a turbulent magnetic field superposed on a large-scale mean field, which we consider to be either spatially-uniform or a draped field derived from a 3D MHD simulation. We find that the ribbon is not double-peaked, in contrast to Giacalone & Jokipii (2015). However, the magnetic mirror force still plays an important role in trapping particles. Furthermore, the ribbon's thickness is considerably larger if the large-scale mean field is draped around the heliosphere. Voyager 1 observations in the VLISM show a turbulent field component that is stronger than previously thought, which we test in our simulation. We find that the inclusion of turbulent fluctuations at scales ≳100 au and power consistent with Voyager 1 observations produces a ribbon whose large-scale structure is inconsistent with IBEX observations. However, restricting fluctuations to <100 au produces a smoother ribbon structure similar to IBEX observations. Different turbulence realizations produce different small-scale features (≲10°) in the ribbon, but its large-scale structure is robust if the maximum fluctuation size is ≲50 au. This suggests that the magnetic field structure at scales ≲50 au is determined by the heliosphere-VLISM interaction and cannot entirely be represented by pristine interstellar turbulence.

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.

Effect of the Interstellar Magnetic Field Draping around the Heliopause on the IBEX Ribbon

Abstract A dominant feature in all-sky maps of energetic neutral atoms (ENAs) from NASA’s Interstellar Boundary EXplorer (IBEX) is a ribbon of enhanced fluxes. This ribbon was not predicted before IBEX, but all the up-to-date IBEX observations support a secondary ENA mechanism for the ribbon formation. There are two different classes of the secondary ENA model; the first class assumes a weak pitch angle scattering of energetic protons in the local interstellar medium (LISM), and the second class assumes strong, but spatially localized, scattering. A recent work by Gamayunov et al. strongly supports a “weak scattering” version, and here we extend the scope of the Gamayunov et al. model by using a more realistic interstellar magnetic field (ISMF) from our global MHD-plasma/kinetic-neutral simulation of the heliosphere–LISM interaction in place of the simple analytical model used previously. The main conclusions of our analysis are summarized as follows. (1) Pitch angle scattering of energetic protons in the LISM is not the primary mechanism that controls the width of ribbon. Instead, the ribbon width is mainly determined by draping of the ISMF around the heliopause. (2) The intensity of the ribbon is controlled by pitch angle scattering of energetic protons in the LISM. A model–IBEX comparison suggests that generation of the small-scale local turbulence is suppressed in the LISM. On the other hand, the large-scale interstellar turbulence (LSIT) alone or an interplay between the LSIT and the case of no scattering leads to a good agreement between the model and IBEX fluxes.

Constraining the Evolution of the Proton Distribution Function in the Heliotail

Abstract We use Interstellar Boundary Explorer (IBEX) measurements of energetic neutral atoms (ENAs) to constrain the proton (mostly pickup ion, PUI) distribution in the heliotail. In our previous study, we solved the Parker transport equation and found that the velocity diffusion coefficient D(v) for PUIs is approximately D(v) ∼ 1.1 × 10−8 km2 s−3 (v/v 0)1.3, assuming the initial proton distribution processed by the termination shock (TS), f p,0, is a kappa distribution with kappa index κ p,0 = 1.63. In this study, we test different forms for f p,0. We find that if f p,0 is kappa-distributed and D(v) = D 0(v/v 0)1.3, any kappa index in the range 1.5 &lt; κ p,0 &lt; 10 is consistent with IBEX data if D 0 ∼ 0.8–1.3 × 10−8 km2 s−3. While the case where D(v) ∝ v 1.3 yields ENA fluxes that appear to best reproduce IBEX data for any κ p,0, it is possible for D(v) to scale close to ∼v 2/3 or ∼v 2 within our uncertainties by changing D 0. We also show that an upstream PUI filled-shell distribution that is heated by a quasi-stationary TS, generating a downstream filled-shell with large cutoff speed, yields an excess of ENAs &gt;2 keV compared to IBEX. However, using a fully kinetic particle-in-cell simulation to process a PUI filled-shell across the TS yields ENA spectra consistent with IBEX, reinforcing the significance of self-consistent, preferential PUI heating and diffusion at the TS. Interestingly, an upstream PUI distribution inferred from the particle-in-cell simulation to reproduce Voyager 2 observations of the nose-ward TS is inconsistent with IBEX observations from the heliotail, suggesting differences in the upstream PUI distribution or TS properties.

