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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. 

The role of pickup ions (PUIs) in the solar wind interaction with the local interstellar medium is investigated with 3D, multifluid simulations. The flow of the mixture of all charged particles is described by the ideal MHD equations, with the source terms responsible for charge exchange between ions and neutral atoms. The thermodynamically distinct populations of neutrals are governed by individual sets of gas dynamics Euler equations. PUIs are treated as a separate, comoving fluid. Because the anisotropic behavior of PUIs at the heliospheric termination shocks is not described by the standard conservation laws (a.k.a. the Rankine–Hugoniot relations), we derived boundary conditions for them, which are obtained from the dedicated kinetic simulations of collisionless shocks. It is demonstrated that this approach to treating PUIs makes the computation results more consistent with observational data. In particular, the PUI pressure in the inner heliosheath (IHS) becomes higher by ∼40%–50% in the new model, as compared with the solutions where no special boundary conditions are applied. Hotter PUIs eventually lead to charge-exchange-driven cooling of the IHS plasma, which reduces the IHS width by ∼15% (∼8–10 au) in the upwind direction, and even more in the other directions. The density of secondary neutral atoms born in the IHS decreases by ∼30%, while their temperature increases by ∼60%. Simulation results are validated with New Horizons data at distances between 11 and 47 au. 

Voyager 1 (V1) has been exploring the heliospheric boundary layer in the very local interstellar medium (VLISM) since August 2012. This study presents a broadband multi-scale analysis of VLSIM magnetic turbulence between 124 and 144 au from the Sun, as observed by V1 during the period from 2013.36 to 2019.0. We use high resolution 48-s data and show the existence of physically relevant fluctuations on scales as small as the ion inertial length in the thermal plasma. In the fine-scale regime below $$\sim 10^{-3}$$ au, an evidence is provided of the intermittent turbulence cascade which retains a significant level of magnetic compressibility. Observed fluctuations are compatible with the presence of filamentary structures and sawtooth-like waveforms of mixed compressible/transverse nature. A striking example of small-scale enhanced turbulence (wavelengths in the range of $$\sim 1-10^3$$ ion inertial lengths) is observed in front of the shock wave that overtook V1 on DOY 237, 2014 at 140 au from the Sun. This event starts on DOY 178, 2014, and suggests the presence of an ion foreshock. Besides, small-scale intermittency has been growing smoothly since 2018.5. Our analysis suggests that local processes are contributing to the production of turbulence in this regime. We identified the range of scales where V1 measurements may be affected by the contribution from pickup ions. On larger scales, coherent wave trains with the correlation time scale in the range of $15-100$ days dominate the spectrum of fluctuations. The spectral analysis is suggestive of a Burgers-like ($$f^{-2}$$) turbulence phenomenology induced by solar activity. Analysis of Coulomb collisional scales shows that the heliospheric boundary layer is not featureless at scales below the mean free path of $$\sim 1$$ au. 

Abstract Large-scale disturbances generated by the Sun’s dynamics first propagate through the heliosphere, influence the heliosphere’s outer boundaries, and then traverse and modify the very local interstellar medium (VLISM). The existence of shocks in the VLISM was initially suggested by Voyager observations of the 2-3 kHz radio emissions in the heliosphere. A couple of decades later, both Voyagers crossed the definitive edge of our heliosphere and became the first ever spacecraft to sample interstellar space. Since Voyager 1’s entrance into the VLISM, it sampled electron plasma oscillation events that indirectly measure the medium’s density, increasing as it moves further away from the heliopause. Some of the observed electron oscillation events in the VLISM were associated with the local heliospheric shock waves. The observed VLISM shocks were very different than heliospheric shocks. They were very weak and broad, and the usual dissipation via wave-particle interactions could not explain their structure. Estimates of the dissipation associated with the collisionality show that collisions can determine the VLISM shock structure. According to theory and models, the existence of a bow shock or wave in front of our heliosphere is still an open question as there are no direct observations yet. This paper reviews the outstanding observations recently made by the Voyager 1 and 2 spacecraft, and our current understanding of the properties of shocks/waves in the VLISM. We present some of the most exciting open questions related to the VLISM and shock waves that should be addressed in the future.