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No meridional plasma flow in the heliosheath transition region

Abstract

Over a two-year period, Voyager 1 observed a gradual slowing-down of radial plasma flow in the heliosheath to near-zero velocity1 after April 2010 at a distance of 113.5 astronomical units from the Sun (1 astronomical unit equals 1.5 × 108 kilometres). Voyager 1 was then about 20 astronomical units beyond the shock that terminates the free expansion of the solar wind and was immersed in the heated non-thermal plasma region called the heliosheath. The expectation from contemporary simulations2,3 was that the heliosheath plasma would be deflected from radial flow to meridional flow (in solar heliospheric coordinates), which at Voyager 1 would lie mainly on the (locally spherical) surface called the heliopause. This surface is supposed to separate the heliosheath plasma, which is of solar origin, from the interstellar plasma, which is of local Galactic origin. In 2011, the Voyager project began occasional temporary re-orientations of the spacecraft (totalling about 10–25 hours every 2 months) to re-align the Low-Energy Charged Particle instrument on board Voyager 1 so that it could measure meridional flow. Here we report that, contrary to expectations, these observations yielded a meridional flow velocity of +3 ± 11 km s−1, that is, one consistent with zero within statistical uncertainties.

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Figure 1: Heliospheric plasma boundaries and flow regions relative to the location of Voyager 1.
Figure 2: Comparison of radial and azimuthal components of heliosheath plasma flow velocity at Voyager 2.
Figure 3: Roll measurements and derived components of heliosheath flow velocity.

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Acknowledgements

This work was supported at The Johns Hopkins University Applied Physics Laboratory by NASA contract NNN06AA01C. We thank J. Aiello for his assistance with our graphical presentation. We are grateful to the staff of the Voyager project for performing the Voyager 1 roll manoeuvres that made our analyses possible.

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R.B.D. performed the data analysis and contributed to the text; S.M.K. contributed to the text; E.C.R. contributed to the text and provided theory interpretation; and M.E.H. analysed elemental composition.

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Correspondence to Robert B. Decker.

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The authors declare no competing financial interests.

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Decker, R., Krimigis, S., Roelof, E. et al. No meridional plasma flow in the heliosheath transition region. Nature 489, 124–127 (2012). https://doi.org/10.1038/nature11441

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