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