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Abstract We introduce the first solar-cycle simulations from our 3D, global MHD-plasma/kinetic-neutrals model, where both hydrogen and helium atoms are treated kinetically, while electrons and helium ions are described as individual fluids. Using Voyager/PWS observations of electron density up to 160 au from the Sun for validation of several different global models, we conclude that the current estimates for the proton density in the local interstellar medium (LISM) need a revision. Our findings indicate that the commonly accepted value of 0.054 cm⁻³may need to be increased to values exceeding 0.07 cm⁻³. We also show how different assumptions regarding the proton velocity distribution function in the outer heliosheath may affect the global solution. A new feature revealed by our simulations is that the helium ion flow may be significantly compressed and heated in the heliotail at heliocentric distances exceeding ∼400 au. Additionally, we identify a Kelvin–Helmholtz instability at the boundary of the slow and fast solar wind in the inner heliosheath, which acts as a driver of turbulence in the heliotail. These results are crucial for inferring the properties of the LISM and of the global heliosphere structure. 

Abstract We present recent advancements in our 3D modeling of the interaction between the solar wind and the local interstellar medium (LISM). The latest model results (Fraternale et al., ApJ, 2023) have raised a question about the electron density of the LISM near the heliopause. We have shown that the presence of helium ions leads to a significant underestimation of this parameter compared to the past simulations and Voyager 1 PWS observations. The latter observations, with over 12 years’ worth of LISM data, offers a robust constraint on our models. Here we present additional simulations in support of the idea that the LISM proton density may need to be revised from approximately 0.054 cm^–3to values around 0.07 cm^–3or higher. Additionally, we have developed and successfully tested a new version of the kinetic code suitable for simulating time-dependent solutions. 

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.