During a geomagnetic storm, solar wind interacts with Earth’s magnetic field, transferring large amounts of energy into the upper atmosphere in the form of electric currents. While some of this energy can fuel auroras, most is transferred into heat in a process called Joule heating, which causes the upper atmosphere to expand. The animation shows that when the storm begins, heat enters the auroral zone. In response, the atmospheric gas above the aurora expands and is lifted to higher altitudes. It then falls in waves that cover the entire globe in a matter of hours.