Magnetic Field Modeling of Hot Channels in Four Flare/Coronal Mass Ejection Events

We investigate the formation and magnetic topology of four flare/coronal mass ejection events with filament-sigmoid systems, in which the sigmoidal hot channels are located above the filaments and appear in pairs before eruption. The formation of hot channels usually takes several to dozens of hours, during which two J-shaped sheared arcades gradually evolve into sigmoidal hot channels and then keep stable for tens of minutes or hours and erupt, while the low-lying filaments show no significant change. We construct a series of magnetic field models and find that the best-fit preflare models contain magnetic flux ropes with hyperbolic flux tubes (HFTs). The field lines above the HFT correspond to the high-lying hot channel, whereas those below the HFT surround the underlying filaments. In particular, the continuous and long field lines representing the flux rope located above the HFT match the observed hot channels well in three events. However, for the SOL2014-04-18 event, the flux bundle that mimics the observed hot channel is located above the flux rope. The flux rope axis lies in a height range of 19.8 and 46 Mm above the photosphere for the four events, among which the flux rope axis in the SOL2012-0712 event has a maximum height, which probably explains why it is often considered as a double-decker structure. Our modeling suggests that the high-lying hot channel may be formed by magnetic reconnections between sheared field lines occurring above the filament before eruption.