Evidence of a Thick Heliopause Boundary Layer Resulting from Active Magnetic Reconnection with the Interstellar Medium

Voyager 1 and 2 data from the vicinity of the heliopause and very local interstellar medium are reexamined to better understand the confounding lack of rotation in the magnetic field (B-field) across the heliopause observed by both Voyagers, despite their very large spatial separations (>100 au). Using three estimates for the orientation of the B-field in the pristine interstellar medium and four models of the heliosphere, we calculate draped interstellar B-field orientations along the model heliopauses and compare those estimates to the Voyager observations. At both Voyagers, expected draped B-fields are inconsistent with the observed B-field orientations after the boundary crossings. Furthermore, we show how the longer-term trends of the observed B-fields at both Voyagers after the crossings actually rotated away from both the expected draped B-field and the pristine interstellar B-field directions. We develop evidence, including an illustrative and analogous set of observations from Magnetospheric Multiscale spacecraft along Earth’s magnetopause, in support of a hypothesis that both Voyagers transited a thick boundary layer of reconnected magnetic flux along the heliopause surface. We estimate that Voyager 1 has not yet fully transited this boundary layer, the radial thickness of which at the Voyager 1 crossing location may be >18 au and likely much thicker. Meanwhile, at Voyager 2's crossing location, the boundary layer is likely much thinner, and for Voyager 2, we present evidence that Voyager 2 might already have transited the boundary layer and entered a region of fields and plasma that were never connected to the Sun—the very local interstellar medium.