A coronal mass ejection is a solar event which emits a burst of plasma (electrons and protons). On Earth, such ejections disrupt the magnetosphere and can deplete the ozone layer. As a CME impacts Earth's magnetosphere, it compresses it on the day side and extends the nightside tail. When the magnetosphere's lines reconnect on the nightside, this creates trillions of watts of power which is directed back towards earth's upper atmosphere. CME's, along with solar flares, can cause particularly strong polar aurora. These events can disrupt radio transmissions, cause power outages (blackouts) and damage satellites and electrical transmission lines.
See also: solar flare