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

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.

• The paper demonstrates that incorporating a dissipation term in MHD simulations reduces magnetic pressure spikes, aligning model outputs with Voyager data.
• It employs a phenomenological decay parameter (approximately 6 years) to convert magnetic energy into plasma heat, resolving prior overestimations of field pile-up.
• Results show refined heliospheric boundaries with improved magnetic field, density, and flow profiles, guiding future enhancements in space plasma modeling.

The Role of Magnetic Field Dissipation in the Inner Heliosheath: Towards Consistency Between Global Heliosphere Models and Voyager Observations

Introduction

Accurately modeling the interaction between the solar wind (SW) and the local interstellar medium (LISM) has remained a persistent challenge, particularly in the region between the termination shock (TS) and the heliopause (HP)—the inner heliosheath (IHS). While global ideal magnetohydrodynamic (MHD) simulations customarily predict a prominent magnetic pile-up within the IHS, in situ Voyager 1 and 2 observations have consistently indicated only a mild, gradual field enhancement. Discrepancy analysis points to unmodeled dissipation of magnetic energy in the current sheet folds unresolved in global models. This work by Korolkov, Baliukin, and Opher (2512.21688) rigorously quantifies the effect of including a physically-motivated magnetic field dissipation process—parameterized by a characteristic decay time—on the structure of the global heliosphere and its alignment with Voyager data.

Shortcomings of Conventional Global Models

Standard MHD treatments, often employing a unipolar solar magnetic field to circumvent the cost of resolving the heliospheric current sheet (HCS) at kinetic scales, systematically overestimate the magnetic pressure near the HP and predict a low-beta boundary layer with unphysical plasma depletion. Conversely, attempts to model the HCS with inadequate resolutions introduce excessive, grid-dependent numerical dissipation, artificially suppressing magnetic fields and violating their physical evolution. Both approaches fail to reconcile with Voyager constraints across both V1 and V2 trajectories.

The origin of this deficiency is the inability of global codes to account for energy release by fast, collisionless reconnection in the highly folded, compressed HCS prevalent throughout the IHS. These reconnection processes, as inferred from both theory and high-resolution kinetic simulations, are responsible for converting magnetic energy to plasma heat and thus limiting the field accumulation.

Phenomenological Modeling of Dissipation

The study introduces a controlled, phenomenological dissipation term into the MHD induction equation. This term damps the magnetic field on a prescribed timescale $\tau$, only within the IHS. A key property is that the dissipated energy is locally converted to plasma internal energy, securing total energy conservation and providing a heating source analogous to macroscopic Joule heating. The term is constructed to reduce only the magnitude, not the topology, of the field, and to satisfy solenoidality ($\nabla\cdot \mathbf{B} = 0$).

Numerical Implementation and Boundary Conditions

A stationary, 3D, single-fluid MHD framework is employed for charged species, with four-fluid treatment for neutrals—a choice justified by comparative analysis with high-fidelity kinetic models. Operator splitting is used to disentangle ideal MHD evolution from dissipative effects. Inner boundary conditions at 1 au utilize heliolatitude-resolved, time-averaged solar wind data, mass flux, and magnetic field derived from IPS, OMNI, and Lyman-$\alpha$ datasets. Outer LISM boundaries match recent consensus parameters.

Impact of Magnetic Dissipation: Model–Observation Comparison

A sweep of the dissipation timescale $\tau$ reveals that a value of approximately 6 years yields optimal agreement with Voyager 2 measurements for magnetic field magnitude, pressure, and proton density within the IHS. The introduction of the dissipation term dramatically suppresses the predicted magnetic pressure spike at the HP, yielding a local plasma $\beta$ of $\sim$4.5 in place of the unphysically magnetically dominated, plasma-depleted layer of the standard model.

Figure 1: Comparison between model and Voyager 2 data for magnetic/thermal pressure, proton density, and flow velocity; solid lines: model with $\tau=6$ yr, dashed: $\tau=\infty$ (no dissipation).

The removal of artificial plasma expulsion and the improved trend agreement in field magnitude underscore the necessity of including a physically motivated global-scale dissipation. Dissipation broadens the TS–HP separation, narrows the IHS, and shifts the TS outward by several au, a behavior validated across a range of $\tau$ values and in both V1/V2 directions. However, while improved, the model retains an IHS thickness above observations, highlighting the residual impact of unmodeled processes (such as heat conduction and time dependence).

Global Structure and Directional Dependence

The assessment of the global heliosphere shows that dissipation effects are inherently three-dimensional. The narrowing of the IHS and the outward movement of the TS are maximized in the tail and V2 directions, with little effect upwind. The positioning of discontinuities responds more sensitively to magnetic dissipation than to the inclusion of other minor energy exchange terms.

Figure 2: Termination shock and heliopause shapes for different dissipation timescales, highlighting their outward shift and IHS thinning with increased dissipation.

Physical Rationale and Theoretical Consistency

The appendices rigorously justify the form of the dissipation term for sector-dominated regions, connecting it to the curl of current density emerging from sheet folding and reconnection theory. They further show that the employed multi-fluid hydrogen treatment introduces at most minor discrepancies compared to full-kinetic treatments in the context of plasma structure, validating the computational approach for global scales.

Figure 3: Schematic illustrating the wavy HCS topology and current system, motivating the localization of dissipation.

Implications and Future Directions

This study demonstrates that only by explicitly modeling large-scale magnetic energy loss—at a rate compatible with reconnection in unresolved HCS structures—do global heliosphere simulations obtain field and density profiles that match observations. The determined dissipation timescale of $\sim$6 years emerges as a benchmark for future MHD-based studies. Nonetheless, the residual IHS width inconsistency and lack of kinetic-scale structures/fluxes indicate the need for greater model complexity, including variable dissipation rates, full kinetic energy partition (particularly for pickup ions), and time-dependent sector geometry. This line of research has considerable implications for interpreting IBEX/IMAP ENA measurements, cosmic ray modulation, and the microphysical modeling of large-scale plasma boundaries.

Conclusion

The inclusion of a tuned, physically motivated magnetic field dissipation term in global IHS models is essential for reconciling simulated and observed heliospheric structure. This work establishes a quantitative link between unmodeled reconnection-driven energy decay and the observed suppression of magnetic pile-up and plasma density depletion. While not a complete solution to all model–data discrepancies, it provides a robust step toward comprehensive, predictive global heliosphere modeling constrained by in situ spacecraft observations and motivates further coupling between global, turbulent dissipation models and local kinetic-scale physics.