Coronal mass ejection

The Sun's corona—the outer atmosphere—occasionally expels large structures containing plasma and magnetic field. These eruptions, called coronal mass ejections, travel outward through the heliosphere and expand. Their speeds and sizes vary widely. A CME carries solar material and an embedded magnetic field, unlike a burst of electromagnetic radiation alone.

Many explosive CMEs arise when stressed magnetic structures in the corona reorganize, releasing stored magnetic energy and driving plasma outward. Magnetic reconnection is one important process in this reorganization. CMEs can also be associated with erupting filaments or prominences—relatively cool, dense material suspended in the corona by magnetic fields. Solar eruptions vary, and no single description captures every event.

A solar flare is a sudden release of electromagnetic radiation from the Sun, whereas a CME is an outward-moving expulsion of plasma and magnetic field. They often occur in the same active region and can be associated, but they are distinct phenomena and either may occur without the other. Because radiation and ejected material travel and interact differently, observing a flare alone does not establish that a CME will strike Earth.

If a CME reaches Earth, its plasma and magnetic field can disturb the magnetosphere. The strength of the resulting geomagnetic storm depends in part on the CME's properties and on the orientation of its magnetic field relative to Earth's. Storms can produce auroras and can also affect radio and satellite navigation, increase drag on low-Earth-orbit spacecraft, and induce currents in power systems. An eruption is not automatically an Earth-directed event, nor does every arrival produce a severe storm.

Spacecraft coronagraphs image the corona by blocking the bright solar disk, helping scientists estimate a CME's size, speed, and direction. Observations farther from the Sun, including measurements near the Sun–Earth L1 region, can identify an approaching disturbance and improve short-term warnings. Forecasts remain uncertain because the CME's path and magnetic-field orientation must be inferred and can evolve during its journey.