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The geomagnetic superstorm of 20 November 2003 with Dst = -422 nT, one of themost intense in history, is not well understood. The superstorm was caused by amoderate solar eruptive event on 18 November, comprehensively studied in ourpreceding Papers I-III. The analysis has shown a number of unusual andextremely complex features, which presumably led to the formation of anisolated right-handed magnetic-field configuration. Here we analyze theinterplanetary disturbance responsible for the 20 November superstorm, comparesome of its properties with the extreme 28-29 October event, and reveal acompact size of the magnetic cloud (MC) and its disconnection from the Sun.Most likely, the MC had a spheromak configuration and expanded in a narrowangle of < 14 degree. A very strong magnetic field in the MC up to 56 nT wasdue to the unusually weak expansion of the disconnected spheromak in anenhanced-density environment constituted by the tails of the preceding ICMEs.Additional circumstances favoring the superstorm were (i) the exact impact ofthe spheromak on the Earth's magnetosphere and (ii) the almost exact southwardorientation of the magnetic field, corresponding to the original orientation inits probable source region near the solar disk center.

solar physics

A Challenging Solar Eruptive Event of 18 November 2003 and the Causes of  the 20 November Geomagnetic Superstorm. IV. Unusual Magnetic Cloud and  Overall Scenario