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

We examine the two kinds of major energetic phenomena that occur in the solar atmosphere: eruptive and confined events. The former describes flares with associated coronal mass ejections (CMEs), while the latter denotes flares without associated CMEs. We find that about 90% of X-class flares are eruptive, but the remaining 10% are confined. To probe why the largest energy releases could be either eruptive or confined, we investigate four X-class events from each of the two types. Both sets of events are selected to have very similar intensities (X1.0 to X3.6) and duration (rise time under 13 minutes and decay time not over 9 minutes) in soft X-ray observations, to reduce any bias due to flare size on CME occurrence. We find that the occurrence of eruption (or confinement) is sensitive to the displacement of the location of the energy release, defined as the distance between the flare site and the flux-weighted magnetic center of the source active region. The displacement is 6 - 17 Mm for confined events but as large as 22 - 37 Mm for eruptive events. This means that confined events occur closer to the magnetic center, while the eruptive events tend to occur close to the edge of active regions. We use the potential field source-surface model to infer the coronal magnetic field above the source active regions and calculate the flux ratio of low (<1.1 Rs) to high (>1.1 Rs) corona. We find that the confined events have a lower ratio (<5.7) than the eruptive events (>7.1). These results imply that a stronger overlying arcade field may prevent energy releases in the low corona from being eruptive, resulting in flares, but without CMEs.