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Why were auroras visible near the tropics during the geomagnetic storm of 1859?

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Space weather Earth's full North Polar auroral oval, in an image taken in ultraviolet light by the U.S. Polar spacecraft over northern Canada, April 6, 1996. In the colour-coded image, which simultaneously shows dayside and nightside auroral activity, the most intense levels of activity are red, and the lowest levels are blue. Polar, launched in February 1996, was designed to further scientists' understanding of how plasma energy contained in the solar wind interacts with Earth's magnetosphere.

The geomagnetic storm of 1859, the largest ever recorded, caused auroras to be visible near the tropics due to the extreme intensity of the event, which was driven by a massive coronal mass ejection (CME) from the Sun.12

During such intense geomagnetic storms, the interaction between the solar wind and Earth's magnetosphere becomes highly efficient through magnetic reconnection. When the magnetic field within a CME possesses a strong southward component, it couples effectively with Earth's northward-pointing geomagnetic field. This process transfers vast amounts of energy, mass, and momentum into the magnetosphere, causing significant disturbances.2

These disturbances energize charged particles and propel them toward the ionosphere, where their precipitation creates auroral displays. In the case of the 1859 storm, the magnitude of this energy transfer was so great that the resulting auroral activity expanded far beyond the polar regions, reaching as far south as the tropics.3412