A curtain of green auroras ripples over hoodoo rock formations near Drumheller, Canada, early Monday, Labor Day in the U.S. The same night, similar shows enlivened skies over many high-latitude countries across the Northern Hemisphere.
Last Friday a solar flare exploded off the sun, launching a giant cloud of charged gas—a coronal mass ejection, or CME—toward Earth. About 48 hours after NASA's Solar Dynamics Observatory had detected the eruption, the CME, travelling faster than a million miles an hour, slammed into Earth's magnetic field, sparking the auroras.
When a CME, or solar wind, enters the upper atmosphere, its charged particles smash into and break up gas molecules, which give off energy in the form of the so-called northern lights (or in the Southern Hemisphere, southern lights).
The colors a sky-watcher sees depend on the type of gas being hit and how high it is. For example, the green aurora pictured was the result of oxygen-atom collisions about 60 to 120 miles (100 to 200 kilometers) up.