The heliopause is the boundary between the hot heliospheric (solar wind) plasma and the relatively cold interstellar plasma. Pressure balance considerations show that there should be a large (factor of 20 to 50) density increase across the heliopause. Here we report electron density measurements from the Voyager 1 and 2 plasma wave instruments near and beyond the heliopause. The plasma density in the outer heliosphere is typically about 0.002 cm−3. The first electron density measured by the Voyager 2 plasma wave instrument in the interstellar medium, 0.039 cm−3 ± 15%, was on 30 January 2019 at a heliocentric radial distance of 119.7 au. The density jump, about a factor of 20, confirms that Voyager 2 crossed the heliopause. The new density is very similar to the first density measured in the interstellar medium by the Voyager 1 plasma wave instrument, 0.055 cm−3, on 23 October 2013 at a radial distance of 122.6 au. These small differences in the densities and radial distances are probably due to the relative locations of the spacecraft in the boundary layer that forms in the interstellar plasma just beyond the heliopause.
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The authors thank J. Richardson and L. Burlaga for providing unpublished PLS and magnetometer data. The research at the University of Iowa was supported by NASA through contract 1622510 with the Jet Propulsion Laboratory.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Stephen Fuselier, G. P. Zank and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Gurnett, D.A., Kurth, W.S. Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments. Nat Astron 3, 1024–1028 (2019). https://doi.org/10.1038/s41550-019-0918-5
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