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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The Voyager 1 data used in this work are archived through the NASA Planetary Data System (https://doi.org/10.17189/1519903). 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 (https://space.physics.uiowa.edu/voyager/data/).
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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.
The authors declare no competing interests.
Peer review information Nature 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 (2021). https://doi.org/10.1038/s41550-021-01363-7