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.
Subscribe to Journal
Get full journal access for 1 year
only $8.25 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
The data used in this paper are archived on an approximately annual basis in the Planetary Data System (https://pds.nasa.gov) and can also be found in the Space Physics Data Facility at https://cdaweb.gsfc.nasa.gov/misc/NotesV.html#VG2_PWS_LR (V2) or https://cdaweb.gsfc.nasa.gov/misc/NotesV.html#VG1_PWS_LR (V1).
Davis, L. E. Jr. Interplanetary magnetic fields and cosmic rays. Phys. Rev. 100, 1440–1444 (1955).
Parker, E. N. Interplanetary Dynamical Processes (Interscience, 1963).
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).
Zank, G. P. Faltering steps into the galaxy: the boundary regions of the heliosphere. Annu. Rev. Astron. Astrophys. 53, 449–500 (2015).
McComas, D. J. et al. IBEX observations of heliospheric energetic neutral atoms: current understanding and future direction. Geophys. Res. Lett. 38, L18101 (2011).
Frisch, P. C., Redfield, S. & Slavin, J. D. The interstellar medium surrounding the Sun. Annu. Rev. Astron. Astrophys. 49, 237–279 (2011).
Zank, G. P. Interaction of the solar wind with the local interstellar medium: theoretical perspective. Space Sci. Rev. 89, 413–688 (1999).
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).
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).
Krimigis, S. M. et al. Search for the exit: Voyager 1 at heliosphere’s border with the galaxy. Science 341, 141–147 (2013).
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).
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).
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).
Krimigis, S. M. et al. Energetic charged particle measurements from Voyager 2 at the heliopause and beyond. Nat. Astron. https://doi.org/10.1038/s41550-019-0927-4 (2019).
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).
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).
Bridge, H. S. et al. The plasma experiment on the 1977 Voyager mission. Space Sci. Rev. 21, 259–287 (1977).
Scarf, F. L. & Gurnett, D. A. A plasma wave investigation for the Voyager mission. Space Sci. Rev. 21, 289–308 (1977).
Gurnett, D. A. et al. Precursors to interstellar shocks of solar origin. Astrophys. J. 809, 121 (2015).
Gurnett, D. A. & Bhattacharjee, A. Introduction to Plasma Physics 2nd edn, 10 (Cambridge Univ. Press, 2017).
Bale, S. D. et al. The source region of an interplanetary type II radio burst. Geophys. Res. Lett. 26, 1573–1576 (1999).
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).
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).
Steinolfson, R. S., Pizzo, V. J. & Holzer, T. Gasdynamic models of the solar wind/interstellar medium interaction. Geophys. Res. Lett. 21, 245–248 (1994).
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).
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).
Gurnett, D. A. & Kurth, W. S. Electron plasma oscillations upstream of the solar wind termination shock. Science 309, 2015–2027 (2005).
Gurnett, D. A. & Kurth, W. S. Intense plasma waves at and near the solar wind termination shock. Nature 454, 78–80 (2008).
Richardson, J. VOYAGER 2 Data up Through October 26, 2018 (MIT Space Plasma Group, 2018); http://web.mit.edu/afs/athena/org/s/space/www/voyager/voyager_data/voyager_data.html; ftp://space.mit.edu/pub/plasma/vgr/v2/ha/key
Washimi, H., Tanaka, T. & Zank, G. P. Time-varying heliospheric distance to the heliopause. Astrophys. J. Lett. 846, L9 (2017).
Pogorelov, N., Heerikhuisen, J. & Zank, G. P. Probing heliospheric asymmetries with an MHD-kinetic model. Astrophys. J. 675, L41–L44 (2008).
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.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
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
The Astrophysical Journal (2021)
Scatter-free acceleration of particles by interaction with plasma shock waves in the interstellar medium
Monthly Notices of the Royal Astronomical Society (2021)
Magnetic Field and Plasma Density Observations of a Pressure Front by Voyager 1 during 2020 in the Very Local Interstellar Medium
The Astrophysical Journal (2021)
The Astrophysical Journal (2020)
Active Femto- and Nano-Objects in Relation to the Solar and Interstellar Winds in Anisotropic Models
Bulletin of the Russian Academy of Sciences: Physics (2020)