The northern lights are caused by collisions between fast-moving
particles (electrons) from space and the oxygen and nitrogen gas in our
atmosphere. These electrons originate in the magnetosphere, the region
of space controlled by Earth’s magnetic field. As they rain into the
atmosphere, the electrons impart energy to oxygen and nitrogen
molecules, making them excited. When the molecules return to their
normal state, they release photons, small bursts of energy in the form
of light. [1]
Astronaut Mike Hopkins, aboard the International Space Station, shared
this picture of the northern lights on Oct. 9, 2013, saying "The pic
doesn't do the northern lights justice. Covered the whole sky. Truly
amazing!": [1]