Lack of relationship between geoeffectiveness and orientations of magnetic clouds with bipolar Bz and unipolar southward Bz

In this study, 38 magnetic clouds (MCs) that caused significant geomagnetic storms (the minimum SYM-H, SHmin, ≤−50 nT) are examined, in which 17 MCs were unipolar Bz in south (S-type) and 21 MCs were bipolar Bz (north-to-south, NS-type, or south-to-north, SN-type). For S-type MC, inclination angle of the axis of the MC, |θ|, is ≥45°, while |θ|<45° for bipolar MC. This paper aims to address a question: is the intensity of a MC-driven storm correlated with the orientations of bipolar and S-type MCs? Our results demonstrate that there is no direct and significant relationship between geoeffectiveness and orientations of bipolar and S-type MCs. In other words, there is no MC preference (bipolar or S-type MC) to regulate the SHmin of the storm. On the whole, the SHmin is found to strongly correlate with southward field Bz (cc=0.96) and with the y component of the solar wind convective electric field (cc=−0.91) but to weakly correlate with solar wind speed (cc=−0.65). This result is consistent with previous studies by Wu and Lepping (2002), J. Geophys. Res. 107 (A10), 1314. doi:10.1029/2001JA000161. By separating MC-driven storms by size into moderate (−100 nT