The Effect of Magnetic Field Dissipation in the Inner Heliosheath: Reconciling Global Heliosphere Model and Voyager Data

TL;DR

This work addresses the mismatch between global ideal MHD heliosphere models and Voyager observations, where magnetic pile-up in the inner heliosheath (IHS) is overestimated. The authors introduce a phenomenological magnetic-dissipation term in the induction equation, $\mathbf{Q}^B = -\mathbf{B}/\tau$, applied in the IHS to capture macroscopic effects of unresolved HCS reconnection without resolving kinetic scales. Calibrating the dissipation timescale to Voyager data, they find $\tau \approx 6$ years yields better agreement with observed magnetic-pressure and proton-density trends, reduces magnetic energy and narrows the IHS, and shifts the termination shock outward by a few AU. This approach preserves magnetic topology and provides a practical path to reconcile global heliospheric models with in situ measurements, while highlighting directions for refining the dissipation mechanism through sector-width dependence and diffusion effects.

Abstract

Global ideal magnetohydrodynamic models of the heliosphere typically predict a greatly exaggerated magnetic field pile-up in the inner heliosheath (IHS), the region between the termination shock and heliopause. However, Voyager 1 and 2 observations show only a gradual increase throughout this region. This mismatch is largely attributed to the simplified assumption of a unipolar solar magnetic field in many global models, which neglects the complex, folded structure of the heliospheric current sheet (HCS). The IHS, especially at low heliolatitudes, contains these compressed sector boundaries, widely considered prime locations for magnetic dissipation via reconnection. To align global model simulations with observations without incurring the prohibitive computational cost of resolving the kinetic-scale current sheet, this work introduces a phenomenological term into the magnetic field induction equation. This term captures the macroscopic effect of magnetic energy dissipation due to unresolved HCS dynamics. It is designed to mitigate the artificial magnetic pile-up, preserve the topological integrity of the magnetic field lines, and avoid explicit magnetic diffusion. This study demonstrates that incorporating a phenomenological dissipation term into global heliospheric models helps to resolve the longstanding discrepancy between simulated and observed magnetic field profiles in the IHS. The proposed mechanism reduces exaggerated magnetic energy (converts it into thermal energy), aligns model output with Voyager measurements of both magnetic field and proton density, and produces the outward shift in termination shock position and a reduction of the IHS thickness. We found that the characteristic time for magnetic field dissipation of about 6 years provides improved agreement with Voyager data in the IHS.

Figures (8)

• Figure 1: Panel (A) shows the magnetic (blue lines) and thermal (red lines) pressure profiles, panel (B) -- proton number density (green lines), and panel (C) -- plasma velocity module (magenta lines) along the V2 trajectory. The colored lines show the distribution of the model parameters with ($\tau$ = 6 years, solid lines) and without ($\tau = \infty$, dashed lines) magnetic dissipation taken into account. The black lines represent 14-day moving averages of V2 MAG (panel A) and PLS (panels B and C) instrument observations. The solid cyan lines represent linear fits to the data in the IHS, while the dashed cyan lines are the same approximations shifted to the model's values to highlight the consistency of the trends. The vertical dotted lines show the model TS and HP distances, while the vertical dashed lines correspond to the actual V2 crossings of the TS and HP.
• Figure 2: The description is the same as for Panel A in Figure \ref{['fig:v1_comparison']}, but comparison with the MAG instrument data along the V1 trajectory is shown.
• Figure 3: The model dependencies of the proton number density (left panel), thermal pressure (middle panel), and magnetic pressure (right panel) for a non-dissipative model ($\tau = \infty$, black lines) and models with dissipation timescales $\tau$ ranging from $\sim$2 to $\sim$10 years (colored lines) on the distance in the upwind direction. The thermal and magnetic pressures are normalized by the value p$_{\rm mag,LISM}$ = B$_{\rm LISM}^2 / (8\pi)$ of the magnetic pressure in the LISM.
• Figure 4: The heliopause (HP) and termination shock (TS) shapes in the BV-plane for models with different magnetic dissipation times: $\tau = \infty$ (no dissipation, black lines), $\tau = 7.07$ years (green lines), and $\tau = 2.36$ years (red lines). The directions towards the Voyager spacecraft (V1 and V2, dashed-dotted black lines) projected onto the BV-plane and the upwind direction (dashed black line) are shown for reference.
• Figure 5: Schematic view of the heliospheric current sheet wavy structure in the inner heliosheath. The plus and minus signs denote the regions of the positive and negative heliospheric magnetic field polarity. The black arrows show the magnetic field direction, and the blue arrows represent the direction of the current density.