Sigmoids as precursors of solar eruptions

Coronal mass ejections (CMEs) appear to originate preferentially in regions of the sun's corona that are sigmoidal, i.e., have sinuous S or reverse-S shapes. Yohkoh salar X-ray images hale been studied before and after a mode st number of earth-directed (halo) CMEs, These images tend to show sigmoida l shapes before the eruptions and arcades, cusps, and transient coronal hol es after. Using such structures as proxies, it has been shown that there is a relationship between sigmoidal shape and tendency to erupt. Regions in t he sun's corona appear sigmoidal because their magnetic fields are twisted. Some of this twist may originate deep inside the sun. However, it is signi ficantly modulated hv the Coriolis force and turbulent convection as this f lux buoys up through the sun's convection zone. As the result of these phen omena, and perhaps subsequent magnetic reconnection, magnetic flux ropes fo rm. These flux ropes manifest themselves as sigmoids in the corona, Althoug h there are fundamental reasons to expect such flux ropes to be unstable, t he physics is not as simple as might first appear, and there exist various explanations for instability Many gaps need to be filled in before the rela tionship between sigmoids and CMEs is well enough understood to be a useful predictive tool.