Nature 318, 426 - 429 (05 December 1985); doi:10.1038/318426a0

Direct observation of He⁺ pick-up ions of interstellar origin in the solar wind

E. Möbius^*, D. Hovestadt^*, B. Klecker^*, M. Scholer^*, G. Gloeckler^† & F. M. Ipavich^†

^*Max-Planck-Institut für Physik und Astrophysik, Institut für extraterrestrische Physik, 8046 Garching, FRG
^†Department of Physics and Astronomy, University of Maryland, College Park, Maryland 20742, USA

Singly-ionized helium with a velocity distribution extending up to double the solar wind velocity has been detected in interplanetary space. This distribution unambiguously determines the source: interstellar neutrals, ionized and accelerated in the solar wind. The observed significant flux increase in early December is due to the gravitational focusing of the interstellar neutral wind on the downwind side of the Sun.

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