coronal mass ejection
A coronal mass ejection (CME) is a huge eruption of material from the Sun's corona into interplanetary space. CMEs are the most energetic of solar explosions and result in the ejection, over the course of several hours, of up to 100 billion kilograms of multi-million-degree plasma at speeds ranging from 10 to 2,000 km/s. They often look like bubbles and, when seen close to the Sun, can appear bigger than the Sun itself, though their density is extremely low.
In contrast to the steady-state solar wind, CMEs originate in regions where the magnetic field is closed and result from the catastrophic disruption of large-scale coronal magnetic structures, such as coronal streamers. CMEs can occur at any time during the solar cycle, but increase in daily frequency from about 0.5 during minimum years to about 2.5 around solar maximum. Fast CMEs – those which outpace the ambient solar wind – give rise to large geomagnetic storms when they encounter Earth's magnetosphere. Such storms, which can disrupt power grids, damage satellite systems, and threaten the safety of astronauts, can result from the passage either of the CME itself or of the shock created by the fast CME's interaction with the slower-moving solar wind.