Abstract
The Interstellar Boundary Explorer continues to observe the outer heliosphere’s response to the large (∼50%) increase in solar wind dynamic pressure at 1 au that began in late 2014 and continues today. The response of the outer heliosphere to this pressure increase resulted in enhanced energetic neutral atom (ENA) emission late in 2016 and much more enhanced emission in early 2017. The time evolution of enhanced emissions provides a measure of the distance to the termination shock and heliosheath over the sky, and the emission intensity provides unique insight into the plasma properties within the heliosheath. The initial ENA brightening was centered on the closest and thinnest region of the inner heliosheath, ∼20° south of the upwind direction. From early 2017 to early 2018, ENA emissions rapidly expanded northward to cover nearly the entire upwind direction, as the pressure increase encompassed heliosheath regions located progressively farther from the Sun. This preferential expansion shows that the next closest regions span the upwind side from the north-port to the south-starboard directions. These are consistent with the heliosphere being shaped by the combined flow and magnetic pressures of the local interstellar medium. The observations fit the expectations of the geometry shown by McComas & Schwadron and are inconsistent with recent suggestions of a roughly spherical heliosphere. The next few years will see the Sun’s enduring 2014 pressure enhancement propagate farther out in the heliosphere, generating increasingly broad regions of enhanced ENA emissions and exposing the directional-dependent distances to the termination shock and heliopause and underlying physics of the global heliospheric interaction.
