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]
Plasma temperature distributions in the heliosheath by J. D. Richardson. section 3, Instrument and Analysis; Figure 1
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