Geomagnetic storms are primarily caused by activity on the Sun that sends energy and charged particles toward Earth, which then interact with Earth's magnetic field and upper atmosphere. Key factors:

• Solar wind and coronal mass ejections (CMEs): When the Sun emits bursts of fast, charged particles, especially in CMEs, they slam into the magnetosphere, compressing it and injecting energy that powers the storm. This is the dominant trigger during solar maximum periods. [sources synthesize from space weather science]
• Interplanetary magnetic field orientation: If the sun’s magnetic field carried in the solar wind (the interplanetary magnetic field) is directed southward, it more easily merges with Earth’s northward field, allowing more energy to transfer into the magnetosphere and amplifying the storm. [general space weather understanding]
• Structure and duration: Geomagnetic storms vary in strength (weak to major) and are usually driven by sustained high-speed solar wind and southward IMF, often lasting hours to many hours. The storm’s main phase is marked by intensified currents in the magnetosphere, strengthening the ring current and depressing the Earth's surface magnetic field. [space weather perspectives]

Process in brief:

• A solar event releases a burst of energetic particles and a magnetic field into space.
• The solar wind carries this material toward Earth.
• When the arriving solar wind couples with Earth’s magnetosphere, energy is pumped in, currents intensify, and the magnetosphere and ionosphere respond, sometimes producing auroras and affecting satellites, GPS, and power grids. [general mechanism]

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