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This study provides a detailed analysis of five distant interplanetary shocks observed by the Solar Wind Around Pluto instrument on board New Horizons, which exhibit the signature of a suprathermal H ^+ pickup ion (PUI) tail in the downstream distribution. These shocks were observed with a PUI data cadence of approximately 24 hr, covering a heliocentric distance range of 23.71–36.75 au. The shock compression ratio varies between approximately 1.4 and 3.2. The H ^+ PUI density and temperature show a gradual increase across the shock, while the H ^+ solar wind density shows erratic behavior without a distinct downstream compression. The H ^+ PUI cooling index variation across the shock displays different characteristics in each shock. This study demonstrates, for the first time, the variation of the number density of downstream H ^+ PUI tails with the shock compression ratio, revealing an increase in tail density with stronger shocks. Additionally, theoretical estimates of reflected PUI number densities derived from the electrostatic cross-shock potential agree very well with the observed H ^+ PUI tail densities for stronger shocks.

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

This study provides a deeper understanding of how the solar wind evolves with increasing distance from the Sun as it encounters an increasing amount of interstellar material. This work extends our prior work by (1) extending the solar wind proton data radial profiles for New Horizons (NH) out to nearly 43 au, (2) quantifying the observed amount of slowing in the solar wind in the outer heliosphere by performing a detailed comparison between the speeds at NH (21–43 au) with speeds at 1 au, and (3) resolving discrepancies between the measured amount of slowing and estimates of the amount of slowing determined from the measured amount of interstellar pickup present in the solar wind. We find that the solar wind density radial profile may decrease at nearly or slightly less than a spherical expansion density profile. However, the temperature profile is well above what would be expected for an adiabatic profile. By comparing outer and inner heliospheric solar wind observations, we find the solar wind speed is reduced by 5%–7% between 30 and 43 au. We find the solar wind polytropic index (γsw) steeply decreases toward zero in the outer heliosphere (21–43 au) with a slope of ∼0.031 au−1. Using both this radial variation in γsw and the measured amount of interstellar pickup ions, we estimate the slowing in the solar wind and obtain excellent agreement with the observed slowing. [ABSTRACT FROM AUTHOR]

In the interaction between the solar wind (SW) and the local interstellar medium, various processes create ions with energies up to ∼10 keV that are out of thermal equilibrium with the “core” population. Wave–particle interactions tend to isotropize the velocity distributions, but the collisionless nature of the SW precludes thermalization. Suprathermal protons can charge-exchange with interstellar hydrogen, producing energetic neutral atoms that are seen by the Interstellar Boundary EXplorer spacecraft. We have developed a model for the presence of several suprathermal populations in the SW downstream of the heliospheric termination shock. The model uses magnetohydrodynamics to satisfy the first three moments of the total ion distribution, and couples these through charge-exchange to neutral hydrogen, conserving mass, momentum, and energy in the combined system. The proton population is separated into a cool core and three suprathermal populations, and hydrogen atoms may charge-exchange with protons from any of those four populations. The phase-space properties of the pick-up ions are selected based on data and theoretical considerations. In this paper we quantify the impact of suprathermal protons on the global structure of the heliosphere by comparing our new model to a traditional Maxwellian fluid model, and a kappa-distribution model. We find that the differences in momentum and energy transfer rates from the protons onto neutral hydrogen between the models leads to different plasma properties in the heliotail, and also changes the size of the heliosphere. Including the energy-dependent charge-exchange cross section into the collision integrals reduces the magnitude of these differences. [ABSTRACT FROM AUTHOR]

In-situ measurements taken by the Voyager 2 spacecraft suggest that the solar wind termination shock is significantly affected by the presence of pickup ions that are produced in the inner heliosphere due to charge exchange between interstellar neutrals and the solar wind ions. We use a fully kinetic particle-in-cell method to self-consistently simulate the shock with all physical properties available from Voyager 2. We have performed a set of simulations with varying velocity distribution functions for the pickup ions, since it was not determined by Voyager’s measurements. We show that the measurements suggest that the pickup ions upstream of the shock are more energetic than generally believed. If their velocity distribution function assumes a filled-shell shape in the wind frame, the maximum cutoff speed for the pickup ions should be ≳650 km s−1 in order to reproduce the measurements, which is almost twice the local wind speed. We suggest that pickup ions upstream of the shock are energized by adiabatic compression of the solar wind plasma as well as due to an enhanced level of turbulence in a broad foreshock region. [ABSTRACT FROM AUTHOR]

In this paper, we present results from our three-dimensional (3D) simulations of the interaction between the solar wind and local interstellar medium with an emphasis on the phase-space properties of energetic neutral atoms (ENAs) along a sight line that intersects the ribbon of enhanced ENA flux seen by NASA’s Interstellar Boundary EXplorer spacecraft. The majority of these ENAs have velocities directed away from the heliosphere, but it is believed that interactions between heliospheric ENAs and ions outside the heliosphere may result in a population of secondary ENAs that return to the heliosphere and generate the ribbon. While we do not consider the ion dynamics that result in secondary ENAs, our analysis is of key importance to the process since the heliospheric ENAs we consider form the source population for those ions. We present the moments of the hydrogen distribution, along with moments parallel and perpendicular to the local magnetic field for the pick-up ions (PUIs) that these neutrals generate. Finally, we present gyro-averaged velocity distributions relative to the local magnetic field for the PUIs created from our simulated H-atoms, along with analytic fits to these distributions in the secondary ENA source region just beyond the heliopause. [ABSTRACT FROM AUTHOR]