= Termination shock
The <termination shock> is the transition where the expanding <solar wind> slows from supersonic flow into the slower, compressed and heated flow of the outer <heliosphere>. It forms because the outward flow encounters pressure from its surroundings. It is distinct from the farther-out <heliopause>, where solar-wind plasma meets the interstellar plasma.
A <shock wave> converts organized flow <kinetic energy> into thermal and nonthermal particle energy. A spacecraft can therefore identify a crossing through changes in the <magnetic field>, plasma and energetic-particle populations, rather than through a visible solid boundary. <Voyager 1> crossed the <termination shock> on 16 December 2004 near 94 <astronomical units> from the <Sun>. Its magnetic-field and particle instruments supplied evidence for entering the <heliosheath>.<Voyager 1 Mission Overview by NASA>[Mission timeline, 16 December 2004]{ref} The original particle report found enhanced low-energy protons but no local maximum of anomalous <cosmic ray> helium, illustrating why detecting a shock and explaining all particle acceleration are separate questions.https://doi.org/10.1126/science.1117684[2005 primary particle report, Voyager 1 explores the termination shock region and the heliosheath beyond, Science 309 (2005), abstract]{ref}
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