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In‐situ observations of cometary pick‐up iIons ≥0.2 AU upstream of comet Halley: ICE observations
Author(s) -
Wenzel K.P.,
Sanderson T. R.,
Richardson I. G.,
Cowley S. W. H.,
Hynds R. J.,
Bame S. J.,
Zwickl R. D.,
Smith E. J.,
Tsurutani B. T.
Publication year - 1986
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl013i008p00861
Subject(s) - comet , physics , solar wind , halley's comet , ion , astronomy , atmosphere (unit) , astrophysics , atmospheric sciences , plasma , meteorology , nuclear physics , quantum mechanics
Burst‐like enhancements of energetic ions were observed by the EPAS instrument on the International Cometary Explorer (ICE) during its closest approach to (28 × 10 6 km upstream of) Comet P/Halley, in late March 1986. The enhancements showed considerable temporal structure and highly anisotropic angular distributions, with strong ion streaming in the antisolar direction. The ion intensity was modulated by the varying solar wind speed (the latter reaching maxima of around 600 km s −1 ), as was found to be the case for heavy cometary ions accelerated by pick‐up in the solar wind flow, during the ICE encounter with Comet P/Giacobini‐Zinner (G‐Z). We therefore conclude that we observed pick‐up ions (most probably ≥ 65 keV oxygen ions) produced by heavy neutrals from Comet Halley. The region of pick‐up ions extended to more than 0.2 AU (∼35 × 10 6 km) from the nucleus at this time, at least three times further than was observed during the Giotto and VEGA encounters. The observations of energetic ions at such large distances suggest the presence, in the neutral atmosphere surrounding the nucleus, of a component with an ionisation scale length of 5‐10 million km, resulting from a relatively high expansion speed of a few km s −1 and/or an ionisation time scale of a few times 10 6 s.