The Voyager 1 and 2 spacecraft are now deep into the heliosheath, the most distant layer of the heliosphere, where the solar wind (a stream of charged particles ejected from the Sun) is still evident but is much slowed by the pressure of interstellar gas. Both Vikings are still sending back data and this paper reports the results of recent manoeuvres in which — after holding a steady course for 25 years — Voyager 1 was periodically rotated through 70° to allow the probe's charged-particle detectors to test predictions from magnetohydrodynamic models that the heliosheath's initially radial flow was being deflected polewards, towards meridional flow. Five such rolls have been completed, the last one on 30 January this year, and the results of the experiment are surprising. The meridional-flow velocity is low — almost zero — suggesting that Voyager 1 is not yet close to the heliopause, the theorectical point where the solar wind slows to a standstill.
- No meridional plasma flow in the heliosheath transition region (Letter p124, doi: 10.1038/nature11441)
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