Why are auroras green at lower altitudes and red at higher altitudes?

Context

While the common explanation mentions oxygen concentration, it lacks depth in explaining the mechanism behind different color auroras. This question delves into the specific processes that excite different wavelengths at different altitudes.

Simple Answer

  • Imagine tiny particles from the sun, called electrons, hitting the air like tiny balls.
  • When electrons hit oxygen at a lower altitude, they make it 'excited' and it glows green.
  • But higher up, the air is thinner and the electrons have more energy.
  • So, when they hit oxygen higher up, they excite it differently and it glows red.
  • It's like hitting a drum lightly to make a soft sound, but hitting it harder makes a louder sound.

Detailed Answer

The aurora borealis, or northern lights, are a captivating display of colorful lights in the sky, primarily caused by charged particles from the sun interacting with the Earth's atmosphere. The color of the aurora is determined by the type of gas and the altitude at which the particles collide. While oxygen is responsible for both green and red auroras, the difference lies in the energy level of the collision and the specific way oxygen atoms react.

When charged particles from the sun enter the Earth's atmosphere, they collide with atoms and molecules in the air. These collisions excite the atoms, causing them to gain energy and jump to a higher energy level. This excited state is unstable, and the atoms quickly return to their normal state by releasing the excess energy in the form of light. The color of the light emitted depends on the type of atom and the amount of energy released.

At lower altitudes, typically between 60 and 150 miles above the Earth's surface, where the atmosphere is denser, the collisions between charged particles and oxygen atoms are more frequent. These collisions cause the oxygen atoms to release green light as they return to their normal state.

However, at higher altitudes, above 150 miles, the atmosphere is much thinner, and the charged particles have more energy. This allows them to excite oxygen atoms to a higher energy level, leading to the emission of red light. The high energy of these collisions can also cause nitrogen atoms to be excited, resulting in the emission of blue or purple light.

Therefore, the color of the aurora is determined by the altitude of the collision and the energy level of the charged particles. The higher the altitude, the more energetic the particles, and the higher the energy level of the excited oxygen atoms. This leads to the emission of red light. Conversely, at lower altitudes, the energy level of the excited oxygen atoms is lower, leading to the emission of green light.

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