Shocks, compressible perturbations, and intermittency in the very local interstellar medium: Voyager 1 and 2 observations and numerical modeling

TL;DR

This work shows that solar-cycle–driven global compressions interacting with the heliopause can reproduce key VLISM perturbations observed by Voyager 1 and 2, including the V1 pf2 pressure front and the subsequent magnetic hump. Using a 3D data-driven MS-FLUKSS multifluid MHD model with PUIs and four neutral hydrogen populations, the authors test two ISMF configurations and four simulations, linking VLISM transients to solar wind dynamics and shock mergers near the HP. A turbulence analysis reveals time-dependent magnetic compressibility extending out to $\sim165\,\mathrm{au}$ on scales $<10$ days, with intermittency concentrated in specific interstellar intervals and a general decline after 2022. The results offer a unified explanation for the observed differences between V1 and V2, provide predictions for future VLISM perturbations (notably around 2024–2026 for V2 and around 2030–2031 for a major SC-25 event), and highlight the need for improved inner boundary conditions and turbulence transport models to better constrain pristine LISM conditions.

Abstract

Voyager spacecraft (V1 and V2) provide unique in situ measurements of perturbations propagating beyond the heliopause through the very local interstellar medium (VLISM), including the shocks and pressure fronts whose origin is debated. In particular, a jump in magnetic field strength, observed by V1 in 2020.4 at 149.3 au from the Sun, was followed by a distinct "hump" and persistently strong magnetic field, both requiring theoretical explanation. This paper offers an interpretation of those observations using a self-consistent, MHD model of the solar wind - LISM interaction driven by the OMNI and interplanetary scintillation data combined with a turbulence analysis of Voyager data. Our simulations convincingly demonstrate that global, solar-cycle-driven compressions, on hitting the heliopause, can reproduce those puzzling V1 observations. They appear to be associated with solar cycle 24, whereas similar interstellar magnetic field structures can occur once per cycle. The turbulence analysis reveals time-dependent magnetic compressibility that persists up to 165 au at scales below 10 days. Turbulence intermittency at scales below 1 hour is mostly confined to specific intervals, possibly associated with a broad foreshock region. The apparent disappearance of intermittency since 2022 reflects the turbulence weakening rather than a fundamental change in VLISM properties. We predict that V1 will record relatively strong magnetic field strengths until $\sim$2030, followed by weaker, infrequent perturbations. At V2, we expect multiple solar-driven compressions before 2026, followed by a major event induced by solar cycle 25 around 2030. New Horizons is expected to cross the termination shock at 80$\pm$ au in 2031.

Figures (15)

• Figure 1: Voyager 1 and 2 MAG data. Panels (a) and (b) show the magnetic field strength as a function of time at V1 and V2, respectively. Panel (c) shows the azimuthal angles as a function of spacecraft heliocentric distance in both the local RTN coordinate system (green, blue curves) and the ecliptic J2000 (HAE) coordinate system (orange and yellow curves). The latter allows the direct comparison between the two spacecraft. Panel (d) shows the elevation angles in the same format as panel (c). We additionally show the range of azimuthal and elevation angles of the far ISMF inferred by zirnstein2016 based on IBEX observations and modeling.
• Figure 2: Space-time distributions of the magnetic field magnitude for simulations A (top panels) and B (bottom panels). The horizontal panel break indicates the moment of time when the BCs at 1 au reach 2024.65 (realization r1) and are restarted from 1980.65 (realization r2). The red line indicates the exact V1 trajectory, while the white curves show the shifted trajectory designed to compensate for the discrepancy in the HP crossing for the two presented realizations.
• Figure 3: Details of the magnetic field magnitude distribution, during the time interval from 2010 to 2025, as obtained from simulations B and B1 with identical BCs.
• Figure 4: Interstellar magnetic field distributions at V1: comparison between the simulation results and V1 data. Panels (a) to (d) show the magnitude distributions for the different cases and different realizations of the 2012-2025 interval within each case. Panel (e) compares results of simulation A and simulation B, highlighting the long-term evolution. Panel (f) shows the individual components of the ISMF and their comparison with observational data. In each panel, the error bars represent the uncertainty associated with a $\pm 1.5$ variation in the V1 trajectory used for data extraction, reflecting the uncertainty in the HP position, while the solid curves show the simulation data extracted along the shifted trajectory shown in Fig. \ref{['fig:ST_B_MF014_MF015']} (white line).
• Figure 5: Tracing the origin of interstellar compressions back to the supersonic SW. Panels (a) and (b) show the space-time distributions of total pressure (for $R>R_{TS}$ only) and the scaled dynamic pressure, $p_\mathrm{dyn} (R/80)^2$ (in the supersonic SW region ($R<R_{TS}$) along the V1 and the NH trajectories, respectively. Panel (c) shows the scaled dynamic pressure of protons measured by NH (black points, Carrington-rotation averaged data) together with the simulated one. The perturbations that result in the interstellar compressions seen by V1, including the "hump" (h), are identified along the NH trajectory and labeled with different symbols.