The description of the local turbulent energy transfer, and the high-resolution ion distributions measured by the Magnetospheric Multiscale mission, together provide a formidable tool to explore the cross-scale connection between the fluid-scale energy cascade and plasma processes at sub-ion scales. When the small-scale energy transfer is dominated by Alfvénic, correlated velocity and PACS numbers: 94.05.-a, 94.05.Lk, 95.30.Qd

The understanding of inertial-scale dynamics in the heliosheath is not yet thorough. Magnetic field fluctuations across the inner heliosheath and the local interstellar medium are here considered to provide accurate and highly resolved statistics over different plasma conditions between 88 and 136 AU. By using the unique in situ 48-s measurements from the Voyager Interstellar Mission, we investigate different fluctuation regimes at the magnetohydrodynamic (MHD) scales, down to the MHD-to-kinetic transition. We focus on a range of scales exceeding five frequency decades (5 × 10 −8 < f < 10 −2 Hz), which is unprecedented in literature analysis. A set of magnetic field data for eight intervals in the inner heliosheath, in both unipolar and sector regions, and four intervals in the local interstellar medium is being used for the analysis. Results are set forth in terms of the power spectral density, spectral compressibility, structure functions and intermittency of magnetic field increments. In the heliosheath, we identify the energy-injection regime displaying a ∼ 1/f energy decay, and the inertial-cascade regime. Here, the power spectrum is anisotropic and dominated by compressive modes, with intermittency that can reach kurtosis values up to ten. In the interstellar medium the structure of turbulence is anisotropic as well, with transverse fluctuations clearly prevailing after May 2015. Here, we show that intermittent features occur only at scales smaller than 10 −6 Hz.

We discuss the observations and simulations related to the interaction of the solar wind (SW) and local interstellar medium (LISM), and the interstellar magnetic field draping around the heliopause (HP). This Letter sheds light on some processes that are not directly seen in the Voyager data. Special attention is paid to the magnetic field behavior at the HP crossing, penetration of shocks, and compression waves across the HP, and their merging in the LISM surrounding it. Modeling identifies forward and reverse shocks propagating through the heliosheath. Voyager data shows that the magnetic field strength experiences a jump at the HP, while the elevation and azimuthal angles are continuous across it. We show that our prior numerical results are in agreement with the Voyager data, if the heliospheric magnetic field is not assumed unipolar. The simulations confirm the importance of taking into account time dependencies of the SW flow, including the presence of transient structures and magnetohydrodynamic instabilities. For the first time, we provide the heliospheric community with the Alfvén speed distribution observed by Voyagers, which shows that it is unexpectedly small and decreases with distance from the HP. This is of critical importance for the identification of physical mechanisms responsible for the Langmuir wave and radio emission generation behind the HP. The data shows that outward-propagating, subcritical shocks traversing the LISM have a rather wide dissipation structure, which raises questions about their ability to reflect electrons as collisionless shocks can do.

Voyager 1 (V1) has been exploring the heliospheric boundary layer in the very local interstellar medium (VLISM) since 2012 August. The measurements revealed a spectrum of fluctuations over a vast range of space and timescales, but the nature of these fluctuations continues to be an intriguing question. Numerous manifestations of turbulence cannot be explained using a single phenomenology. Weak shocks and compressions are the prominent features of the VLISM. We use high-resolution (48 s) measurements to perform a multiscale analysis of turbulence at V1 between the years of 2013.36 and 2019.0 (124–144 au from the Sun). On relatively large scales, wave trains of mixed compressible/transverse nature with the correlation scale in the range of 15–100 days dominate the spectrum of fluctuations. The observed magnetic field profiles are suggestive of a Burgers-like (f −2) turbulence phenomenology induced by solar activity. We demonstrate that the level of large-scale compressible fluctuations is still significant in late 2018. We analyze the turbulence down to small scales comparable to the ion inertial length and show that magnetic compressibility is always large on these scales. Besides the shock-induced turbulence measured from 2014.486, the intensity and intermittency of small-scale fluctuations have been growing smoothly since 2018.5. Our analysis suggests that local processes are contributing to the production of turbulence on small scales. We present the estimates of transport coefficients in the plasma traversed by V1. The range of scales is identified where V1 measurements are affected by the contribution from pickup ions.

This review summarizes the current state of research aiming at a description of the global heliosphere using both analytical and numerical modeling efforts, particularly in view of the overall plasma/neutral flow and magnetic field structure, and its relation to energetic neutral atoms. Being part of a larger volume on current heliospheric research, it also lays out a number of key concepts and describes several classic, though still relevant early works on the topic. Regarding numerical simulations, emphasis is put on magnetohydrodynamic (MHD), multi-fluid, kinetic-MHD, and hybrid modeling frameworks. Finally, open issues relating to the physical relevance of so-called “croissant” models of the heliosphere, as well as the general (dis)agreement of model predictions with observations are highlighted and critically discussed.