A small and round heliosphere suggested by magnetohydrodynamic modelling of pick-up ions

Abstract

As the Sun moves through the surrounding partially-ionized medium, neutral hydrogen atoms penetrate the heliosphere, and through charge-exchange with the supersonic solar wind, create a population of hot pick-up ions (PUIs). Until recently, the consensus was that the shape of the heliosphere, is comet-like. The Termination Shock crossing by Voyager 2 (V2) demonstrated that the heliosheath (HS) (the region of shocked solar wind) pressure is dominated by PUIs; however, their impact on the global structure of the heliosphere has not been explored. Here we use a novel magnetohydrodynamic model that treats the PUIs as a separate fluid from the thermal component of the solar wind. The depletion of PUIs, due to charge exchange with the neutral H atoms of the ISM in the HS, cools the heliosphere, “deflating” it and leading to a narrower HS and a smaller and rounder shape, confirming the shape suggested by Cassini observations. The new model reproduces both the properties of the PUIs, based on the New Horizon, and solar wind ions, based on the V2 spacecraft observations, as well as the solar like magnetic field data outside the heliosphere at Voyager 1(V1) and V2.