What causes a geomagnetic storm to begin suddenly?

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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.

A geomagnetic storm begins suddenly due to the arrival of high-pressure plasma from the Sun, typically associated with coronal mass ejections or high-speed solar wind streams.12

When the leading edge of this high-pressure region reaches Earth, it compresses the magnetosphere earthward. This sudden motion increases the strength of the magnetopause current, which causes an abrupt, global increase in the horizontal component of the geomagnetic field observed at the ground. This brief interval, lasting only a few minutes, is known as the sudden-commencement phase. Following this, the storm enters its initial phase, during which the increased pressure persists for several hours, maintaining a larger-than-normal surface magnetic field.12