Why Can We Only See the Aurora Near the North and South Poles?
How Earth's Magnetic Field Lines Steer the Wind from the Sun
Green and red curtains of light across the night sky: the aurora. Many of us have seen it in photos or videos, but the places where you can actually see it are said to be a few limited regions near the North and South Poles. Why can't the aurora be seen everywhere in the world? Why does it appear in particular places?
The places named as aurora spots are Norway, Iceland, Alaska and northern Canada, all far to the north on a globe. In the Southern Hemisphere, too, it is said to be seen in the south of New Zealand and near the Antarctic continent.
Near the equator, or at mid-latitudes such as Japan, the aurora is almost never seen unless very special conditions come together. The light could just as well appear over any part of Earth, so why is it so lopsided?
The key to this puzzle is said to lie in the relationship between the invisible "wind" blowing from the Sun and Earth, a giant magnet.
This stream is called the "solar wind," and it is thought to be made of tiny charged particles such as electrons and protons.
Earth behaves like a giant magnet, and its field lines bunch together near the North and South Poles.
Let's look at how field lines guide particles to the poles, step by step.
The invisible "wind" blowing from the Sun
The Sun gives off not only light and heat but also a stream of tiny charged particles, such as electrons and protons, which flows constantly into space. This stream is called the "solar wind," and it is thought to travel through space at a very high speed, several hundred km per second. Earth is exposed to this solar wind all the time.
Earth, a "giant magnet," bends the particles' paths
Thanks to electric currents flowing inside it, Earth is said to create magnetic field lines like those of a giant bar magnet in space. Because solar wind particles carry an electric charge, when they meet field lines they cannot travel straight, and are thought to be bent to follow the lines. The region shaped by Earth's magnetism is called the "magnetosphere," and it is said to act like a shield that protects the surface from most solar wind particles.
They deliberately steer much of it to "exits" at the North and South Poles.
Why do particles gather near the poles?
Recall the experiment with a bar magnet. If you sprinkle iron filings around a magnet, they gather thickly near its N and S poles. This is because the field lines crowd together toward the two ends (poles) of the magnet. Earth's field lines have the same shape: they crowd together near the North and South Poles.
Solar wind particles, bent to follow the field lines, are guided into the sky near the North and South Poles, where the lines crowd together. There they hit oxygen and nitrogen in the atmosphere, and give off light that we see as the aurora.
When a big explosion occurs on the Sun (such as a solar flare), a stronger solar wind than usual is said to sometimes reach Earth. At such times, an aurora normally seen only near the poles is occasionally observed from lower-latitude regions such as Japan.
What you can check for yourself
- Prepare a bar magnet and some iron filings (or a compass)
- Place a sheet of paper over the magnet and sprinkle a thin layer of iron filings on the paper (or move the compass a little at a time around the magnet)
- Watch how the filings gather thickly near the N and S poles (or the compass needle swings sharply)
- Imagine that Earth's field lines likewise gather near the North and South Poles
With a simple magnet experiment, you can see with your own eyes the shape of field lines that are normally invisible.
Summary
The aurora is concentrated near the North and South Poles because charged particles blowing from the Sun (the solar wind) are bent along Earth's magnetic field lines, or so it is thought. Earth's field lines crowd together near the North and South Poles, just like the pattern of iron filings around a bar magnet, so solar wind particles are collected in these two regions and give off light as they hit the atmosphere.
The beautiful light of the aurora is the result of a grand physical phenomenon woven by the wind from the Sun and Earth, a giant magnet.
Why the aurora looks different in colour depending on place and height is explained in this article.
For another familiar phenomenon born from Earth's tilt and motion, see also Why is Autumn Equinox Day sometimes September 23 and sometimes 22?
