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Paper: Exceptions to the rule: the X-flares of AR 2192 Lacking Coronal Mass Ejections
Volume: 504, Coimbra Solar Physics Meeting: Ground-based Solar Observations in the Space Instrumentation Era
Page: 203
Authors: Thalmann, J. K.; Su, Y.; Temmer, M.; Veronig, A. M.
Abstract: NOAA Active region (AR) 2192, that was present on the Sun in October 2014, was the largest region which occurred since November 1990 (see Figure 1). The huge size accompanied by a very high activity level, was quite unexpected as it appeared during the unusually weak solar cycle 24. Nevertheless, the AR turned out to be one of the most prolific flaring ARs of cycle 24. It produced in total 6 X, 29 M, 79 C flares during its disk passage from October 18-29, 2014 (see Figure 2). Surprisingly, all flares greater than GOES class M5 and X were confined, i.e. had no coronal mass ejections (CME) associated. All the flare events had some obvious similarity in morphology, as they were located in the core of the AR and revealed only minor separation motion away from the neutral line but a large initial separation of the conjugate flare ribbons. In the paper by Thalmann et al. (2015) we describe the series of flares and give details about the confined X1.6 flare event from October 22, 2014 as well as the single eruptive M4.0 flare event from October 24, 2014. The study of the X1.6 flare revealed a large initial separation of flare ribbons together with recurrent flare brightenings, which were related to two episodes of enhanced hard X-ray emission as derived from RHESSI observations. This suggests that magnetic field structures connected to specific regions were repeatedly involved in the process of reconnection and energy release. Opposite to the central location of the sequence of confined events within the AR, a single eruptive (M4.0) event occurred on the outskirt of the AR in the vicinity of open magnetic fields. Our investigations revealed a predominantly north-south oriented magnetic system of arcade fields overlying the AR that could have preserved the magnetic arcade to erupt, and consequently kept the energy release trapped in a localized volume of magnetic field high up in the corona (as supported by the absence of a lateral motion of the flare ribbons and the recurrent brightenings within them). We conclude that the background magnetic field configuration is an essential parameter for deriving the “eruptiveness” of flare events. Sun et al. (2015) supports this conclusion and derived for this AR a quite slow decay of the strength of the overlying magnetic field (decay index; see Török & Kliem 2005). Interestingly, our magnetic field modellings revealed no flux rope inherent to the AR, indicating that further investigations are needed. In a recent paper by Veronig $ Polanec (2015), who investigated in more detail the X-flares using also ground-based observations in Hα from Kanzelhöhe Observatory (Pötzi et al. 2015), it was shown that such confined events could be explained by the emerging-flux model, where newly emerging small flux tubes reconnect with pre-existing large coronal loops.
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