Erin is writing a paper on the northern lights. In which sentence should she add a definition to help clarify her ideas? The northern lights are colorful swirls of light that flicker in the nighttime sky in the upper part of the northern hemisphere. This natural occurrence happens when fast-moving, electrically charged particles (electrons) from space collide with the oxygen and nitrogen gas in our atmosphere. These electrons originate in the magnetosphere. As they rain into the atmosphere, the electrons give energy to oxygen and nitrogen molecules, making them excited. When the molecules return to their normal state, they release small bursts of energy in the form of light.

When billions of these collisions occur and enough photons are released, the oxygen and nitrogen in the atmosphere let out enough light for the eye to detect it. This ghostly glow can light up the night sky in a dance of colors. But since the northern lights are much dimmer than sunlight, they cannot be seen from the ground in the daytime.

The color of the northern lights depends on which gas is being excited by the electrons and on how much energy is being exchanged. These molecules also let out ultraviolet light, which can only be detected by special cameras on satellites.