Which phenomenon is primarily responsible for auroras near Earth's poles?

Correct — C. Auroras are caused by charged particles from the Sun colliding with atoms in the upper atmosphere.

Disturbances in the solar wind alter the particles' paths in Earth's magnetosphere, and the magnetic field guides electrons and protons down around the poles.

The struck oxygen and nitrogen glow. Solar wind plus magnetic field → option (c).

Why the others are wrong

• (a)
  Earthquake waves — Earthquake waves travel through the solid Earth. They carry no charged particles from the Sun and make no light in the upper atmosphere.
• (b)
  Lightning discharges — Lightning is an electrical discharge in thunderclouds, down in the weather. Auroras glow around 100 km up and are driven by the Sun.
• (d)
  Volcanic ash dispersion — Volcanic ash is fine rock dust spread by winds. It has nothing to do with the Sun's charged particles, which are what light an aurora.

Auroras form in the auroral zone, a band about 6° of latitude wide centred near 67° north and south. Northern ones are the aurora borealis, southern ones the aurora australis.

Colour depends on the gas and the height. Atomic oxygen gives green (557.7 nm) lower down and red (630 nm) higher up. Molecular nitrogen gives blue.

Key facts

• Auroras are caused by charged particles from the Sun colliding with atoms in the upper atmosphere.
• Earth's magnetic field guides these particles towards the polar regions.
• Atomic oxygen produces the common green and the high-altitude red, molecular nitrogen the blue.
• The northern lights are the aurora borealis, the southern lights the aurora australis.

Common traps

• Crediting lightning: lightning is lower-atmosphere weather, while auroras are driven by the Sun.
• Forgetting the magnetic field: it is what concentrates auroras around the poles.

No directly related past PYQ was found.