Aurora
An atmospheric glow produced when charged particles from the Sun are guided by Earth's magnetic field and collide with gases in the upper atmosphere near the poles.
Earth's magnetic field has flipped hundreds of times; the last reversal was about 780,000 years ago. During reversals the field can weaken to under 10% of today’s strength, letting auroras appear near the equator. Since 1840, Earth’s field has already weakened by about 9%.
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