The solar wind blows outwards from the Sun and forms a bubble of solar material in the interstellar medium. The heliopause (HP) is the boundary that divides the hot tenuous solar wind plasma in the heliosheath from the colder, denser very local interstellar medium (VLISM). The Voyager 2 plasma experiment observed the HP crossing from the solar wind into the VLISM on 5 November 2018 at 119 au. Here we present the first measurements of plasma at and near the HP and in the VLISM. A plasma boundary region with a width of 1.5 au is observed before the HP. The plasma in the boundary region slows, heats up and is twice as dense as typical heliosheath plasma. A much thinner boundary layer begins about 0.06 au inside the HP where the radial speed decreases and the density and magnetic field increase. The HP transition occurs in less than one day. The VLISM is variable near the HP and hotter than expected. Voyager 2 observations show that the temperature is 30,000–50,000 K, whereas models and observations predicted a VLISM temperature of 15,000–30,000 K.
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Data from the Voyager Plasma experiment are available at http://web.mit.edu/space/www/voyager.html
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The work at MIT is supported by NASA. Magnetic field and GCR data are shown courtesy of the Voyager Magnetometer (N. Ness, P.I.) and Cosmic Ray Subsystem (E. Stone, P.I.) teams. We thank G. S. Gordon Jr and L. A. Finck for development of, and assistance with, the plasma analysis.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks George Livadiotis, Brian Wood and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Richardson, J.D., Belcher, J.W., Garcia-Galindo, P. et al. Voyager 2 plasma observations of the heliopause and interstellar medium. Nat Astron 3, 1019–1023 (2019) doi:10.1038/s41550-019-0929-2
Nature Astronomy (2019)