What Causes the Aurora Borealis' Colors?

Aurora - Wikipedia

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Aurora Borealis Science, Science Behind Northern Lights

What causes the different shapes and colours of the aurora? - BBC Sky at Night Magazine

What are the Northern Lights / Aurora Borealis? – Norwegian Centre for Space Weather (NOSWE)

The Colors of the Aurora (U.S. National Park Service)

All About the Northern Lights in the United States

What Causes the Aurora - Lee Petersen

Why there are Different Aurora Colours Explained by Aurora Nights

The Northern and Southern Lights - What is an Aurora? - Universe Today

Aurora Northern Lights - Where and when to see the Aurora Borealis