Interplanetary conditions leading to superintense geomagnetic storms (Dst ≤-250 nT) during solar cycle 23

The interplanetary causes of superintense geomagnetic storms (superstorms, Dst <= -250 nT) that occurred during solar cycle 23 are studied. Eleven superstorms occurred during the cycle, five close to solar maximum (2000-2001) and six in the post-maximum/declining phase (2003-2004). About 1/3 of the superstorms were caused by magnetic clouds (MCs), 1/3 by a combination of sheath and MC fields, and 1/3 by sheath fields alone. The interplanetary parameter best correlated with peak Dst was the time-integrated E-y during the storm main phase (in contrast with peak B-s and/or peak E-y for less intense geomagnetic storms). The range of peak Dst for these storms was -263 to -422 nT. The storm main phase durations had a range of 3-33 h. We conclude from this study that: (1) only MCs and/or interplanetary sheaths had fields intense enough and with long enough durations to cause superstorms; (2) superstorms occurred only in the maximum and declining phases; (3) the total energy transferred from the solar wind to the magnetosphere is best correlated with the time-integrated solar wind E-y parameter.