Hinode's observational result on the saturation of magnetic helicity injected into the solar atmosphere and its relation to the occurrence of a solar flare

This paper is intended to present key features of the evolution toward the onset of a solar flare, using Hinode's observations on the photospheric magnetic field. Hinode can provide the temporal development of the photospheric field as a vector quantity, which shows that magnetic shear is initially developed in a flare-productive active region (AR10930), and then decreases toward the onset of an X-class flare. The magnetic helicity in this active region first increases rapidly, while it become saturated in the late phase. We explain the physical origin of these features in terms of the emergence of a magnetic flux tube into the solar atmosphere, and investigate how they relate to the occurrence of a flare. We also discuss how the magnetic helicity evolves in different types of flares, showing that its evolution can be used for flare prediction.