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Lessons / Space around Earth

Space around Earth · 4 min · 4 steps

Why the sky lights up: aurora

Plays here

Goal. Explain where aurorae appear and why, and read a live aurora forecast and the Kp index.

Steps

  1. Predict

    Where on Earth do aurorae appear most often?

    • Right at the geographic North and South Poles
    • In rings around the magnetic poles
    • Along the equator
    Show the answer

    In rings around the magnetic poles. In rings around the magnetic poles, the auroral ovals.

    The glow is NOAA's live forecast of where aurora may be seen in about the next hour. It forms a ring around the north magnetic pole, over Alaska, Canada, Iceland, Scandinavia and Siberia.

  2. Around the south magnetic pole there is a second ring: the aurora australis, the southern lights, seen from Antarctica and sometimes from Tasmania and southern New Zealand.

  3. Charged particles from the Sun are steered by the Earth's magnetic field towards these rings. They hit oxygen and nitrogen 100 to 300 km up and make them glow: green and red from oxygen, blue and purple from nitrogen.

  4. The legend shows the Kp index, from 0 to 9. In a strong storm the ring grows towards the equator: in May 2024 aurorae were seen in Mexico and recorded by cameras in the Himalayas of northern India.

The common mistake

Myth

Aurorae are sunlight reflecting off polar ice.

Fact. Aurorae are light given off by gases in the upper atmosphere, 100 to 300 km up, when charged particles from the Sun crash into them. They are far above the clouds and have nothing to do with ice.

Check yourself

Question 1

What makes an aurora glow?

  • Sunlight reflecting off ice
  • Particles from the Sun hitting gases high in the atmosphere
  • Lightning near the poles
Show the answer

Particles from the Sun hitting gases high in the atmosphere. Charged particles steered by the magnetic field excite oxygen and nitrogen, which then give off light.

Question 2

What does a high Kp index mean?

  • The magnetic field is strongly disturbed, so aurorae spread further from the poles
  • The air at the poles is very cold
  • The Moon is full
Show the answer

The magnetic field is strongly disturbed, so aurorae spread further from the poles. Kp measures how much the magnetic field is shaken. During storms the auroral rings widen towards the equator.

Question 3

Which gas makes the common green aurora?

  • Oxygen
  • Neon
  • Carbon dioxide
Show the answer

Oxygen. Oxygen about 100 to 250 km up gives the familiar green light; higher up, oxygen glows red.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
EHa-Vc-2
US Next Generation Science Standards
MS-PS2-5
England national curriculum
KS3 Physics: magnetism (Earth's magnetism, compass and navigation)