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 &lt; γ &lt; 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.

Simulation of the Solar Wind Dynamic Pressure Increase in 2014 and Its Effect on Energetic Neutral Atom Fluxes from the Heliosphere

Abstract In late 2014, the solar wind dynamic pressure increased by ∼50% over a relatively short time (∼6 months). In early 2017, the Interstellar Boundary Explorer (IBEX) observed an increase in heliospheric energetic neutral atom (ENA) fluxes from directions near the front of the heliosphere. These enhanced ENA emissions resulted from the increase in SW pressure propagating through the inner heliosheath (IHS), affecting the IHS plasma pressure and emission of ∼keV ENA fluxes. We expand on the analysis by McComas et al. on the effects of this pressure change on ENA fluxes observed at 1 au using a three-dimensional, time-dependent simulation of the heliosphere. The pressure front has likely already crossed the termination shock (TS) in all directions, but ENA fluxes observed at 1 au will change over the coming years, as the TS, heliopause, and IHS plasma pressure continue to change in response to the SW pressure increase. Taken in isolation, the pressure front creates a “ring” of increasing ENA fluxes projected in the sky that expands in angular radius over time, as a function of the distances to the heliosphere boundaries and the ENA propagation speed. By tracking the position of this ring over time in our simulation, we demonstrate a method for estimating the distances to the TS, heliopause, and ENA source region that can be applied to IBEX data. This will require IBEX observations at 4.3 keV up through ∼2020, and longer times at lower ENA energies, in order to observe significant changes from the heliotail.

Heliosphere Responds to a Large Solar Wind Intensification: Decisive Observations from IBEX

Abstract Our heliosphere—the bubble in the local interstellar medium produced by the Sun’s outflowing solar wind—has finally responded to a large increase in solar wind output and pressure in the second half of 2014. NASA’s Interstellar Boundary Explorer (IBEX) mission remotely monitors the outer heliosphere by observing energetic neutral atoms (ENAs) returning from the heliosheath, the region between the termination shock and heliopause. IBEX observed a significant enhancement in higher energy ENAs starting in late 2016. While IBEX observations over the previous decade reflected a general reduction of ENA intensities, indicative of a deflating heliosphere, new observations show that the large (∼50%), persistent increase in the solar wind dynamic pressure has modified the heliosheath, producing enhanced ENA emissions. The combination of these new observations with simulation results indicate that this pressure is re-expanding our heliosphere, with the termination shock and heliopause already driven outward in the locations closest to the Sun. The timing between the IBEX observations, a large transient pressure enhancement seen by Voyager 2, and the simulations indicates that the pressure increase propagated through the heliosheath, reflected off the heliopause, and the enhanced density of the solar wind filled the heliosheath behind it before generating significantly enhanced ENA emissions. The coming years should see significant changes in anomalous cosmic rays, galactic cosmic radiation, and the filtration of interstellar neutral atoms into the inner heliosphere.

Interstellar Neutral Helium in the Heliosphere from IBEX Observations. V. Observations in IBEX-Lo ESA Steps 1, 2, and 3

Abstract Direct-sampling observations of interstellar neutral (ISN) He by the Interstellar Boundary Explorer (IBEX) provide valuable insight into the physical state of and processes operating in the interstellar medium ahead of the heliosphere. The ISN He atom signals are observed at the four lowest ESA steps of the IBEX-Lo sensor. The observed signal is a mixture of the primary and secondary components of ISN He and H. Previously, only data from one of the ESA steps have been used. Here, we extend the analysis to data collected in the three lowest ESA steps with the strongest ISN He signal, for the observation seasons 2009–2015. The instrument sensitivity is modeled as a linear function of the atom impact speed onto the sensor’s conversion surface separately for each ESA step of the instrument. We find that the sensitivity increases from lower to higher ESA steps, but within each of the ESA steps it is a decreasing function of the atom impact speed. This result may be influenced by the hydrogen contribution, which was not included in the adopted model, but seems to exist in the signal. We conclude that the currently accepted temperature of ISN He and velocity of the Sun through the interstellar medium do not need a revision, and we sketch a plan of further data analysis aiming at investigating ISN H and a better understanding of the population of ISN He originating in the outer heliosheath.