The image shows the aurora stretching across the entire horizon, with the moon in the upper right portion of the picture, according to the description from NASA. Jupiter is visible to the right of the moon, and if you look closely, you can see the Big Dipper right in the middle of the aurora.
According to the Geophysical Institute at the University of Alaska Fairbanks, an aurora is caused by the collision of electrons from space with atoms and molecules of gasses (like oxygen and nitrogen) from the earth's atmosphere. This collision results in a transfer of energy to the oxygen's electrons, and, as a result, quick bursts of light are emitted. A great number of these collisions create the light that's visible to the naked eye.
Casado, who's a coordinator of the Tierras y Estrellas project, was able to capture the entire horizon because he took multiple photos and stitched them together digitally.