What are the sudden-commencement, initial, main, and recovery phases of a geomagnetic storm?

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geomagnetic field The magnetic field of a bar magnet has a simple configuration known as a dipole field. Close to Earth's surface this field is a reasonable approximation of the actual field.

A geomagnetic storm is a disturbance of Earth's magnetosphere caused by solar activity. The progression of a storm is characterized by four distinct phases:

Sudden-commencement phase: Lasting only a few minutes, this phase involves a sudden global increase in the horizontal component of the geomagnetic field.1
Initial phase: This follows the sudden commencement and consists of the persistent increase of the horizontal field component, typically lasting two to six hours.1
Main phase: Lasting 12 to 48 hours, this phase is marked by a decrease in the horizontal component of the field. This occurs as the magnetosphere becomes inflated or injected with incoming plasma.1
Recovery phase: In this final stage, the injected plasma slowly drains into the atmosphere or the interplanetary medium over several days, allowing the geomagnetic field to return to its pre-storm state.1