A geomagnetic storm is a worldwide disturbance of Earth's magnetic field that follows 2 to 3 days after violent activity on the Sun, such as solar flares or coronal mass ejections. A high energy plasma ejected from a solar disturbance sets up large currents in Earth's magnetosphere, causing a rapid rise of about 0.2% in the magnetic field at the surface. The plasma subsequently moves around Earth, with attendant auroral and geomagnetic effects, while the field value drops to about 0.5% below its normal value, recovering over several days. A geomagnetic storm's main phase, which can last as long as 2 to 2½ days in the case of a severe storm, is characterized by multiple intense substorms.