A Comparative Study Between Eruptive X-class Flares Associated with Coronal Mass Ejections and Confined X-class Flares

Following the traditional naming of “eruptive flare” and “confined flares” but not implying a causal relationship between flare and coronal mass ejection (CME), we refer to the two kinds of large energetic phenomena occurring in the solar atmosphere as eruptive event and confined event, respectively: the former type refers to flares with associated CMEs, while the later type refers to flares without associated CMEs. We find that about 90% of X-class flares, the highest class in flare intensity size, are eruptive, but the rest 10% confined. To probe the question why the largest energy release in the solar corona could be either eruptive or confined, we have made a comparative study by carefully investigating 4 X-class events in each of the two types with a focus on the differences in their magnetic properties. Both sets of events are selected to have very similar intensity (X1.0 to X3.6) and duration (rise time less than 13 minutes and decaying time less than 9 minutes) in soft X-ray observations, in order to reduce the bias of flare size on CME occurrence. We find no difference in the total magnetic flux of the photospheric source regions for the two sets of events. However, we find that the occurrence of eruption (or confinement) is sensitive to the displacement of the location of the energy release, which is defined as the distance between the flare site and the flux-weighted magnetic center of the source active region. The displacement is 6 to 17 Mm for confined events, but is as large as 22 to 37 Mm for eruptive events, compared to the typical size of about 70 Mm for active regions studied. In other words, confined events occur closer to the magnetic center while the eruptive events tend to occur closer to the edge of active regions. Further, we have used potential-field source-surface model (PFSS) to infer the 3-D coronal magnetic field above source active regions. For each event, we calculate the coronal flux ratio of low corona (< 1.1 R⊙) to high corona (≥ 1.1 R⊙). Department of Computational and Data Sciences, College of Science, George Mason University, 4400 University Dr., MSN 6A2, Fairfax, VA 22030, USA, Email: [email protected], [email protected] School of Earth & Space Sci., Univ. of Sci. & Tech. of China, Hefei, Anhui 230026, China, Email: [email protected]