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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 27, issue 2 | Copyright
Ann. Geophys., 27, 869-875, 2009
https://doi.org/10.5194/angeo-27-869-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  19 Feb 2009

19 Feb 2009

Proton beam velocity distributions in an interplanetary coronal mass ejection

E. Marsch1, S. Yao2, and C.-Y. Tu2 E. Marsch et al.
  • 1Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany
  • 2Department of Geophysics, Peking University, 100871 Beijing, China

Abstract. The plasma and magnetic-field instruments on the Helios 2 spacecraft, which was on 3 April 1979 located at 0.68 AU, detected an interplanetary coronal mass ejection (ICME) that revealed itself by the typical signature of magnetic field rotation. The solar wind flow speed ranged between 400 and 500 km/s. We present here some detailed proton velocity distributions measured within the ICME. These cold distributions are characterized by an isotropic core part with a low temperature, T≤105 K, but sometimes reveal a broad and extended hot proton tail or beam propagating along the local magnetic field direction. These beams lasted only for about an hour and were unusual as compared with the normal ICME protons distribution which were comparatively isotropic. Furthermore, we looked into the velocity and field fluctuations in this ICME and found signatures of Alfvén waves, which might be related to the occurrence of the hot proton beams. However, it cannot be excluded that the beam originated from the Sun.

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