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The heliosheath, usually meaning the inner heliosheath, is the region of shocked solar wind between the termination shock and the heliopause. Solar wind plasma entering it has been slowed and compressed, and its flow turns as it approaches the boundary with the interstellar medium. The termination shock and heliopause are different boundaries: crossing the shock does not yet mean leaving solar-wind material.

The Voyager 2 plasma experiment provided direct measurements around several shock crossings between 30 August and 1 September 2007. The observed thermal plasma was cooler than a simple picture of complete conversion of the incoming flow energy into thermal-ion heat would suggest. The researchers inferred that much of the energy instead went into pickup ions or other energetic particles.^[ref]

The instrument used four Faraday cups, three facing approximately into the flow. Currents recorded at different particle energies and viewing directions were fitted with moving Maxwell-Boltzmann velocity distributions to estimate proton temperatures. This measures a selected part of the particle population; it does not directly measure all the energetic ions that may carry the remaining energy.^[ref]

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]