Magnitude and direction of the local interstellar magnetic field inferred from Voyager 1 and 2 interstellar data and global heliospheric model

Abstract: In this Letter, we provide constraints on the direction and magnitude of the pristine (i.e., unperturbed by the interaction with the Sun) local interstellar magnetic field. The constraints are based on analysis of the interstellar magnetic field components at the heliopause measured by magnetometer instruments on board Voyager 1 and 2 spacecraft. The analysis was performed with the help of our kinetic-magnetohydrodynamical (MHD) model of the global heliosphere. The model shows that the solar-induced disturbances of the interstellar magnetic field are extended relatively far from the Sun up to 400-500 AU. The field is draped around the heliopause and compressed. By comparison of the model results with Voyager data we found that the model provides results comparable with the data for the interstellar magnetic field of $B_{LISM}$ = 3.7-3.8 $\mu$G in magnitude and directed towards $\approx$125$^\circ$ in longitude, and $\approx$37$^\circ$ in latitude in the heliographic inertial (HGI) coordinate system.

• The paper uses Voyager 1 and 2 data combined with a kinetic-MHD model to constrain the magnitude and direction of the local interstellar magnetic field.
• The model, aligned with Voyager data, suggests the IsMF magnitude is 3.7-3.8 μG, directed near 125° longitude and 37° latitude in the heliospheric inertial system.
• The findings show how the IsMF significantly shapes the heliosphere and suggest future research on magnetic field dissipation in the inner heliosheath.

Analysis of the Local Interstellar Magnetic Field Using Voyager Data and a Global Heliospheric Model

The paper by Izmodenov et al. presents a focused examination of the local interstellar magnetic field's (IsMF) magnitude and direction using data from the Voyager 1 and 2 spacecraft, augmented by a sophisticated kinetic-magnetohydrodynamical (MHD) model of the heliospheric interaction with the local interstellar medium (LISM). This research provides critical insights into the structure of the magnetic field as it interacts with the heliosphere, offering predictions aligned with observational data.

Methodological Framework

The study employs the Voyagers' magnetometer data to derive constraints on the pristine IsMF. By integrating the data with a detailed MHD model, the researchers examined how solar-induced disturbances extend the IsMF's influence over a considerable distance. The model suggests notable draping and compression of the magnetic field around the heliopause, with perturbations affecting LISM magnetic field measurements up to 400-500 AU from the Sun.

Key Findings

The model aligns closely with data for a magnetic field magnitude of approximately 3.7-3.8 $\mu$G, directed at about 125$^\circ$ longitude and 37$^\circ$ latitude in the heliographic inertial coordinate system. This consistency with observational data enhances the robustness of the model, supporting the proposed IsMF parameters.

Observational and Model Compliances

1. Heliopause Crossing Distances: The model predicts the heliopause's distance at Voyager crossing points with reasonable precision. The estimations for heliopause crossing differ marginally from actual distances by 2.5 AU, which is negligible and could be attributed to slight variations in proton or hydrogen atom number densities.
2. Termination Shock (TS) Asymmetry: The model highlights the intriguing asymmetry of the termination shock distances recorded by Voyager 1 and 2, revealing that the shape of the heliopause is modulated significantly by the magnetic field's inclination.
3. Magnetic Field Comparison: Close examination of magnetic field data immediately beyond the heliopause shows strong agreement with the model. The $B_R$, $B_T$, and $B_N$ components are relatively well-predicted, although the model shows a more consistent $B_R$ component across longer periods than observed in the data.
4. Radial and Tangential Velocity Components: The velocity components measured by Voyagers are in qualitative agreement with model outputs, particularly for Voyager 1, reflecting either validation or necessity for refinement, depending on the exact match for Voyager 2.

Theoretical and Practical Implications

The results contribute to a refined understanding of the heliosphere's interaction with LISM, suggesting that the magnetic field's local topography and dynamics play a pivotal role in shaping the heliosphere. The research also implies that future models may explore potential dissipation mechanisms within the inner heliosheath, indicated by discrepancies in magnetic field magnitudes.

Future Directions

As the current model aligns well but not perfectly, the research suggests opportunities for further refinement, particularly in examining the misalignment for velocity components and investigating dissipation phenomena. Continued exploration of larger parameter spaces could help in refining the understanding of magnetic field interactions at these astronomical scales.

Overall, Izmodenov et al. provide a substantive contribution to space science, elucidating the interplay between solar and interstellar forces and enhancing prediction and modeling techniques foundational for further explorations in astrophysical and heliospheric research.