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Why does a southward magnetic field in a coronal mass ejection increase its effects on Earth?

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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.

A southward magnetic field within a coronal mass ejection (CME) significantly increases its impact on Earth because it facilitates the efficient transfer of energy, mass, and momentum from the solar wind into Earth’s magnetosphere.1

This process occurs through magnetic reconnection, a mechanism where magnetic field lines of different orientations break and reconnect. Earth’s geomagnetic field is generally northward-pointing. When a CME arrives with a strong southward magnetic field component, it is oppositely directed to Earth’s field at the dayside magnetopause. This alignment allows the interplanetary magnetic field to couple efficiently with the geomagnetic field, creating "open" field lines that permit solar plasma to enter the magnetosphere.123

This interaction triggers intense geomagnetic storms, which can disrupt satellite operations, power transmission lines, and radio communications, while also intensifying auroral displays.143