Unified Picture of the Local Interstellar Magnetic Field from Voyager and IBEX

Abstract

Prior to the Voyagers’ heliopause crossings, models and the community expected the magnetic field to show major rotations across the boundary. Surprisingly, the field showed no significant change in direction from the heliospheric Parker Spiral at either Voyager location. Meanwhile, a major result from the IBEX mission is the derived magnitude and direction of the interstellar field far from the Sun (∼1000 au) beyond the influence of the heliosphere. Using a self-consistent model fit to IBEX ribbon data, Zirnstein et al. reported that this “pristine” local interstellar magnetic field has a magnitude of 0.293 nT and direction of 227° in ecliptic longitude and 34.°6 in ecliptic latitude. These values differ by 27% (51%) and 44° (12°) from what Voyager 1 (2) currently observes (as of ∼2022.75). While differences are to be expected as the field undrapes away from the heliosphere, the global structure of the draping across hundreds of astronimcal units has not been reconciled. This leads to several questions: How are these distinct sets of observations reconcilable? What is the interstellar magnetic field’s large-scale structure? How far out would a future mission need to go to sample the unperturbed field? Here, we show that if realistic errors are included for the difficult-to-calibrate radial field component, the measured transverse field is consistent with that predicted by IBEX, allowing us to answer these questions through a unified picture of the behavior of the local interstellar magnetic field from its draping around the heliopause to its unfolding into the pristine interstellar medium.