Skip to content

Auroras

Electricity and magnetism · level 6, lesson 6 of 6

A question to start

In March 2024, a huge burst of particles left the Sun and swept past Mars. A few days later, Perseverance looked up at the night sky and saw it glowing faintly green. How can a sky glow?

The idea

The Sun throws out charged particles: a steady stream, the solar wind, and sudden bursts in solar storms. When fast particles hit atoms high in a planet's air, they knock the atoms' electrons up to higher energy levels. As the electrons drop back, the atoms give out light: an aurora.

Oxygen atoms give a green line at exactly 557.7 nm. That is the green of auroras on Earth, and it is the green Perseverance saw.

  • On Earth, the magnetic field steers the particles along its field lines towards the poles, so auroras glow in rings round the magnetic poles.
  • On Mars, there is no global field to steer them. During a strong solar storm, the fastest particles can rain in everywhere, and the whole night sky glows.
Auroras Mars Earth
Where they glow can fill the whole night sky in a solar storm in rings round the magnetic poles
Green oxygen line 557.7 nm 557.7 nm
First seen in visible light 2024, from the ground by Perseverance seen by people for thousands of years

NASA's MAVEN orbiter had discovered this whole-sky kind of aurora on Mars in 2014, but only in ultraviolet light. The 2024 sighting was the first aurora ever seen from the surface of another planet.

Earth: rings at the polesMars: the whole sky

Worked examples

1. The photon. At 557.7 nm, each photon carries \(hc/\lambda = 1240/557.7 = 2.22\) eV.

2. Why the poles. Charged particles spiral along field lines. Earth's field lines dip into the ground near the poles, so that is where the particles reach the air.

3. Planning to see it. The scientists knew the solar storm was heading for Mars days ahead, so they pointed the rover's camera and spectrometer at the right part of the sky in time.

Try it

  1. What makes the air glow in an aurora?
  2. Which atom gives the green 557.7 nm light?
  3. Why are Earth's auroras near the poles?
  4. Why can a Martian aurora fill the whole sky?
Answers
  1. Fast charged particles hit atoms high in the air, lifting their electrons; the atoms give out light as the electrons drop back.
  2. Oxygen.
  3. The magnetic field guides the particles along its lines, which reach the air near the poles.
  4. Mars has no global field to steer the particles, so they can arrive anywhere.

Common mistake

"No magnetic field means no auroras." Mars has auroras, just of different kinds. Without a global field, they are not confined to the poles.

Where it leads