Past analysis has shown that the heliosphere structure can be deduced from correlations between long-scale solar wind pressure evolution and energetic neutral atom emissions. However, this required spatial and temporal averaging that smoothed out small or dynamic features of the heliosphere. In late 2014, the solar wind dynamic pressure increased by roughly 50% over a period of 6 months, causing a time and directional-dependent rise in around 2–6 keV energetic neutral atom fluxes from the heliosphere observed by the Interstellar Boundary Explorer. Here, we use the 2014 pressure enhancement to provide a simultaneous derivation of the three-dimensional heliospheric termination shock (HTS) and heliopause (HP) distances at high resolution from Interstellar Boundary Explorer measurements. The analysis reveals rippled HTS and HP surfaces that are oblique with respect to the local interstellar medium upwind direction, with significant asymmetries in the heliosphere structure compared to steady-state heliosphere models. We estimate that the heliosphere boundaries contain roughly ten astronomical unit-sized spatial variations, with slightly larger variations on the HTS surface than the HP and a large-scale, southwards-directed obliquity of the surfaces in the meridional plane. Comparisons of the derived HTS and HP distances with Voyager observations indicate substantial differences in the heliosphere boundaries in the northern versus southern hemispheres and their motion over time.

This study reports the first high-time-resolution observations of interstellar pickup ions (PUIs) in the outer heliosphere, including the first high-resolution observations of PUIs mediating shocks collected anywhere. These new data were enabled by a clever flight software reprogramming of the Solar Wind Around Pluto (SWAP) instrument on New Horizons to provide ∼30 minutes resolution as compared to the previous ∼24 hr time resolution. This time resolution is sufficient to resolve the shock structures and quantify the particle heating across these shocks. In the ∼10 months of initial data, we observed seven relatively small shocks, including one reverse shock. We find that the PUIs are preferentially compressed and heated across the shocks, indicating compression ratios from ∼1.2–1.8, with little heating for values less than ∼1.5 and progressively more PUI heating for larger compression ratios. In contrast, core solar wind properties did not show consistent changes across the shocks, indicating that these particles (1) participate little in the large-scale fluid-like interactions of the outer heliosphere’s combined solar wind and PUI plasma and (2) cannot be used to characterize PUI-mediated shocks as prior studies sought to do. All six forward shock crossings showed gradual increases in PUI pressure over shock widths of ∼0.05–0.13 au, which is roughly three decades larger than characteristic particle scales such as the PUI gyroradii. The new high-resolution observations and results described here are important for understanding shocks in the outer heliosphere, the termination shock, and more broadly for PUI-mediated shocks across many astrophysical systems.

In this study, we estimate the heliospheric termination shock (HTS) compression ratio at multiple directions in the sky from a quantitative comparison of the observed and simulated inner heliosheath (IHS) energetic neutral atom (ENA) fluxes. We use a 3D steady-state simulation of the heliosphere to simulate the ENA fluxes by postprocessing the MHD plasma using a multi-Maxwellian distribution for protons in the IHS. The simulated ENA fluxes are compared with time exposure–averaged IBEX-Hi data for the first 3 yr of the mission. The quantitative comparison is performed by calculating the fractional difference in the spectral slope between the observed and simulated ENA fluxes for a range of compression ratios, where the simulated ENA spectrum is varied as a function of downstream pickup ion temperature as a function of compression ratio. The estimated compression ratio in a particular direction is determined by the minimum value of the fractional difference in spectral slope. Our study shows that the compression ratio estimated by this method is in close agreement with the large-scale compression ratio observed by Voyager 2 in its travel direction. Also, the compression ratio in other directions near the ecliptic plane is similar to the compression ratio at the Voyager 2 direction. The weakest shock compression is found to be on the port side of the heliosphere at direction (27°, 15°). This is the first study to estimate the HTS compression ratio at multiple directions in the sky from IBEX data.

We present statistical comparisons between energetic neutral atom (ENA) fluxes obtained using a global simulation of the heliosphere and data collected by the Interstellar Boundary Explorer (IBEX) spacecraft. The simulation of the inner heliosheath (IHS) ENA flux is based on a 3D steady-state heliosphere, while the data are from the IBEX-Hi instrument over the time period 2009–2015. The statistical comparison is performed by calculating the chi-square value between the simulated ENA fluxes and the data for each line of sight in the sky. A comparison with exposure-averaged data for solar minimum and solar maximum conditions is also performed to see the effect of solar wind (SW) properties on the IHS ENA fluxes. The model matches well with the data in the flanks and parts of the nose of the heliosphere, whereas the match is poor in the downwind tail, ribbon, and polar regions. We interpret these results to mean that (i) heliosheath plasma in the polar region consists of advected fast (or slow) SW during the solar minimum (or maximum) condition, and (ii) heliospheric termination shock parameters are likely different over the poles. A poor match at around 30° north and south of the downwind direction is likely due to the existence of a mixture of plasma that comes from fast and slow SW. While our results are consistent with a single heliotail, the shape of the heliosphere continues to be an area of active research, and more data and further modeling are needed to determine its true structure.

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.

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<p>Nonthermal energetic pickup ions (PUIs), created in the heliosphere by charge exchange between solar wind ions and interstellar neutral atoms, play an essential role in understanding solar wind evolution in the outer heliosphere and the structure and dynamics of the global heliosphere. New Horizons spacecraft, launched in 2006, is now located at about 55 au from the Sun, exploring the outer heliosphere, and is the only spacecraft equipped with proper instruments to measure nonthermal energetic pickup ions (PUIs) in the outer heliosphere for the first time. Its observations showed that energetic PUIs dominate the internal pressure of the outer heliosphere, with PUI pressures larger than the thermal solar wind and magnetic pressures outside ~ 20 au. At these distances, PUIs contribute substantially to heating and slowing down the solar wind. Moreover, New Horizons observations showed that PUIs mediate shock waves in the outer heliosphere. Here, we give an overview of the energetic particles in the outer heliosphere and their effect on shocks. We present the in situ observations of the hydrogen and Helium PUIs made by New Horizons' SWAP and PEPSSI instruments. Finally, we present some of the most important open questions related to the outer heliosphere that future studies and space missions should address.</p>

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.