Solar Physics Division Meeting 2000, June 19-22
Session 9. Living with a Star 3: Space Weather Aspects
Oral, Chair: D. H. Hathaway, Tuesday, June 20, 2000, 1:30-3:00, 3:30-4:00pm, Forum

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[9.07] What Causes Deceleration of Coronal Mass Ejection

J. Lin, T. G. Forbes (University of New Hampshire)

More and more observational data show evidence of deceleration in some coronal mass ejections (CMEs) (e.g. Sheeley et al. 1999). Sheeley et al. (1999) classified CMEs into two categories: (1) Gradual CMEs, which have speeds of 400-600 km/s, usually form when prominences erupt; and (2) impulsive CMEs, which have speeds of 500-1000 km/s, are often associated with flares. CMEs in the second category typically undergo a period of deceleration after their initial acceleration, while CMEs in the first category do not. What could cause the deceleration of the fast CMEs? After ruling out the effects of projection and gravity, Sheeley et al. (1999) suggested that the mass swept up by the shock accompanying the ejection may be the reason. Here we suggest another possibility, namely the rapid formation of an extensive current sheet below the CME. Such a current sheet tends to form for both slow and fast CMEs, but in the case of a fast CME the growth of the sheet is much more rapid than its dissipation by magnetic reconnection. In the case of a slow CME the growth and reconnection processes are more evenly balanced, and thus the current sheet is not as extensive. Consequently, deceleration is less likely to occur in a slow CME.

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