Evidence for helically kinked magnetic flux ropes in solar eruptions

Images of the X-ray corona near the solar disk's center were examined for large, transient brightenings of the type known to be associated with H alpha filament eruptions and coronal mass ejections. Many of the brightenings were sigmoid (S-shaped). The measured ratios of length to width of the sigmoid features are shown to be consistent with ratios predicted by a simple model based on a kinked and twisted magnetic flux rope. Many of the studied sigmoid brightenings evolved into arcades of bright loops. Such arcades are often associated with coronal mass ejections, and it is suggested that the cause of the ejections is an MHD helical kink instability in the H alpha filament/coronal arcade complexes. Reverse-S brightenings outnumbered forward-S brightenings by six to one in the northern hemisphere. Forward-S brightenings were similarly predominant in the south. This hemispherical segregation suggests that the magnetic fields in the transient features are systematically twisted. Globally, the implied distribution of magnetic helicity is similar to the distribution that Martin et al. discovered in quiescent H alpha filaments.