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E. Möbius is the first author of a 1985 study detecting interstellar helium after it became ionized and was carried into the solar wind. The observed pickup ions had a velocity distribution extending to approximately twice the solar-wind speed. The authors also related a seasonal increase in flux to gravitational focusing of neutral interstellar material downstream of the Sun.^[ref]

A related primary report describes the time-of-flight mass spectrometry used by a satellite instrument to observe energetic, singly ionized helium between September and December 1984. Measuring particle identity and energy makes the population distinguishable from the ordinary bulk solar wind; the interpretation then uses the shape of the observed spectrum.^[ref]

An idealized velocity argument explains the upper-speed scale. Let the solar wind move at speed $V$ relative to the Sun. An initially slow neutral atom becomes an ion with speed roughly $V$ in the wind's reference frame. If its velocity direction is redistributed while that magnitude remains similar, transforming back to the Sun's reference frame can produce speeds up to $2V$. This is a physical signature of pickup and acceleration, rather than an assertion that every detected ion moves at that maximum.

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]

A pickup ion is an ion created from a neutral particle that initially moves differently from the surrounding magnetized plasma, and is then accelerated and carried along by that plasma's electromagnetic fields. In the solar wind, interstellar neutral atoms can be ionized by sunlight or charge exchange. Their new charge couples them to the interplanetary magnetic field and the solar wind's motional electric field.

For an idealized uniform flow of speed $V$, with magnetic field perpendicular to the flow, a newly ionized atom initially at rest relative to the Sun has speed $V$ in the flow frame. It gyrates about the field while its guiding center moves with the flow. Transforming back to the solar frame gives speeds between zero and $2V$ during a gyro-orbit. Subsequent scattering modifies this initial ring distribution; pickup does not mean immediate thermal equilibrium with ordinary solar-wind ions.

The 1985 discovery paper reported singly ionized helium extending to about twice the solar-wind speed, and a seasonal enhancement associated with gravitational focusing of interstellar neutrals.^[ref] These velocity and composition measurements distinguish newly picked-up material from the main thermal flow.