Sun-Earth connection Event of Super Geomagnetic Storm on March 31, 2001: the Importance of Solar Wind Density

An X1.7 flare at 10:15 UT and a halo CME with a projected speed of 942 km/s erupted from NOAA solar active region 9393 located at N20W19, were observed on 2001 March 29. When the CME reached the Earth, it triggered a super geomagnetic storm (hereafter super storm). We find that the CME always moved towards the Earth according to the intensity-time profiles of protons with different energies. The solar wind parameters responsible for the main phase of the super storm occurred on March 31, 2001 is analyzed taking into account the delayed geomagnetic effect of solar wind at the L1 point and using the SYM-H index. According to the variation properties of SYM-H index during the main phase of the super storm, the main phase of the super storm is divided into two parts. A comparative study of solar wind parameters responsible for the two parts shows the evidence that the solar wind density plays a significant role in transferring solar wind energy into the magnetosphere, besides the southward magnetic field and solar wind speed.