Coronal mass ejections (CMEs) are large eruptions of plasma and magnetic field from the solar corona. They are often associated with solar flares but can also occur without any flare activity. CMEs can cause geomagnetic storms that disrupt the Earth’s magnetosphere and ionosphere, and can even induce currents in electrical power grids.
A CME typically starts with a rise in soft X-ray emission from the active region where it originates. This is followed by an expansion of the hot plasma into interplanetary space, sometimes reaching as far as Earth. The event is usually accompanied by a release of energy in the form of a shock wave. The typical speed of a CME is around 1000 km/s, but they can range from slow (a few hundred km/s) to extremely fast (>2000 km/s).
The most powerful CME ever observed was on July 23, 2012, associated with an X5.7 class flare. It had a peak velocity of 3100 km/s and expelled about 10^13 kg (about one-third the mass of Mount Everest) of plasma into space.
CMEs can interact with each other, or with material already present in interplanetary space, such as dust particles or planetary atmospheres. When they impact upon planets or moons, they can create spectacular light shows known as aurorae. CME interactions are also thought to be responsible for some types of radio emission from Jupiter’s atmosphere known as Decametric Bursts.