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 demonstrated that the heliosheath (the region of shocked solar wind) pressure is dominated by PUIs; however, the impact of the PUIs 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 hydrogen atoms of the interstellar medium in the heliosheath, cools the heliosphere, ‘deflating’ it and leading to a narrower heliosheath 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 Horizons observations, and the solar wind ions, based on the Voyager 2 spacecraft observations as well as the solar-like magnetic field data outside the heliosphere at Voyager 1 and Voyager 2.
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Our model is the OH module of SWMF and is available at http://csem.engin.umich.edu/tools/swmf/. The data produced by the model that support the findings of this study are available from the corresponding author upon reasonable request.
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We thank the staff at NASA Ames Research Center for the use of the Pleiades supercomputer under the award SMD-16-7616 and SMD-18-1875 and especially N. Carney. M.O. acknowledge discussions with A. Michael and M. Kornbleuth. M.O. and J.D. were partially supported by NASA grants NNH13ZDA001N-GCR and NNX14AF42G. A.L. acknowledges support from the Breakthrough Prize Foundation.
The authors declare no competing interests.
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Opher, M., Loeb, A., Drake, J. et al. A small and round heliosphere suggested by magnetohydrodynamic modelling of pick-up ions. Nat Astron 4, 675–683 (2020). https://doi.org/10.1038/s41550-020-1036-0
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