Solar source of the largest geomagnetic storm of cycle 23

The largest geomagnetic storm of solar cycle 23 occurred on 2003 November 20 with a Dst index of −472 nT, due to a coronal mass ejection (CME) from active region 0501. The CME near the Sun had a sky‐plane speed of ∼1660 km/s, but the associated magnetic cloud (MC) arrived with a speed of only 730 km/s. The MC at 1 AU (ACE Observations) had a high magnetic field (∼56 nT) and high inclination to the ecliptic plane. The southward component of the MC's magnetic field was made up almost entirely of its axial field because of its east‐south‐west (ESW) chirality. We suggest that the southward pointing strong axial field of the MC reconnected with Earth's front‐side magnetic field, resulting in the largest storm of the solar cycle 23.