Geomagnetic storms are intervals of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial injection of energy into the magnetosphere-ionosphere system, to an intensified ring current, strong enough to exceed some key threshold of the quantifying storm time Dst index. We have studied all the 9 great magnetic storms (peak Dst < -200 nT) observed during the rise and maximum of solar cycle 23 (from 1997 to early 2001), in order to identify their solar and interplanetary causes. Apart of one storm occurred during the period without observations from the Solar and Heliospheric Observatory (SOHO), all of them were related to coronal mass ejections observed by the Large Angle and Spectroscopic Coronagraph (LASCO). The sources of interplanetary southward magnetic field, Bs, responsible for the occurrence of the storms were related to the intensified shock/sheath field, interplanetary magnetic cloud's field, or the combination of sheath-cloud or sheath-ejecta field. It called our attention the fact that one of the events was related to a slow CME, with CME expansion speed not greater than 550 km/s. The purpose of this paper is to address the main sources of large geomagnetic disturbances using the current satellite capability available. As a general conclusion, we found that shock/sheath compressed fields are the most important interplanetary causes of great magnetic storms during this period.

In this work we present a comparison between the solar origins of two solar-terrestrial connection events occurring on April 1999 and February 2000, respectively. On April 13 (1999), a coronal mass ejection (CME) appeared for the first time in LASCO-C2 coronagraph field of view at 03:30 UT. The possible solar disc source of this CME was located at coordinates N25 E05, and it consisted on flaring activity together with a filament eruption as seen in EIT images. The geomagnetic storm with a Dst peak of -91 nT registered on April, 17 was attributed to this solar surface activity. On February 8, 9 and 10 (2000) three Earth-directed halo CMEs were recorded by LASCO coronagraphs. The interaction of two of them caused the intense interplanetary geomagnetic storm registered on February 12 (Dst = -110 nT). The CMEs detected on February 8 and 10 were launched from AR 8858 and the source of the activity was attributed to two solar flares of X-ray class M1.3 and C7.3 respectively. The CME detected on February 9 was originated in AR 8853 and flaring activity was also identified as the solar source for this event. In this study we discuss the solar origins of the events that gave rise to coronal and interplanetary disturbances and as a consequence produced geomagnetic activity at the Earth. This work is part of a complete study that is searching the solar origin, the interplanetary aspects and the magnetospheric effects of these events.