Heidelberg 2006 – wissenschaftliches Programm
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EP: Extraterrestrische Physik
EP 18: Sonne: Energetische Teilchen, Sonnenwind und CMEs
EP 18.5: Fachvortrag
Donnerstag, 16. März 2006, 12:15–12:30, B
The torus instability in coronal mass ejections — •Bernhard Kliem1 and Tibor Török2 — 1Astrophysikalisches Institut Potsdam — 2Mullard Space Science Laboratory, University College London
We model coronal mass ejections (CMEs) as expanding toroidal current rings. The ring is unstable against expansion if the external poloidal field Bex decreases sufficiently rapidly with distance R from torus centre. For steep profiles of Bex(R), representative of active regions, the expansion accelerates initially nearly exponentially, followed by a nearly linear further expansion. For only slightly supercritical profiles of Bex(R), representative of the quiet Sun, the acceleration profile increases very slowly with R so that a nearly constant acceleration is observed during the expansion over many initial radii R0. The two apparently disparate classes of fast and slow CMEs are thus described in a uniform manner by the model. While the photospheric line tying of flux ropes acts stabilizing with regard to the torus instability, it raises the acceleration and extends the radial range of significant acceleration in comparison to an untied, freely expanding ring if the instability occurs. It also enforces an overproportional expansion of the minor radius with the consequence that a cavity and hence the classical three-part structure of CMEs are formed. We discuss the relationship of the torus instability to the helical kink instability in CMEs. The two related ideal MHD instabilities can explain the onset of CMEs and their essential properties (rise profiles, helical shape, three-part structure), leaving the formation of unstable flux ropes as the main open question of a flux rope model for CMEs.