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Persistent plasma waves in interstellar space detected by Voyager 1


In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium1. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 au of interstellar space, revealing a large-scale density gradient2,3 and turbulence4 outside of the heliopause. Previous studies of the plasma density relied on the detection of discrete plasma oscillation events triggered ahead of shocks propagating outwards from the Sun, which were used to infer the plasma frequency and, hence, density5,6. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 data that persists from 2017 onwards and enables a steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling distance of 0.03 au. We find au-scale density fluctuations that trace interstellar turbulence between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and they could be clarified by new findings from Voyager or a future interstellar mission. The emission’s persistence suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.

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Fig. 1: Voyager 1 composite plasma wave spectrum.
Fig. 2: Weak narrowband plasma waves in the Voyager 1 dynamic spectrum.
Fig. 3: Peak frequency versus time of the narrowband plasma wave emission.
Fig. 4: De-fluctuated dynamic spectrum of the narrowband plasma wave emission.

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Data availability

The Voyager 1 data used in this work are archived through the NASA Planetary Data System ( Data and examples of the PWS data processing algorithms are also available through the University of Iowa Subnode of the PDS Planetary Plasma Interactions Node (


  1. 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).

    Article  ADS  Google Scholar 

  2. 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).

    Article  ADS  Google Scholar 

  3. Kurth, W. S. & Gurnett, D. A. Observations of a radial density gradient in the very local interstellar medium by Voyager 2. Astrophys. J. Lett. 900, L1 (2020).

    Article  ADS  Google Scholar 

  4. Lee, K. H. & Lee, L. C. Interstellar turbulence spectrum from in situ observations of Voyager 1. Nat. Astron. 3, 154–159 (2019).

    Article  ADS  Google Scholar 

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

    Article  ADS  Google Scholar 

  6. Gurnett, D. A. et al. A foreshock model for interstellar shocks of solar origin: Voyager 1 and 2 observations. Astron. J. 161, 11 (2021).

    Article  ADS  Google Scholar 

  7. Cairns, I. H. & Robinson, P. A. Theory for low-frequency modulated Langmuir wave packets. Geophys. Res. Lett. 19, 2187–2190 (1992).

    Article  ADS  Google Scholar 

  8. Hospodarsky, G. B. et al. Fine structure of Langmuir waves observed upstream of the bow shock at Venus. J. Geophys. Res. 99, 13363–13372 (1994).

    Article  ADS  Google Scholar 

  9. 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).

    Article  ADS  Google Scholar 

  10. Kim, T. K., Pogorelov, N. V. & Burlaga, L. F. Modeling shocks detected by Voyager 1 in the local interstellar medium. Astrophys. J. Lett. 843, L32 (2017).

    Article  ADS  Google Scholar 

  11. Huchra, J. P. & Geller, M. J. Groups of galaxies. I. Nearby groups. Astrophys. J. 257, 423–437 (1982).

    Article  ADS  Google Scholar 

  12. Pedregosa, F. et al. Scikit-learn: machine learning in Python. J. Mach. Learn. Res. 12, 2825–2830 (2011).

    MathSciNet  MATH  Google Scholar 

  13. Redfield, S. & Falcon, R. E. The structure of the local interstellar medium. V. Electron densities. Astrophys. J. 683, 207–225 (2008).

    Article  ADS  Google Scholar 

  14. Salpeter, E. E. Electron density fluctuations in a plasma. Phys. Rev. 120, 1528–1535 (1960).

    Article  ADS  MathSciNet  Google Scholar 

  15. Perkins, F. & Salpeter, E. E. Enhancement of plasma density fluctuations by nonthermal electrons. Phys. Rev. 139, 55–62 (1965).

    Article  ADS  Google Scholar 

  16. Dougherty, J. P. & Farley, D. T. A theory of incoherent scattering of radio waves by a plasma. Proc. R. Soc. Lond. A 259, 79–99 (1960).

    Article  ADS  Google Scholar 

  17. Carlson, H. C., Wickwar, V. B. & Mantas, G. P. Observations of fluxes of suprathermal electrons accelerated by HF excited instabilities. J. Atmos. Terr. Phys. 44, 1089–1100 (1982).

