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

## Abstract

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.

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## 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.

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## Acknowledgements

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

Authors

### Contributions

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.

### Corresponding author

Correspondence to Stamatios M. Krimigis.

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### Competing interests

The authors declare no competing interests.

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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). https://doi.org/10.1038/s41550-019-0927-4

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