Coronal flux rope catastrophe caused by photospheric flux emergence

Photospheric magnetic flux emergence has been often observed in solar active regions and considered to be closely related to solar explosive phenomena such as flares and CMEs. However, in view of the fact that photospheric flux emergence and associated flares or CMEs differ remarkably in timescale, their relationship must be implemented by a catastrophic process. To demonstrate this point, we investigate the catastrophe of a coronal flux rope system caused by photospheric flux emergence. The initial magnetic field is taken to be a two-dimensional force-free field in spherical geometry, consisting of an isolated flux rope and a bipolar background field surrounding it. A flux emergence is then introduced somewhere on the photosphere, and it causes a variation of the background field and the formation of a current sheet between the emerging arcade and the original background field. It is shown that as the total flux of the emerging arcade increases, a catastrophe may take place for the system, depending on the location and field orientation of the emerging arcade and whether there exists a magnetic reconnection across the newly formed current sheet. Although the main conclusions are superficially similar to those obtained by previous studies, we stress an entirely new concept that the photospheric flux emergence plays the role of a trigger rather than a driver of the associated flare and CME by forcing a highly complex system to reach its catastrophic point.