CMEs travelling faster than the background solar wind speed can generate a shock wave.

When these flux ropes reconfigure, the denser filament or prominence can collapse back to the solar surface and be quietly reabsorbed, or a CME may result. CMEs can also occur from locations where relatively cool and denser plasma is trapped and suspended by magnetic flux extending up to the inner corona - filaments and prominences. These types of CMEs usually take place from areas of the Sun with localized fields of strong and stressed magnetic flux such as active regions associated with sunspot groups. This can result in the sudden release of electromagnetic energy in the form of a solar flare which typically accompanies the explosive acceleration of plasma away from the Sun – the CME. The more explosive CMEs generally begin when highly twisted magnetic field structures (flux ropes) contained in the Sun’s lower corona become too stressed and realign into a less tense configuration – a process called magnetic reconnection. They expand in size as they propagate away from the Sun and larger CMEs can reach a size comprising nearly a quarter of the space between Earth and the Sun by the time it reaches our planet. Slower CMEs can take several days to arrive. The fastest Earth-directed CMEs can reach our planet in as little as 15-18 hours. CMEs travel outward from the Sun at speeds ranging from slower than 250 kilometers per second (km/s) to as fast as near 3000 km/s. They can eject billions of tons of coronal material and carry an embedded magnetic field (frozen in flux) that is stronger than the background solar wind interplanetary magnetic field (IMF) strength. Coronal Mass Ejections (CMEs) are large expulsions of plasma and magnetic field from the Sun’s corona.

Corona weather in august 2016