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1. Heliosphere in the Local Interstellar Medium

   https://doi.org/10.3847/25c2cfeb.d7c10e40  

   Pogorelov, Nikolai V; Arge, Charles N; Bera, Ratan K; Burlaga, Leonard F; Fraternale, Federico; Gedalin, Michael; Heerikhuisen, Jacob; Kim, Tae K; Linker, Jon; Roytershtein, Vadim; et al (August 2023, Bulletin of the AAS)

   The Sun moves with respect to the local interstellar medium (LISM) and modifies its properties to heliocentric distances as large as 1 pc. The solar wind (SW) is affected by penetration of the LISM neutral particles, especially H and He atoms. Charge exchange between the LISM atoms and SW ions creates pickup ions (PUIs) and secondary neutral atoms that can propagate deep into the LISM. Neutral atoms measured at 1 au can provide us with valuable information on the properties of pristine LISM. New Horizons provides us with unique measurements of pickup ions in the SW region where they are thermodynamically dominant. Voyager 1 and 2 spacecraft perform in-situ measurements of the LISM perturbed by the presence of the heliosphere and relate them to the unperturbed region. The Interstellar Boundary Explorer (IBEX) makes it possible identify the 3-D structure of the heliospheric interface. We outline the main challenges in the physics of the SW–LISM interaction. The physical processes that require a focused attention of the heliospheric community are discussed from the theoretical perspective and space missions necessary for their investigation. We emphasize the importance of data-driven simulations, which are necessary for the interpretation and explanation of spacecraft data. 

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2. Transport of Interstellar Neutral Helium throughout the Heliosphere

   https://doi.org/10.3847/2041-8213/ac313c  

   Fraternale, Federico; Pogorelov, Nikolai V.; Heerikhuisen, Jacob (November 2021, The Astrophysical Journal Letters)

   Abstract A number of physical processes accompanying the solar wind interaction with the local interstellar medium (LISM) are governed by charge exchange between ions and neutral atoms of interstellar origin. A new, 3D, MHD-plasma/kinetic-neutral model is developed that self-consistently includes both neutral hydrogen and helium atoms, and their feedback on the plasma, through charge exchange and photoionization. Focusing on the transport of interstellar neutral helium, quantitative estimates are provided for bulk properties, deflection angles, and velocity distribution functions (VDFs) along the upwind direction. It is shown that the average deflection of secondary He atoms born in the outer heliosheath (OHS) from their original direction in the LISM is ∼12° in front of the heliopause, and occurs in the directions parallel to the plane formed by the velocity and magnetic field vectors in the unperturbed LISM. While these properties are consistent with Interstellar Boundary Explorer observations of the “warm breeze,” we show that charge exchange in the OHS leads to remarkable deviations of their VDF from the Maxwellian distribution. He atom filtration in the OHS results in a significant temperature anisotropy and VDF asymmetries, even for the primary helium atoms that experience no charge exchange at all. This is an entirely kinetic phenomenon that shows that primary He atoms observed at 1 au have distributions substantially different from those in the LISM. 

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3. Turbulence in the Outer Heliosphere

   https://doi.org/10.1007/s11214-022-00914-2  

   Fraternale, Federico; Adhikari, Laxman; Fichtner, Horst; Kim, Tae K.; Kleimann, Jens; Oughton, Sean; Pogorelov, Nikolai V.; Roytershteyn, Vadim; Smith, Charles W.; Usmanov, Arcadi V.; et al (September 2022, Space Science Reviews)

   Abstract The solar wind (SW) and local interstellar medium (LISM) are turbulent media. Their interaction is governed by complex physical processes and creates heliospheric regions with significantly different properties in terms of particle populations, bulk flow and turbulence. Our knowledge of the solar wind turbulence nature and dynamics mostly relies on near-Earth and near-Sun observations, and has been increasingly improving in recent years due to the availability of a wealth of space missions, including multi-spacecraft missions. In contrast, the properties of turbulence in the outer heliosphere are still not completely understood. In situ observations by Voyager and New Horizons , and remote neutral atom measurements by IBEX strongly suggest that turbulence is one of the critical processes acting at the heliospheric interface. It is intimately connected to charge exchange processes responsible for the production of suprathermal ions and energetic neutral atoms. This paper reviews the observational evidence of turbulence in the distant SW and in the LISM, advances in modeling efforts, and open challenges. 

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4. Three-dimensional Modeling of the Solar Wind and Local Interstellar Medium Interaction with Pickup Ions in the Presence of Heliospheric Current Sheet

   https://doi.org/10.1088/1742-6596/2742/1/012010  

   Bera, R K; Fraternale, F; Pogorelov, N V (April 2024, Journal of Physics: Conference Series)

   Our three-dimensional, time-dependent, multi-fluid model has been used to investigate the solar wind (SW)–local interstellar medium (LISM) interaction with pickup ions (PUIs) treated as a separate fluid. A non-zero, but fixed, angle between the Sun’s magnetic and rotation axis is adopted. The flow of the plasma mixture (thermal SW protons, PUIs, and electrons), is described by the system of ideal magnetohydrodynamic equations with the source terms responsible for charge exchange between ions and neutral atoms. Different populations of neutral atoms are governed by the individual sets of the Euler equations. As the standard Rankine–Hugoniot relations are not appropriate to describe the anisotropic behavior of PUIs at the termination shock, we use a kinetically-derived set of boundary conditions at it. We extend our previous work [1] and perform these new simulations on a Cartesian grid. This approach allows us to maintain a uniform grid resolution in all directions, without compromising resolution, at large distances from the Sun. The possibility of transition of the SW flow to a stochastic regime in the region between the termination shock and heliopause is further investigated. 

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5. Magnetohydrodynamic/Kinetic Modeling of the Interaction between the Solar Wind and the Local Interstellar Medium with He+, He2+, and Pickup H+ ions and H, He Atoms

   Fraternale, Federico; Pogorelov, Nikolai; Bera, Ratan (February 2025, https://doi.org/10.48550/arXiv.2502.01623)

   The interaction of the solar wind with the local interstellar medium (LISM) spans a wide range of interacting particle populations, energies, and scales. Sophisticated models are required to capture the global picture, interpret near-Earth observations, and ultimately understand the properties of the LISM at distances of thousands of AUs, where the medium is presumed to be unperturbed by this interaction. We present a new extension of our MHD-plasma/kinetic-neutral heliospheric model, implemented within the Multi-Scale Fluid- Kinetic Simulation Suite (MS-FLUKSS). The new model treats singly and doubly charged helium ions, pickup protons, and electrons as separate, self-consistently coupled populations, interacting through six charge exchange processes and photoionization with kinetically treated neutral hydrogen and helium atoms. In this paper, we provide detailed information on the implementation, including new fits for the charge-exchange cross sections, and demonstrate the functionality and performance of the new code 

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