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AGU: Geophysical Research Letters

 

Index Terms

  • Magnetospheric Physics: Solar wind interactions with unmagnetized bodies
  • Planetology: Comets and Small Bodies: Atmospheric and ionospheric composition and chemistry
  • Planetology: Comets and Small Bodies: Interactions with solar wind plasma and fields
  • Planetology: Comets and Small Bodies: Plasma and MHD instabilities

Abstract

GEOPHYSICAL RESEARCH LETTERS, VOL. 13, NO. 8, PP. 861-864, 1986
doi:10.1029/GL013i008p00861

In‐situ observations of cometary pick‐up iIons ≥0.2 AU upstream of comet Halley: ICE observations

K.‐P. Wenzel

Space Science Department of ESA, ESTEC, Noordwijk, NL

T. R. Sanderson

Space Science Department of ESA, ESTEC, Noordwijk, NL

I. G. Richardson

Blackett Laboratory, Imperial College, London, UK

S. W. H. Cowley

Blackett Laboratory, Imperial College, London, UK

R. J. Hynds

Blackett Laboratory, Imperial College, London, UK

S. J. Bame

Los Alamos National Laboratory, Los Alamos, NM, USA

R. D. Zwickl

Los Alamos National Laboratory, Los Alamos, NM, USA

E. J. Smith

Jet Propulsion Laboratory, Pasadena, CA, USA

B. T. Tsurutani

Jet Propulsion Laboratory, Pasadena, CA, USA

Burst‐like enhancements of energetic ions were observed by the EPAS instrument on the International Cometary Explorer (ICE) during its closest approach to (28 × 106 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 × 106 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 106s.

Received 19 June 1986; accepted 11 July 1986; .

Citation: Wenzel, K.‐P., T. R. Sanderson, I. G. Richardson, S. W. H. Cowley, R. J. Hynds, S. J. Bame, R. D. Zwickl, E. J. Smith, and B. T. Tsurutani (1986), In‐situ observations of cometary pick‐up iIons ≥0.2 AU upstream of comet Halley: ICE observations, Geophys. Res. Lett., 13(8), 861–864, doi:10.1029/GL013i008p00861.

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