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.^[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.^[ref]