  The super-storm of November 20, 2003 was associated with a high speed coronal
mass ejection which originated in the NOAA AR 10501 on November 18. This
coronal mass ejection had severe terrestrial consequences leading to a
geomagnetic storm with DST index of -472 nT, the strongest of the current solar
cycle. In this paper, we attempt to understand the factors that led to the
coronal mass ejection on November 18. We have also studied the evolution of the
photospheric magnetic field of NOAA AR 10501, the source region of this coronal
mass ejection. For this purpose, the MDI line-of-sight magnetograms and vector
magnetograms from Solar Flare Telescope, Mitaka, obtained during November,
17-19, 2003 were analysed. In particular, quantitative estimates of the
temporal variation in magnetic flux, energy and magnetic field gradient were
estimated for the source active region. The evolution of these quantities was
studied for the 3-day period with an objective to understand the pre-flare
configuration leading up to the moderate flare which was associated with the
geo-effective coronal mass ejection. We also examined the chromospheric images
recorded in H-alpha from Udaipur Solar Observatory to compare the flare
location with regions of different magnetic field and energy. Our observations
provide evidence that the flare associated with the CME occurred at a location
marked by high magnetic field gradient which led to release of free energy
stored in the active region.
