Energetic charged particle measurements from Voyager 2 at the heliopause and beyond

Article metrics


The long-anticipated encounter by Voyager 2 (V2) of the region between the heliosphere and the very local interstellar medium (VLISM) occurred toward the end of 2018. Here, we report measurements of energetic (>28 keV) charged particles on V2 from the interface region between the heliosheath, dominated by heated solar wind plasma, and the VLISM, expected to contain cold non-solar plasma and the Galactic magnetic field. The number of particles of solar origin began a gradual decrease on 7 August 2018 (118.2 au), while those of Galactic origin (Galactic cosmic rays) increased ~20% in number over a period of a few weeks. An abrupt change occurred on 5 November when V2 was located at 119 au, with a decrease in the number of particles at energies of >28 keV and a corresponding increase in the number of Galactic cosmic rays of energy E > 213 MeV. This signature of the transition to the VLISM resembles, but is very different from, that observed on Voyager 1 at ~121.6 au, associated with the putative crossing of the heliopause some six years earlier.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Overview of GCR intensities and low-energy heliospheric ions, electrons and ACRs.
Fig. 2: Time history of composition of heliospheric ions near and beyond the HP.
Fig. 3: Angular distributions of heliospheric ACR at V1 and V2.
Fig. 4: Hot plasma properties in the HS near the HP and beyond at V1 and V2.
Fig. 5: Comparison of GCR and HS particles at V1 and V2 over about the same distance scale of 2.74 au surrounding the respective HP crossings.
Fig. 6: Schematic view of hot plasma anisotropies within the HS and upstream from the HP at V1 and V2.
Fig. 7: Concept of the global heliosphere summarizing the findings of V1 and V2.
Fig. 8: Propagation of the solar wind dynamic pressure from 1 au to the locations of V1 and V2 during crossings of the TS and HP.

Data availability

The V1 and V2 LECP measurements, including the in-situ LECP ion data used in this study, can be accessed through NASA’s public Planetary Data System (https://pds.nasa.gov/), while the solar wind dynamic pressure measurements can be accessed through the OMNI web page (ftp://spdf.gsfc.nasa.gov/pub/data/omni/low_res_omni/). Any other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


  1. 1.

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

  2. 2.

    Parker, E. N. The stellar-wind regions. Astrophys. J. 134, 20–27 (1961).

  3. 3.

    Witte, M., Banaszkiewicz, M. & Rosenbauer, H. Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment. Space Sci. Rev. 78, 289–296 (1996).

  4. 4.

    Krimigis, S. M. et al. Search for the exit: Voyager 1 at heliosphere’s border with the Galaxy. Science 341, 144–147 (2013).

  5. 5.

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

  6. 6.

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

  7. 7.

    Fisk, L. A. & Gloeckler, G. On whether or not Voyager 1 has crossed the heliopause. Astrophys. J. 789, 9 (2014).

  8. 8.

    Fisk, L. A. & Gloeckler, G. The Fisk & Gloeckler model for the nose region of the heliosheath: another model for Ed Stone to test. J. Phys. Conf. Ser. 767, 012008 (2016).

  9. 9.

    Decker, R. B. et al. Mediation of the solar wind termination shock by non-thermal ions. Nature 454, 67–70 (2008).

  10. 10.

    Krimigis, S. M. et al. The Low Energy Charged Particle (LECP) experiment on the Voyager spacecraft. Space Sci. Rev. 21, 329–354 (1977).

  11. 11.

    Decker, R. B., Krimigis, S. M., Roelof, E. C. & Hill, M. E. No meridional plasma flow in the heliosheath transition region. Nature 489, 124–127 (2012).

  12. 12.

    Richardson, J. D., Belcher, J. W., Garcia-Galindo, P. & Burlaga, L. F. Voyager 2 plasma observations of the heliopause and interstellar medium. Nat. Astron. https://doi.org/10.1038/s41550-019-0929-2 (2019).

