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How do coronal mass ejections cause intense geomagnetic storms?

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Diagram of Earth's atmosphere layers-troposphere, stratosphere, mesosphere, and thermosphere-showing how temperature and air pressure change with increasing altitude, with key features like the ozone layer and pressure percentages at various heights. — Ionosphere and magnetosphere The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.

Coronal mass ejections (CMEs) are large eruptions of magnetized plasma from the Sun that trigger intense geomagnetic storms through the efficient transfer of energy, mass, and momentum into Earth’s magnetosphere.12

The primary mechanism for this interaction is magnetic reconnection. When a CME approaches Earth, its internal magnetic field often contains a strong, long-lived southward component. Because Earth’s geomagnetic field points northward, this southward orientation allows the CME’s magnetic field to couple efficiently with the magnetosphere.123

This process leads to several critical developments:
* Energy Transfer: Reconnection allows solar plasma to enter the magnetosphere, where it is accelerated and energized.2
* Ring Current Formation: The resulting dawn-to-dusk electric field across the magnetosphere energizes charged particles, which become trapped in closed drift paths around the planet, forming a symmetrical ring current. This current reduces the magnetic field at Earth’s surface, characterizing the main phase of a geomagnetic storm.34
* Substorms: Intense magnetospheric substorms often accompany these events, further injecting particles into the inner magnetosphere and causing significant space weather disturbances.3