Interplanetary conditions causing intense geomagnetic storms (Dst ≤-100 nT) during solar cycle 23 (1996-2006)

[1] The interplanetary causes of intense geomagnetic storms and their solar dependence occurring during solar cycle 23 (1996-2006) are identified. During this solar cycle, all intense (Dst <= -100 nT) geomagnetic storms are found to occur when the interplanetary magnetic field was southwardly directed (in GSM coordinates) for long durations of time. This implies that the most likely cause of the geomagnetic storms was magnetic reconnection between the southward IMF and magnetopause fields. Out of 90 storm events, none of them occurred during purely northward IMF, purely intense IMF By fields or during purely high speed streams. We have found that the most important interplanetary structures leading to intense southward Bz ( and intense magnetic storms) are magnetic clouds which drove fast shocks (sMC) causing 24% of the storms, sheath fields (Sh) also causing 24% of the storms, combined sheath and MC fields (Sh+MC) causing 16% of the storms, and corotating interaction regions ( CIRs), causing 13% of the storms. These four interplanetary structures are responsible for three quarters of the intense magnetic storms studied. The other interplanetary structures causing geomagnetic storms were: magnetic clouds that did not drive a shock (nsMC), non magnetic clouds ICMEs, complex structures resulting from the interaction of ICMEs, and structures resulting from the interaction of shocks, heliospheric current sheets and high speed stream Alfven waves. During the rising phase of the solar cycle, sMC and sheaths are the dominant structure driving intense storms. At solar maximum, sheath fields, followed by Sh+MCs and then by sMC were responsible for most of the storms. During the declining phase, sMC, Sh and CIR fields are the main interplanetary structures leading to intense storms. We have also observed that around 70% of the storms follow the interplanetary criteria of Ey >= 5 mV/m for at least 3 h. Around 90% of the storms used in the study followed a less stringent set of criteria: Ey >= 3 mV/m for at least 3 h. Finally, we obtain the approximate rate of intense magnetic storms per solar cycle phases: minimum/rising phase 3 storms. year(-1), maximum phase 8.5 storms. year(-1), and declining/minimum phases 6.5 storms. year(-1).