Charateristics of strong geomagnetic storms (Dsl < -100 nT) along 23rd solar cyles

Geomagnetic storm event is causing geomagnetic field changes usually expressed with magnetograms of H, D and Z components. By using the Dst index data, there was identified 91 of Strong geomagnetic storms (Dst < -100 nT) during 1996-2007 consisting of 71 or 78.02% were sudden commencement types and rest 20 or 21.98 % were gradual storm types. There are also found that the best initial phase duration lies between 0-2 hours after onset time is 46 cases or 64.78% of 71 cases total. While the main phase duration lies between 6-12 hours after the onset time is 30 cases or 42.25% of the total 71 cases. This value is important as a recommendation in developing automatic detection of SC type geomagnetic storms at space weather program in the Space Science Center, LAPAN.