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 relative to the Sun. An initially slow neutral atom becomes an ion with speed roughly 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 . This is a physical signature of pickup and acceleration, rather than an assertion that every detected ion moves at that maximum.
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