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Astron. Astrophys. 316, 493-498 (1996)

Ulysses observations of open and closed magnetic field lines within a coronal mass ejection

V. Bothmer1,*, M.I. Desai1, R.G. Marsden1, T.R. Sanderson1, K.J. Trattner1, K.-P. Wenzel1, J.T. Gosling2, A. Balogh3, R.J. Forsyth3, and B.E. Goldstein4

1 Space Science Department of ESA, Postbus 299, 2200 AG Noordwijk, The Netherlands (e-mail: Bothmer@estsck.estec.esa.nl)
2 Los Alamos National Laboratory, Los Alamos, New Mexico, USA
3 Blackett Laboratory, Imperial College, London, UK
4 Jet Propulsion Laboratory, Pasadena, California, USA

Received 11 March 1996 / Accepted 6 June 1996

Abstract. During the rapid passage from the Sun's south to north polar regions, the Ulysses spacecraft encountered in February 1995, at img2.gif S, a coronal mass ejection (CME) exactly at the time when it crossed from high speed solar wind coronal hole flow into low speed streamer belt flows. We have investigated this CME, which was superimposed on an energetic particle event associated with a corotating interaction region (CIR), using energetic particle, plasma and magnetic field measurements. Ulysses' entry into the CME was accompanied by a strong decrease in the intensity of 1-3 MeV protons. The leading portion of the CME with a helical magnetic flux rope topology characteristic of magnetic clouds apparently consisted of closed magnetic loops as indicated by counterstreaming suprathermal electron fluxes along the interplanetary magnetic field (IMF) and bi-directional streaming 0.4-0.7 MeV ions. In contrast, the absence of counterstreaming electrons and ions, the presence of sunward streaming 0.4-5 MeV ions and sunward bursts of suprathermal electrons at energies from img3.gif 40 eV up to several 100 eV inside another portion of the CME suggest that here the magnetic field lines were ``open'' i.e., with only one end rooted in the solar corona. These field lines were most likely connected to the reverse shock of a CIR beyond Ulysses. We suggest that 3-dimensional reconnection processes are responsible for the formation of magnetic flux rope CMEs from rising coronal loops leading to open and closed field topologies.

Key words: Sun: corona - particle emission - magnetic field - interplanetary medium

*Present address: IFKKI, Kiel, Germany

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