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Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments


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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Fig. 1: Trajectories of V1 and V2 in inertial (x, y, z) ecliptic coordinates.
Fig. 2: Typical electric-field waveforms of electron plasma oscillations and their associated electric-field power spectrum as detected by V1.
Fig. 3: Frequency–time spectrogram of electron plasma oscillations detected by the V1 PWS wideband receiver, and the corresponding local magnetic-field strength from the magnetometer.
Fig. 4: Three panel plots of the electric-field intensities from the V2 PWS 16-channel spectrum analyser.
Fig. 5: Comparison of the plasma number densities measured by V1 and V2.

Data availability

The data used in this paper are archived on an approximately annual basis in the Planetary Data System ( and can also be found in the Space Physics Data Facility at (V2) or (V1).


  1. 1.

    Davis, L. E. Jr. Interplanetary magnetic fields and cosmic rays. Phys. Rev. 100, 1440–1444 (1955).

    ADS  Article  Google Scholar 

  2. 2.

    Parker, E. N. Interplanetary Dynamical Processes (Interscience, 1963).

  3. 3.

    Axford, W. I. Introductory lecture—The heliosphere. In Physics of the Outer Heliosphere: Proc. 1st COSPAR Colloq. held in Warsaw, Poland, 19–22 September 1989 (eds Grzedzielski, S. & Page, D. E.) 7–15 (Pergamon, 1990).

  4. 4.

    Zank, G. P. Faltering steps into the galaxy: the boundary regions of the heliosphere. Annu. Rev. Astron. Astrophys. 53, 449–500 (2015).

    ADS  Article  Google Scholar 

  5. 5.

    McComas, D. J. et al. IBEX observations of heliospheric energetic neutral atoms: current understanding and future direction. Geophys. Res. Lett. 38, L18101 (2011).

    ADS  Article  Google Scholar 

  6. 6.

    Frisch, P. C., Redfield, S. & Slavin, J. D. The interstellar medium surrounding the Sun. Annu. Rev. Astron. Astrophys. 49, 237–279 (2011).

    ADS  Article  Google Scholar 

  7. 7.

    Zank, G. P. Interaction of the solar wind with the local interstellar medium: theoretical perspective. Space Sci. Rev. 89, 413–688 (1999).

    ADS  Article  Google Scholar 

  8. 8.

    Ajello, J. M., Stewart, A. I., Thomas, G. E. & Garps, A. Solar cycle study of interplanetary Lyman-alpha variations: Pioneer Venus Orbiter sky background results. Astrophys. J. 317, 964–968 (1987).

    ADS  Article  Google Scholar 

  9. 9.

    Stone, E. C. et al. Voyager 1 observes low energy galactic cosmic rays in a new region depleted of heliospheric ions. Science 341, 150–153 (2013).

    ADS  Article  Google Scholar 

  10. 10.

    Krimigis, S. M. et al. Search for the exit: Voyager 1 at heliosphere’s border with the galaxy. Science 341, 141–147 (2013).

    ADS  Article  Google Scholar 

  11. 11.

    Burlaga, L. F., Ness, N. F. & Stone, E. C. Magnetic field observations as Voyager 1 enters the heliosheath depletion region. Science 341, 147–150 (2013).

    ADS  Article  Google Scholar 

  12. 12.

    Gurnett, D. A., Kurth, W. S., Burlaga, L. F. & Ness, N. F. In situ observations of interstellar plasma with Voyager 1. Science 341, 1489–1492 (2013).

    ADS  Article  Google Scholar 

  13. 13.

    Stone, E. C., Cummings, A. C., Heikkila, B. C. & Lal, N. Cosmic ray measurements from Voyager 2 as it crossed into interstellar space. Nat. Astron. (2019).

  14. 14.

    Krimigis, S. M. et al. Energetic charged particle measurements from Voyager 2 at the heliopause and beyond. Nat. Astron. (2019).

  15. 15.

    Burlaga, L. F. et al. Magnetic field and particle measurements made by Voyager 2 at and near the heliopause. Nat. Astron. (2019).

