= Heliosheath
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.<Cool heliosheath plasma and deceleration of the upstream solar wind at the termination shock by John D. Richardson and others>[Abstract, Nature 454, pages 63–66]{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.<Plasma temperature distributions in the heliosheath by J. D. Richardson>[section 3, Instrument and Analysis; Figure 1]{ref}
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