  Photospheric magnetic field not only plays important roles in building up
free energy and triggering solar eruptions, but also has been observed to
change rapidly and permanently responding to the coronal magnetic field
restructuring due to coronal transients. The Helioseismic and Magnetic Imager
instrument (HMI) on board the newly launched Solar Dynamics Observatory (SDO)
produces seeing-free full-disk vector magnetograms at consistently high
resolution and high cadence, which finally makes possible an unambiguous and
comprehensive study of this important back-reaction process. In this study, we
present a near disk-center, GOES-class X2.2 flare occurred at NOAA AR 11158 on
2011 February 15 using the magnetic field measurements made by HMI. We obtained
the first solid evidence of an enhancement in the transverse magnetic field at
the flaring magnetic polarity inversion line (PIL) by a magnitude of 70%. This
rapid and irreversible field evolution is unequivocally associated with the
flare occurrence, with the enhancement area located in between the two
chromospheric flare ribbons. Similar findings have been made for another two
major flare events observed by SDO. These results strongly corroborate our
previous suggestion that the photospheric magnetic field near the PIL must
become more horizontal after eruptions.
  In-depth studies will follow to further link the photospheric magnetic field
changes with the dynamics of coronal mass ejections, when full Stokes inversion
is carried out to generate accurate magnetic field vectors.
