Why do gases high in the sky glow during an aurora?

An aurora can look like a green curtain being folded across the stars. There is no enormous lamp painting a cloud from below. The light is being made high in the atmosphere, where tiny collisions connect the behavior of atoms to disturbances in the space around Earth.

The Sun continually sends out the solar wind, a stream of charged particles carrying a magnetic field. Its interaction with Earth's magnetic environment can transfer and store energy around our planet. When that energy is released, some of it accelerates electrons. Earth's magnetic field guides their motion toward the upper atmosphere, especially along routes that reach regions around the magnetic poles.

An arriving electron can collide with an oxygen atom or a nitrogen molecule and transfer energy to it. The gas particle is then in an excited state: it has more internal energy than before. Later it can return to a lower-energy state and release a photon, a small packet of light. The incoming electron and the emitted photon are different things. The collision supplies energy; the gas releases part of it as light.

The gas and the change in its energy determine which colors can appear. Oxygen produces familiar green and red emissions. Nitrogen contributes blue, pink and other colors. Altitude matters because different particles and collision conditions occur at different heights. A display can contain several glowing layers whose colors overlap in our view, rather than a single sheet of one substance.

Why does this light often form curtains or arcs instead of covering the whole sky evenly? The particles do not arrive everywhere in equal numbers. Magnetic structure guides them into changing regions, often forming a broad oval around each magnetic pole. Patterns within those regions can brighten and move as the supply and energy of particles change. The visible shape tells a story about large-scale magnetic conditions, while every bit of emitted light comes from a microscopic event.

A gas-discharge sign offers a nearby connection: energetic electrons can also make gas emit characteristic colors inside a tube. An aurora has no glass tube and is driven by a very different environment, but both let us see energy passing into atoms and molecules and leaving as light. The moving sky is a vast collection of these otherwise invisible exchanges.