    Article  ADS  Google Scholar 

  18. Vierinen, J. et al. Radar observations of thermal plasma oscillations in the ionosphere. Geophys. Res. Lett. 44, 5301–5307 (2017).

    Article  ADS  Google Scholar 

  19. Meyer-Vernet, N., Issautier, K. & Moncuquet, M. Quasi-thermal noise spectroscopy: the art and the practice. J. Geophys. Res. 122, 7925–7945 (2017).

    Article  Google Scholar 

  20. Rickett, B. J. Radio propagation through the turbulent interstellar plasma. Annu. Rev. Astron. Astrophys. 28, 561–605 (1990).

    Article  ADS  Google Scholar 

  21. Spangler, S. R. & Gwinn, C. R. Evidence for an inner scale to the density turbulence in the interstellar medium. Astrophys. J. Lett. 353, L29 (1990).

    Article  ADS  Google Scholar 

  22. Bhat, N. D. R., Cordes, J. M., Camilo, F., Nice, D. J. & Lorimer, D. R. Multifrequency observations of radio pulse broadening and constraints on interstellar electron density microstructure. Astrophys. J. 605, 759–783 (2004).

    Article  ADS  Google Scholar 

  23. Rickett, B., Johnston, S., Tomlinson, T. & Reynolds, J. The inner scale of the plasma turbulence towards PSR J1644–4559. Mon. Not. R. Astron. Soc. 395, 1391–1402 (2009).

    Article  ADS  Google Scholar 

  24. Lee, K. H. & Lee, L. C. Turbulence spectra of electron density and magnetic field fluctuations in the local interstellar medium. Astrophys. J. 904, 66 (2020).

    Article  ADS  Google Scholar 

  25. Burlaga, L. F., Florinski, V. & Ness, N. F. Turbulence in the outer heliosheath. Astrophys. J. 854, 20 (2018).

    Article  ADS  Google Scholar 

  26. Cordes, J. M., Weisberg, J. M., Frail, D. A., Spangler, S. R. & Ryan, M. The galactic distribution of free electrons. Nature 354, 121–124 (1991).

    Article  ADS  Google Scholar 

  27. Krishnakumar, M. A., Mitra, D., Naidu, A., Joshi, B. C. & Manoharan, P. K. Scatter broadening measurements of 124 pulsars at 32 Mhz. Astrophys. J. 804, 23 (2015).

    Article  ADS  Google Scholar 

  28. Ocker, S. K., Cordes, J. M. & Chatterjee, S. Electron density structure of the local galactic disk. Astrophys. J. 897, 124 (2020).

    Article  ADS  Google Scholar 

  29. Zank, G. P., Nakanotani, M. & Webb, G. M. Compressible and incompressible magnetic turbulence observed in the very local interstellar medium by Voyager 1. Astrophys. J. 887, 116 (2019).

    Article  ADS  Google Scholar 

  30. Fraternale, F. & Pogorelov, N. V. Waves and turbulence in the very local interstellar medium: from macroscales to microscales. Astrophys. J. 906, 75 (2021).

    Article  ADS  Google Scholar 

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S.K.O., J.M.C., S.C. and S.R.S. acknowledge support from the National Aeronautics and Space Administration (NASA 80NSSC20K0784). S.K.O., J.M.C. and S.C. also acknowledge support from the National Science Foundation (NSF AAG-1815242) and are members of the NANOGrav Physics Frontiers Center, which is supported by the NSF award PHY-1430284. The research at the University of Iowa was supported by NASA through Contract 1622510 with the Jet Propulsion Laboratory.

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Authors and Affiliations



S.K.O. conducted the data analysis and wrote the initial draft of the paper. J.M.C., S.C., S.R.S. and S.K.O. are NASA Outer Heliosphere Guest Investigators on the Voyager Interstellar Mission. D.A.G. is the Principal Investigator of the Voyager PWS investigation and W.S.K. is a co-investigator of Voyager PWS and was responsible for the initial processing of the data at the University of Iowa. All authors contributed to the discussion of the results and commented on the draft.

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Correspondence to Stella Koch Ocker.

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The authors declare no competing interests.

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Peer review informationNature Astronomy thanks G. P. Zank and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Ocker, S.K., Cordes, J.M., Chatterjee, S. et al. Persistent plasma waves in interstellar space detected by Voyager 1. Nat Astron 5, 761–765 (2021).

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