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

TL;DR

This paper analyzes the unusual magnetic cloud responsible for the 2003 geomagnetic superstorm, revealing its compact size, spheromak structure, and the specific conditions that led to the extreme geomagnetic disturbance.

Contribution

It provides a detailed analysis of the magnetic cloud's structure, size, and impact conditions, offering new insights into the causes of the superstorm.

Findings

Magnetic cloud had a spheromak configuration with a size <14 degrees.

Magnetic field in the cloud reached up to 56 nT due to weak expansion.

Exact impact and southward orientation caused the superstorm.

Abstract

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.