  We examine two successive flare eruptions (X5.4 and X1.3) on 2012 March 7 in
the NOAA active region 11429 and investigate the magnetic field reconfiguration
associated with the two eruptions. Using an advanced non-linear force-free
field (NLFFF) extrapolation method based on the SDO/HMI vector magnetograms, we
obtain a stepwise decrease in the magnetic free energy during the eruptions,
which is roughly 20%-30% of the energy of the pre-flare phase. We also
calculate the magnetic helicity, and suggest that the changes of the sign of
the helicity injection rate might be associated with the eruptions. Through the
investigation of the magnetic field evolution, we find that the appearance of
the "implosion" phenomenon has a strong relationship with the occurrence of the
first X-class flare. Meanwhile, the magnetic field changes of the successive
eruptions with implosion and without implosion were well observed.
