Astrophysics

To an Instability and Beyond

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Science  27 Jun 2003:
Vol. 300, Issue 5628, pp. 2005
DOI: 10.1126/science.300.5628.2005b

Just like Buzz Lightyear speeding through the atmosphere in his own bubble, the Sun creates a Buzz-like bubble of supersonic solar wind that rams into the interstellar medium. This tapering bubble is called the heliosphere and essentially defines the edge of the solar system. The gas and particles in the solar upwind are supersonic until they reach the termination shock at about 90 astronomical units (AU, where 1 AU is the mean distance between the Sun and Earth). The wind then travels subsonically and starts to flow in the direction of the interstellar medium in the heliosheath until it reaches the heliopause, where the interstellar and solar winds meet at about 110 AU.

Opher et al. developed a three-dimensional magnetohydrodynamic code, BATS-R-US, to discover that a shear velocity instability forms just beyond the termination shock, causing a jet and sheetlike feature to oscillate up and down in the heliosheath. Such an instability could alter the entry of interstellar particles and the mixing between the interstellar and solar winds, which might alter the make-up of the solar system. Voyager 1 is approaching the termination shock at a rate of about 3 AU per year and may soon be able to observe this instability.—LR

Astrophys. J. 591, L61 (2003).

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