  16. 16.

    Richardson, J. D., Belcher, J. W., Garcia-Galindo, P. & Burlaga, L. F. Voyager 2 plasma observations of the heliopause and interstellar medium. Nat. Astron. (2019).

  17. 17.

    Bridge, H. S. et al. The plasma experiment on the 1977 Voyager mission. Space Sci. Rev. 21, 259–287 (1977).

    ADS  Article  Google Scholar 

  18. 18.

    Scarf, F. L. & Gurnett, D. A. A plasma wave investigation for the Voyager mission. Space Sci. Rev. 21, 289–308 (1977).

    ADS  Article  Google Scholar 

  19. 19.

    Gurnett, D. A. et al. Precursors to interstellar shocks of solar origin. Astrophys. J. 809, 121 (2015).

    ADS  Article  Google Scholar 

  20. 20.

    Gurnett, D. A. & Bhattacharjee, A. Introduction to Plasma Physics 2nd edn, 10 (Cambridge Univ. Press, 2017).

  21. 21.

    Bale, S. D. et al. The source region of an interplanetary type II radio burst. Geophys. Res. Lett. 26, 1573–1576 (1999).

    ADS  Article  Google Scholar 

  22. 22.

    Gurnett, D. A., Kurth, W. S., Allendorf, S. C. & Poynter, R. L. Radio emission from the heliopause triggered by an interplanetary shock. Science 262, 199–203 (1993).

    ADS  Article  Google Scholar 

  23. 23.

    Burlaga, L. F., Ness, N. F., Gurnett, D. A. & Kurth, W. S. Evidence for a shock in interstellar plasma: Voyager 1. Astrophys. J. Lett. 778, L3 (2013).

    ADS  Article  Google Scholar 

  24. 24.

    Steinolfson, R. S., Pizzo, V. J. & Holzer, T. Gasdynamic models of the solar wind/interstellar medium interaction. Geophys. Res. Lett. 21, 245–248 (1994).

    ADS  Article  Google Scholar 

  25. 25.

    Baranov, V. B. & Malama, Yu. G. Model of the solar wind interaction with the local interstellar medium: numerical solution of the self-consistent problem. J. Geophys. Res. 98, 157–163 (1993).

    Article  Google Scholar 

  26. 26.

    Fuselier, S. A. & Cairns, I. H. The 2-3 kHz heliospheric radiation, the IBEX ribbon, and the three-dimensional shape of the heliopause. Astrophys. J. 771, 83 (2013).

    ADS  Article  Google Scholar 

  27. 27.

    Gurnett, D. A. & Kurth, W. S. Electron plasma oscillations upstream of the solar wind termination shock. Science 309, 2015–2027 (2005).

    ADS  Article  Google Scholar 

  28. 28.

    Gurnett, D. A. & Kurth, W. S. Intense plasma waves at and near the solar wind termination shock. Nature 454, 78–80 (2008).

    ADS  Article  Google Scholar 

  29. 29.

    Richardson, J. VOYAGER 2 Data up Through October 26, 2018 (MIT Space Plasma Group, 2018);;

  30. 30.

    Washimi, H., Tanaka, T. & Zank, G. P. Time-varying heliospheric distance to the heliopause. Astrophys. J. Lett. 846, L9 (2017).

    ADS  Article  Google Scholar 

  31. 31.

    Pogorelov, N., Heerikhuisen, J. & Zank, G. P. Probing heliospheric asymmetries with an MHD-kinetic model. Astrophys. J. 675, L41–L44 (2008).

    ADS  Article  Google Scholar 

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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.

Author information




D.A.G. is the Principal Investigator of the Voyager PWS investigation and W.S.K. the Co-Investigator. D.A.G. is responsible for the overall conduct of the investigation and wrote the initial draft of the paper. W.S.K. is responsible for the data processing and initial identification of the events discussed in the paper and has contributed comments and corrections to the paper.

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Correspondence to D. A. Gurnett.

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The authors declare no competing interests.

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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).

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