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In this paper, we address the formation of a magnetic flux rope (MFR) thaterupted on 2012 July 12 and caused a strong geomagnetic storm event on July 15.Through analyzing the long-term evolution of the associated active regionobserved by the Atmospheric Imaging Assembly and the Helioseismic and MagneticImager on board the Solar Dynamics Observatory, it is found that the twistedfield of an MFR, indicated by a continuous S-shaped sigmoid, is built up fromtwo groups of sheared arcades near the main polarity inversion line half daybefore the eruption. The temperature within the twisted field and shearedarcades is higher than that of the ambient volume, suggesting that magneticreconnection most likely works there. The driver behind the reconnection isattributed to shearing and converging motions at magnetic footpoints withvelocities in the range of 0.1--0.6 km s$^{-1}$. The rotation of the precedingsunspot also contributes to the MFR buildup. Extrapolated three-dimensionalnon-linear force-free field structures further reveal the locations of thereconnection to be in a bald-patch region and in a hyperbolic flux tube. Abouttwo hours before the eruption, indications for a second MFR in the form of anS-shaped hot channel are seen. It lies above the original MFR that continuouslyexists and includes a filament. The whole structure thus makes up a stabledouble-decker MFR system for hours prior to the eruption. Eventually, afterentering the domain of instability, the high-lying MFR impulsively erupts togenerate a fast coronal mass ejection and X-class flare; while the low-lyingMFR remains behind and continuously maintains the sigmoidicity of the activeregion.

Formation of a Double-decker Magnetic Flux Rope in the Sigmoidal Solar Active Region 11520