Magnetic build-up and precursors of CMES

CMEs are fundamentally magnetic phenomena, thus to improve CME forecast we have to find out more about the characteristics of the small and large-scale magnetic field in and around their source region prior to CME occurrence. In this paper we show examples of the magnetic evolution of CME-prolific active regions using SOHO/MDI data. It appears that CMEs are preceded by magnetic evolution during which the helicity of the source region is increasing due to twisted flux emergence, shearing motions between opposite polarity footpoints of subsequently emerging bipoles and, in a smaller extend, by the differential rotation acting on the emerged flux. Furthermore, we find short-term magnetic precursors of CME events, typically a combination of major flux emergence, cancellation and fast shearing motions in active regions with strong concentrated magnetic fields prior to h are-related CMEs and small-scale cancellation events along the magnetic inversion line in decayed active regions with low magnetic flux density prior to filament eruption-related CMEs. We make an overview of recent studies on magnetic helicity and suggest that such analyses will be able to provide a key to unlock the secrets of CME buildup and initiation.