  13. 13.

    Burlaga, L. F. et al. Magnetic field and particle measurements made by Voyager 2 at and near the heliopause. Nat. Astron. https://doi.org/10.1038/s41550-019-0920-y (2019).

  14. 14.

    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. https://doi.org/10.1038/s41550-019-0928-3 (2019).

  15. 15.

    Gurnett, D. A. & Kurth, W. S. Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments. Nat. Astron. https://doi.org/10.1038/s41550-019-0918-5 (2019).

  16. 16.

    Dialynas, K., Krimigis, S. M., Mitchell, D. G., Decker, R. B. & Roelof, E. C. The bubble-like shape of the heliosphere observed by Voyager and Cassini. Nat. Astron. 1, 0115 (2017).

  17. 17.

    Dialynas, K., Krimigis, S. M., Decker, R. B. & Mitchell, D. G. Plasma pressures in the heliosheath from Cassini ENA and Voyager 2 measurements: validation by the Voyager 2 heliopause crossing. Geophys. Res. Lett. 46, 7911–7919 (2019).

  18. 18.

    Burlaga, L. F. & Ness, N. F. Observations of the interstellar magnetic field in the outer heliosheath: Voyager 1. Astrophys. J. 829, 134 (2016).

  19. 19.

    Zirnstein, E. J. et al. Local interstellar magnetic field determined from the Interstellar Boundary Explorer ribbon. Astrophys. J. Lett. 818, L18 (2016).

  20. 20.

    Strauss, R. D. et al. Modelling anomalous cosmic ray oxygen in the heliosheath. Astron. Astrophys. 522, A35 (2010).

  21. 21.

    Roelof, E. C., Krimigis, S. M., Hill, M. E. & Decker, R. B. Evidence from Voyager 1/2 that anomalous cosmic rays are accelerated in the heliosheath ‘reservoir’. In Proc. AGU Fall Meeting SH31D-06R (AGU, 2016).

  22. 22.

    Jokipii, J. R. & Giacalone, J. The theory of anomalous cosmic rays. Space Sci. Rev. 83, 123–136 (1998).

  23. 23.

    Krimigis, S. M., Roelof, E. C., Decker, R. B. & Hill, M. E. Zero outward flow velocity for plasma in a heliosheath transition layer. Nature 474, 359–361 (2011).

  24. 24.

    Richardson, J. D. & Decker, R. B. Voyager 2 observations of plasmas and flows out to 104 au. Astrophys. J. 792, 126 (2014).

  25. 25.

    Florinski, V., Jokipii, J. R., Alouani-Bibi, F. & le Roux, J. A. Energetic particle anisotropies at the heliospheric boundary. Astrophys. J. Lett. 776, L37 (2013).

  26. 26.

    Krimigis, S. M. Voyager energetic particle observations at interplanetary shocks and upstream of planetary bow shocks: 1977–1990. Space Sci. Rev. 59, 167–201 (1992).

  27. 27.

    Krimigis, S. M. et al. Voyager 1 exited the solar wind at a distance of ~85 au from the Sun. Nature 426, 45–48 (2003).

  28. 28.

    Richardson, J. & Wang, C. The global nature of solar cycle variations of the solar wind dynamic pressure. Geophys. Res. Lett. 26, 561–564 (1999).

  29. 29.

    Le Chat, G. et al. The solar wind energy flux. Sol. Phys. 279, 197–205 (2012).

  30. 30.

    Izmodenov, V. V., Malama, Y. & Ruderman, M. S. Modeling of the outer heliosphere with the realistic solar cycle. Adv. Space Res. 41, 318–324 (2008).

  31. 31.

    Izmodenov, V. V. & Alexashov, D. B. Three-dimensional kinetic-MHD model of the global heliosphere with the heliopause-surface fitting. Astrophys. J. Supp. Ser. 220, 32 (2015).

  32. 32.

