Ions Measured by Voyager 1 Outside the Heliopause to ~28 au and Implications Thereof

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Abstract
We report 40–139 keV in situ ion measurements from the Voyager 1/Low Energy Charged Particle (LECP) instrument, over the time period from 2000 to the first 3 months of 2020, toward assessing energetic ion flow properties inside the heliosheath (HS) and upstream of the heliopause (HP). We identify an average radial outflow of ions at ∼46, 67, and 109 keV of ∼2.33 × 10−3, ∼1.37 × 10−3, and ∼7.27 × 10−4 ( cm 2 sr s keV ) –1, respectively, from the beginning of 2013 up to 2020, i.e., over a spatial range of ∼28 au past the HP. The raw 40–139 keV rates, uncorrected for background, show distinct intensity dropouts for particles with 90° pitch angles, in concert with the >211 MeV Galactic cosmic-ray anisotropy events, reported after Voyager 1 exited the HS in 2012 August. The intensity spectrum is consistent with a power-law form in energy with a spectral index of ∼−1.4, similar to energy spectra measured by both Voyager 1 and 2/LECP inside the HS, providing strong indication that these particles correspond to an ion population leaking from the HS into interstellar space, most likely due to the flux tube interchange instability at the boundary. Future modeling efforts should incorporate this ion population, which provides the framework for establishing a communication between the HS and the very local interstellar medium.

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