Helicity-driven sigmoid evolution and its role in CME initiation

1. Helicity-driven sigmoid evolution and its role in CME initiation David Alexander, Rice University Evolution and eruption of a soft X-ray sigmoid associated with an instability arising from the continuous injection of helicity by the rotation of the underlying sunspot (from Alexander et al., 2005) SOHO/MDI Magnetograms showing the evolution of a long-lived active region spanning 4 months: note the counter-clockwise rotation of the main spot group (from Tian et al., 2005)

2. Helicity-driven sigmoid evolution and its role in CME initiation David Alexander, Rice University The proposed effort builds upon recent observations of rotating sunspots in TRACE white light images and their apparent association with soft X-ray sigmoids evolution and flare production. The role played by the rotating spots, and the associated injection of helicity, in the generation of flares and CMEs is not yet fully understood although there have been a number of intriguing studies by various groups. Despite the delays caused by the transition of the original grant to Rice University, following the PIs move there, and the setting up of a contract with Lockheed Martin co-investigator, Dr. Rich Nightingale, significant progress has been made in our study of rotating sunspots resulting in several presentations at scientific meetings and the production of a number of relevant papers (see below). As part of the setting up process the PI hired post-doc Dr. Lirong Tian at Rice University in June of 2004. The number of rotating sunspot events observed with TRACE has significantly increased and these are currently being prepared for a detailed velocity analysis. Preliminary studies of some of the largest events have shown a clear association between sunspot rotation and the production of X-class flares (Nightingale and Metcalf, 2005). This has been augmented by a SOHO/MDI study of 104 active regions exhibiting a magnetic d-configuration and utilizing the apparent rotation of the sunspot and the tilt angle to the equator to determine the relationship between twist and writhe helicity (Tian et al., 2005a). In a complementary study we explored the behavior of the magnetic field and associated coronal activity of a single long-lived (over 4 solar rotations) active region to show that the sense of helicity and the sense of sunspot rotation are consistent with an emergence of a twisted W-fluxtube which has been subjected to the kink instability in the sub-photosphere. This has been written up for Solar Physics and is currently under review (Tian et al., 2005b). One of the primary aims of the original proposal was examine the role of the coronal helicity injection implicit in the sunspot rotation as the ‘proximate cause’ for the sigmoid evolution. An initial study of two sunspot rotation cases and using a “stackplot” velocity analysis has indicated that the evolution of the sunspot rotation, i.e. the duration over which it injects helicity into the corona, can determine whether a coronal sigmoid reaches an instability limit and therefore erupts or not (Alexander et al., 2005). This study is to be augmented by detailed local correlation tracking techniques and vector magnetic field measurements to better determine the helicity injected into individual coronal structures. Publications 1. Magnetic twist and writhe of delta active regions, L.Tian, D. Alexander, Y. Liu & J.Yang, Solar Phys., 2005a, in press. 2. Formation of a twisted and kink-driven fluxtube for a long-lived active region: AR 9632, L. Tian, Y. Liu, J.Yang & D. Alexander, Solar Phys., 2005b, in review. 3. The role of sunspot rotation in driving sigmoid eruptions, D. Alexander, R. Nightingale, T. R. Metcalf, & D. S. Brown, 2005, in preparation. 4. TRACE Observations of Many Active Regions with X-flares and Rotating Sunspots in the Current Solar Cycle, R. Nightingale & T. R. Metcalf, 2005, in preparation. 5. Sunspot and sunspot group rotation of NOAA AR 9684, L. Tian, D. Alexander, Y. Liu & J. Yang, 2005c, in preparation.