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Domination of heliosheath pressure by shock-accelerated pickup ions from observations of neutral atoms


The solar wind blows an immense magnetic bubble, the heliosphere, in the local interstellar medium (mostly neutral gas) flowing by the Sun1. Recent measurements by Voyager 2 across the termination shock, where the solar wind is slowed to subsonic speeds before entering the heliosheath, found that the shocked solar wind plasma2 contains only 20 per cent of the energy released by the termination shock, whereas energetic particles3 above 28 keV contain only 10 per cent; 70 per cent of the energy is unaccounted for, leading to speculation2,3 that the unmeasured pickup ions or energetic particles below 28 keV contain the missing energy. Here we report the detection and mapping of heliosheath energetic (4–20 keV) neutral atoms produced by charge exchange of suprathermal ions with interstellar neutral atoms. The energetic neutral atoms come from a source 60° wide in longitude straddling the direction of the local interstellar medium. Their energy spectra resemble those of solar wind pickup ions, but with a knee at 11 keV instead of 4 keV, indicating that their parent ions are pickup ions energized by the termination shock. These termination-shock-energized pickup ions contain the missing 70 per cent of the energy dissipated in the termination shock, and they dominate the pressure in the heliosheath.

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Figure 1: The flow direction of energetic neutral atoms in the ecliptic plane.
Figure 2: The temporal variation of the 6.8 keV hydrogen ENA flux.
Figure 3: The map of the heliosheath source of ENAs versus ecliptic longitude.
Figure 4: Energy spectra of the particles.

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We thank R. Mewaldt for suggestions, G. Gloeckler and E. Roelof for discussions, D. Curtis for instrument development and explanations of calibration data, and the Voyager team for sharing their results before publication. This research was supported in part by the National Aeronautics and Space Administration.

Author Contributions L.W. analysed the data and wrote the paper with direction and help from R.P.L. R.P.L. designed the Suprathermal Electron instrument and is the co-investigator in charge. D.E.L. assisted with the experimental work. J.G.L. is the principal investigator of the IMPACT investigation on STEREO that includes STE.

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Correspondence to Linghua Wang.

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This file contains Supplementary Discussion; Supplementary Figure S1 with Legend; Supplementary Table 1 with Legend; and additional references. The Supplementary Discussion includes more detailed instrumentation for detection of energetic neutral atoms, data analysis methods and limitation. The Supplementary Figure S1 shows an example of energetic neutral atom observations by STE. The Supplementary Table 1 summarizes the changes in the energy densities and number densities of various plasma/energetic particle populations and magnetic fields across the termination shock. (PDF 258 kb)

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Wang, L., Lin, R., Larson, D. et al. Domination of heliosheath pressure by shock-accelerated pickup ions from observations of neutral atoms. Nature 454, 81–83 (2008).

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