An interplanetary origin of great geomagnetic storms: Multiple magnetic

An event of multiple magnetic cloud ( Multi-MC) in interplanetary space on March 31, 2001 caused the largest geomagnetic storm with D-st = - 387 nT during the 23(th) solar maximum (2000 similar to 2001). By analyzing the data from the ACE spacecraft, we describe the characteristics of the magnetic fields and plasma within this Multi-MC at 1 AU. We also identify its solar sources, namely coronal mass ejections (CMEs). by utilizing the observations from the Solar and Heliospheric Observatory (SOHO) and the GOES satellite. In this instance, the Multi-MC became more effective with the extraordinary enhanced southward magnetic fields, and largely extended the duration of the D., storm. The observations and the theoretical analysis reveal that the strength of the magnetic fields including the southward component increased several times due to the compression between the sub-clouds in the Multi-MC, and therefore intensified the corresponding effectiveness of geomagnetism. Thus, we suggest that the compression between the sub-clouds of Multi-MC should be a mechanism in causing great geomagnetic storms. In addition, we find that the CMEs, which formed the Multi-MC, seemed not to originate from the same solar region.