CME Initiation, Evolution, and Interplanetary Consequences
HAO scientists have carried out 3D isothermal MHD simulations of the evolution of the large scale coronal magnetic field as a twisted magnetic flux tube is driven (slowly) through the lower boundary into a pre-existing coronal arcade field.
MHD SIMULATIONS OF CME INITIATION AND DYNAMIC EVOLUTION
It is found that depending on how rapidly the ambient coronal magnetic field declines with height, the emerging flux rope may lose confinement and erupt through either the onset of the torus instability or the kink instability (Figure 1). In the former case the erupting flux rope primarily shows an outward expansion while in the latter case the flux rope develops significant writhing or rotation. Both types of eruptive behavior have been observed. (Fan & Gibson, 2007.)
Both cases found formation of a current sheet of sigmoid morphology prior to the onset of the eruption. The current sheet intensifies during the eruption and reconnections in the current sheet produce post-reconnection loops with cusp-shaped tops. Some sigmoid shaped dipped fields are found to remain beneath the post-reconnection loops during the early phase of the eruption in both cases (Figure 2). These results explain the observed presence of X-ray sigmoids in CME source regions, and the transition from X-ray sigmoid brightening to cusp-shaped post-flare X-ray loops, which in some cases are seen to straddle a remaining under-lying X-ray sigmoid, during an eruption.
CONNECTION TO MAGNETIC CLOUDS
Connecting interplanetary coronal mass ejections (ICMEs) to their coronal pre-eruption source requires a clear understanding of how that source may have evolved during eruption. Gibson and Fan (2006a, 2006b) presented a three-dimensional MHD simulation of the eruption of a kinked flux rope, which showed how, in the course of eruption, a coronal flux rope may writhe and reconnect both internally and with surrounding fields in a manner that leads to a partial ejection of only part of the rope as a CME. Gibson and Fan (2008) explicitly determined how such evolution during eruption would lead to alterations of the magnetic connectivity, helicity, orientation, and topology of the ejected portion of the rope so that it differs significantly from that of the pre-eruption rope. These changes complicate how ICMEs embedded in the solar wind relate to their solar source. In particular, the location and evolution of transient coronal holes (Figure 3), topology of magnetic clouds ("tethered spheromak") (Figure 4), and likelihood of interacting ICMEs would differ significantly from what would be predicted for a CME which did not undergo writhing and partial ejection during eruption.