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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 26, issue 10
Ann. Geophys., 26, 3077–3088, 2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: SOHO 20 – Transient events on the Sun and in the...

Ann. Geophys., 26, 3077–3088, 2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

  15 Oct 2008

15 Oct 2008

Why are CMEs large-scale coronal events: nature or nurture?

L. van Driel-Gesztelyi1,2,3, G. D. R. Attrill1, P. Démoulin2, C. H. Mandrini4, and L. K. Harra1 L. van Driel-Gesztelyi et al.
  • 1University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
  • 2Observatoire de Paris, LESIA, FRE 2461(CNRS), 92195 Meudon Principal Cedex, France
  • 3Konkoly Observatory of Hungarian Academy of Sciences, Budapest, Hungary
  • 4Instituto de Astronomía y Física del Espacio, CONICET-UBA, CC. 67, Suc. 28, 1428 Buenos Aires, Argentina

Abstract. The apparent contradiction between small-scale source regions of, and large-scale coronal response to, coronal mass ejections (CMEs) has been a long-standing puzzle. For some, CMEs are considered to be inherently large-scale events – eruptions in which a number of flux systems participate in an unspecified manner, while others consider magnetic reconnection in special global topologies to be responsible for the large-scale response of the lower corona to CME events. Some of these ideas may indeed be correct in specific cases. However, what is the key element which makes CMEs large-scale? Observations show that the extent of the coronal disturbance matches the angular width of the CME – an important clue, which does not feature strongly in any of the above suggestions. We review observational evidence for the large-scale nature of CME source regions and find them lacking. Then we compare different ideas regarding how CMEs evolve to become large-scale. The large-scale magnetic topology plays an important role in this process. There is amounting evidence, however, that the key process is magnetic reconnection between the CME and other magnetic structures. We outline a CME evolution model, which is able to account for all the key observational signatures of large-scale CMEs and presents a clear picture how large portions of the Sun become constituents of the CME. In this model reconnection is driven by the expansion of the CME core resulting from an over-pressure relative to the pressure in the CME's surroundings. This implies that the extent of the lower coronal signatures match the final angular width of the CME.

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