NASA - National Aeronautics and Space Administration

As the Sun's ionized and magnetized particles are passing by Earth they impart mechanical energy which is transformed into magnetic energy by compressing the tail. The tail field lines eventually merge (or "reconnect") and slingshot particles towards and away from Earth, thereby converting magnetic into particle energy. This energy finds itself along field lines and powers the aurora on the one hand, and down the tail via the expulsion of a plasma blob, a plasmoid, on the other. This storage-and-release process of solar wind energy by the magnetosphere is called a substorm. (Click on image to view animation.) Credit: NASA