Coronal mass ejection (CME)

Alternative title: CME

coronal mass ejection (CME), Sun: coronagraph of a coronal mass ejection, taken by Solar and Heliospheric Observatory spacecraft, December, 2002 [Credit: SOHO/ESA/NASA]Sun: coronagraph of a coronal mass ejection, taken by Solar and Heliospheric Observatory spacecraft, December, 2002SOHO/ESA/NASAlarge eruption of magnetized plasma from the Sun’s outer atmosphere, or corona, that propagates outward into interplanetary space. The CME is one of the main transient features of the Sun. Although it is known to be formed by explosive reconfigurations of solar magnetic fields through the process of magnetic reconnection, its exact formation mechanism is not yet understood.

Fast CMEs drive interplanetary shocks in the solar wind and cause the most intense geomagnetic storms on Earth. The main drivers of space weather, geomagnetic storms are disturbances in Earth’s magnetosphere that can have significant impact on both ground- and space-based technological systems. Their formation process, three-dimensional structure, evolution as they propagate through interplanetary space, relationship with solar flares, and impact on Earth’s space environment are important areas of solar and space physics research.

Observations and appearance

Encke’s comet: coronal mass ejection [Credit: NASA/GSFC Conceptual Image Lab]Encke’s comet: coronal mass ejectionNASA/GSFC Conceptual Image LabBefore the invention of the coronagraph (an instrument that places an occulting disk in front of the Sun to block out its bright light), the Sun’s corona was visible for only a few minutes during total solar eclipses, when the Moon acted as the occulting disk. With the advent of space-based solar astronomy in the early 1970s, high-resolution and relatively continuous observations of the Sun’s corona could be made, allowing for the routine observation of CMEs.



CMEs are very large and dynamic structures that can contain more than 1015 grams of solar material. They can have a radial size of 0.25 astronomical unit (AU; 37 million km, or 23 million miles) when they pass by Earth, which is 1 AU (150 million km, or 93 million miles) from the Sun. CMEs that are launched toward Earth are called halo CMEs because as they approach Earth, they appear larger than the Sun, making a “halo” of bright coronal emission completely around it.

The occurrence rate of CMEs generally follows the 11-year solar cycle of sunspot activity, and CMEs occur more frequently and are most intense around solar maximum. CMEs cause the largest geomagnetic storms. There are two main types of geomagnetic storms: recurrent and nonrecurrent storms. Recurrent storms are caused by features on the Sun called coronal holes that live for several months and generate corotating interaction regions (disturbances in the solar wind where the fast solar wind from the coronal holes catches up with the slow solar wind) that repeat on the 27-day solar rotation period. Nonrecurrent storms occur sporadically throughout the solar rotation but are primarily driven by CMEs. Corotating interaction regions are most commonly observed during the declining phase of the solar cycle (the few years after solar maximum) into solar minimum, whereas CMEs are seen most often during solar maximum.

CMEs and space weather



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