    Opher, M., Drake, J. F., Zieger, B. & Gombosi, T. I. Magnetized jets driven by the Sun: the structure of the heliosphere revisited. Astrophys. J. Lett. 800, L28 (2015).

  33. 33.

    Galli, A. et al. The roll-over of heliospheric neutral hydrogen below 100 eV: observations and implications. Astrophys. J. 821, 107 (2016).

  34. 34.

    Galli, A. et al. The downwind hemisphere of the heliosphere: eight years of IBEX-Lo observations. Astrophys. J. 851, 2 (2017).

  35. 35.

    Opher, M., Loeb, A., Drake, J. F. & Gabor, T. A predicted small and round heliosphere. Preprint at https://arxiv.org/abs/1808.06611 (2019).

  36. 36.

    Pogorelov, N. V., Stone, E. C., Florinski, V. & Zank, G. P. Termination shock asymmetries as seen by the Voyager spacecraft: the role of the interstellar magnetic field and neutral hydrogen. Astrophys. J. 668, 611–624 (2007).

  37. 37.

    McComas, D. J., Dayeh, M. A., Funsten, H. O., Livadiotis, G. & Schwadron, N. A. The heliotail revealed by the Interstellar Boundary Explorer. Astrophys. J. 771, 77 (2013).

  38. 38.

    Krimigis, S. M., Mitchell, D. G., Roelof, E. C. & Decker, R. B. ENA (E > 5 keV) images from Cassini and Voyager ‘ground truth’: suprathermal pressure in the heliosheath. AIP Conference Proc. 1302, 79–85 (2010).

  39. 39.

    Krimigis, S. M. et al. Characteristics of hot plasma in the Jovian magnetosphere: results from the Voyager spacecraft. J. Geophys. Res. 86, 8227–8257 (1981).

  40. 40.

    Hamilton, D. C., Gloeckler, G., Krimigis, S. M. & Lanzerotti, L. J. Composition of non-thermal ions in the Jovian magnetosphere. J. Geophys. Res. 86, 8301–8318 (1981).

  41. 41.

    Krimigis, S. M. et al. General characteristics of hot plasma and energetic particles in the Saturnian magnetosphere: results from the Voyager spacecraft. J. Geophys. Res. 88, 8871–8892 (1983).

  42. 42.

    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).

  43. 43.

    Decker, R. B. et al. Voyager 1 in the foreshock, termination shock, and heliosheath. Science 309, 2020–2024 (2005).

  44. 44.

    Stone, E. C. et al. Voyager 1 explores the termination shock region and the heliosheath beyond. Science 309, 2012–2020 (2005).

Download references


We are grateful to L. Burlaga and J. Richardson on the Voyager team, who shared their data with us before publication. Work at the Johns Hopkins University Applied Physics Laboratory is supported by NASA contract NNN06AA01C and by subcontract at the University of Maryland and Fundamental Technologies. We thank S. Nylund, J. Gunther, J. Manweiler and S. Lasley for their assistance in the data processing efforts. This paper is dedicated to the members of the original LECP team who are no longer with us, T. Armstrong, I. Axford and C. Y. Fan. We are grateful to the original Voyager Program Scientist at NASA headquarters, M. Mitz, whose advocacy of state-of-the-art instrumentation for Voyager resulted in comprehensive measurements through all the years of this pioneering mission.

Author information

All authors were actively involved in aspects of this manuscript. S.M.K. contributed most of the text, and R.B.D. and S.M.K. carried out most of the data analysis; R.B.D. and E.C.R. contributed to the text and provided theory and interpretations; K.D. provided the solar wind pressure analysis.

Correspondence to Stamatios M. Krimigis.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Krimigis, S.M., Decker, R.B., Roelof, E.C. et al. Energetic charged particle measurements from Voyager 2 at the heliopause and beyond. Nat Astron 3, 997–1006 (2019) doi:10.1038/s41550-019-0927-4

Download citation

Further reading