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Zero outward flow velocity for plasma in a heliosheath transition layer


Voyager 1 has been in the reservoir of energetic ions and electrons that constitutes the heliosheath since it crossed the solar wind termination shock1,2,3 on 16 December 2004 at a distance from the Sun of 94 astronomical units (1 au = 1.5 × 108 km). It is now 22 au past the termination shock crossing4. The bulk velocity of the plasma in the radial–transverse plane has been determined5 using measurements of the anisotropy of the convected energetic ion distribution6. Here we report that the radial component of the velocity has been decreasing almost linearly over the past three years, from 70 km s−1 to 0 km s−1, where it has remained for the past eight months. It now seems that Voyager 1 has entered a finite transition layer of zero-radial-velocity plasma flow, indicating that the spacecraft may be close to the heliopause, the border between the heliosheath and the interstellar plasma. The existence of a flow transition layer in the heliosheath contradicts current predictions7—generally assumed by conceptual models—of a sharp discontinuity at the heliopause.

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Figure 1: Directional velocity measurements from Voyager 1 at the edge of the heliosphere.
Figure 2: The heliosphere and its boundaries in the general direction of Voyager 1.


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This work was supported at The Johns Hopkins University Applied Physics Laboratory by NASA. We are grateful to J. Aiello (for assistance with our graphical presentation) and R. McNutt (for a historical summary of heliosheath terminology).

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S.M.K. contributed most of the text; E.C.R. contributed to the text and provided theory interpretation; and R.B.D. performed the data analysis with the assistance of M.E.H.

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Correspondence to Stamatios M. Krimigis.

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The authors declare no competing financial interests.

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Krimigis, S., Roelof, E., Decker, R. et al. Zero outward flow velocity for plasma in a heliosheath transition layer. Nature 474, 359–361 (2011).

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