Details

Autor(en) / Beteiligte

• Decker, Robert B.
• Krimigis, Stamatios M.
• Roelof, Edmond C.
• Hill, Matthew E.

Titel

No meridional plasma flow in the heliosheath transition region

Ist Teil von

• Nature (London), 2012-09, Vol.489 (7414), p.124-127

Ort / Verlag

London: Nature Publishing Group UK

Erscheinungsjahr

2012

Quelle

EBSCOhost Psychology and Behavioral Sciences Collection

Beschreibungen/Notizen

• The radially outward flow of plasma from the Sun is expected to be deflected when it meets the flow of interstellar plasma through which the Solar System moves, but the spacecraft Voyager 1 unexpectedly finds that the deflected, meridional, flow is consistent with zero within the transition region. Voyager 1's roll in the heliosheath 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. Over a two-year period, Voyager 1 observed a gradual slowing-down of radial plasma flow in the heliosheath to near-zero velocity 1 after April 2010 at a distance of 113.5 astronomical units from the Sun (1 astronomical unit equals 1.5 × 10 8 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 simulations 2 , 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.