Here's NASA's background on it:
These ever-shifting displays of coloured ribbons, curtains, rays, and spots are most visible near the North (aurora borealis) and South (aurora australis) Poles as charged particles streaming from the Sun (the solar wind) interact with Earth's magnetic field, resulting in collisions with atoms of oxygen and nitrogen in the upper atmosphere. The atoms are excited by these collisions, and typically emit photons as a means of returning to their original energy state. The photons form the aurora that we see. The most commonly observed colour of aurora is green, caused by photons (light) emitted by excited oxygen atoms at wavelengths centered at 0.558 micrometers, or millionths of a meter [...]
While aurora are generally only visible close to the poles, severe magnetic storms impacting the Earth's magnetic field can shift them towards the equator. This striking aurora image was taken during a geomagnetic storm that was most likely caused by a coronal mass ejection from the Sun on May 24, 2010. The ISS was located over the Southern Indian Ocean at an altitude of 350 km, with the astronaut observer most likely looking towards Antarctica (not visible) and the South Pole.