Rapid Cavity Formation and Expansion in CME's

Bernhard Kliem (Potsdam University/Mullard Space Science Laboratory)

Coronal mass ejections (CMEs) observed in the outer corona and solar wind by white-light coronagraphs often show a three-part structure consisting of a bright front enclosing a cavity, with a bright core in the center representing prominence/filament material. This structure is supposed to exist in general, even if a three-part structure is not clearly developed. The cavity is thought to represent the cross section of the CME flux rope in the plane of sky. Preexisting cavities are observed only around some quiescent erupting prominences, but usually not in active regions. Observations of CME cavities in the inner corona, where most of them appear to form, have become possible only with the STEREO and SDO missions. I will present such data which reveal a very rapid formation and expansion of "EUV cavities" in fast and impulsively commencing eruptions early in the phase of main CME acceleration and impulsive flare rise. This cavity expansion appears to be the trigger of large-scale EUV waves (i.e., they are not a "flare blast wave"). Different from the white-light observations, the EUV cavity initially appears to be larger than the CME flux rope. However, it evolves into the white-light cavity subsequently. MHD simulations of flux rope eruptions conform to this picture of cavity formation in the ambient flux and subsequent approach of flux rope size and cavity size for parameters that also favor fast and impulsive CMEs. The initial expansion of ambient flux can be understood as the reverse of the well known pinch effect, driven by decreasing flux rope current as the rope rises. Similar to the upward flux expansion in a CME, the cross-sectional expansion is caused by both ideal MHD effects and magnetic reconnection, which adds flux to the rope.