For those who want to know more: terms, numbers and links to textbooksFrom middle-school science to topics under active research, with the level of each part clearly marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics"
- High school+High-school "Physics," or advanced material and columns in textbooks
- UniversityNot taught in high school; university-level content (magnetospheric physics)
- ResearchNot yet taught as settled fact even at university; topics researchers are investigating now
Middle schoolTerms: words around the aurora
- Solar wind: a stream of charged particles blowing from the Sun.
- Magnetic field lines: invisible paths along which magnetic force acts around a magnet.
- Magnetosphere: the region of space reached by Earth's magnetism.
High schoolChecking with a formula: how far can a particle stray from a field line?
Let's work out the radius of the circle a solar wind particle makes around a field line (the Larmor radius). If it is much smaller than Earth, the particle cannot leave the field line and is guided along it to the poles.
| In symbols | r ＝ (m × v) ÷ (q × B) |
| In words | radius of the circle ＝ particle mass × speed ÷ (particle's electric charge × magnetic field strength) |
| Where the formula comes from | It is a balance between the Lorentz force and circular motion. The force from the magnetic field exactly matches the force needed to keep moving in a circle. |
| Symbol m: mass of a proton (in kilograms) | Said to be 1.67 times 10 to the power of minus 27 |
| Symbol v: speed of the solar wind (in metres per second) | As a rough guide, 5 times 10 to the power of 5 |
| Symbol q: electric charge of a proton (in coulombs) | Said to be 1.6 times 10 to the power of minus 19 |
| Symbol B: strength of the magnetic field near Earth (in tesla) | As a rough guide, 3 times 10 to the power of minus 8 |
| Numerator (number part) | 1.67 × 5 ＝ 8.35 |
| Denominator (number part) | 1.6 × 3 ＝ 4.8 |
| Radius (number part; unit is 10 to the power of 5 metres, i.e. about 174 km) | 8.35 ÷ 4.8 ≒ 1.74 |
| How many times is Earth's radius of 6400 km larger? | 6400 ÷ 174 ≒ 37 |
The circle a particle traces is only about 1/40 of Earth's radius. So the particle cannot stray far from a field line, and is carried to the North and South Poles, where the field lines dive into the ground. This smallness is why the aurora gathers at the poles.
As a side note, let's also work out how long the solar wind itself takes to reach Earth.
Time taken (s) ＝ Sun–Earth distance (km) ÷ solar wind speed (km/s)
| Sun–Earth distance | Said to be about 150 million km |
| Solar wind speed | As a rough guide, about 500 km per second |
| Time taken (s) | 150000000 ÷ 500 ＝ 300000 |
| Convert to hours | 300000 ÷ 3600 ≒ 83.3 |
| Convert to days | 83.3 ÷ 24 ≒ 3.5 |
| Result | The solar wind reaches Earth after about 3.5 days |
It is said that the aurora becomes active a few days after a big explosion is seen on the Sun, and this is thought to be related to how long the solar wind takes to reach Earth.
* The solar wind speed is said to vary from about 300 to 800 km per second depending on conditions, so this calculation is a rough guide.
High school+Why charged particles spiral around field lines
When a charged particle moves through a magnetic field, it feels a force (the Lorentz force) perpendicular to both its direction of motion and the direction of the field line. Because of this force, a charged particle is thought to move in a spiral, winding around the field line.
UniversityThe magnetosphere, a region that protects Earth
In magnetospheric physics, researchers study the shape of the region reached by Earth's magnetism (the magnetosphere) and how it interacts with the solar wind. The magnetosphere keeps most solar wind particles from reaching the surface directly, but in regions near the poles called "cusps," particles are thought to get in relatively easily.
📖 Derivations and further reading: Japanese Wikipedia, "Magnetosphere"
ResearchWhat is still unclear
- At what height, and by what mechanism, the colour and shape of the aurora are determined in detail is a topic still under close study in magnetospheric physics.
- Research is also under way on techniques to predict "space weather" more accurately, which is how solar activity affects satellites, power grids and more.
- Studies also compare Earth's aurora with the auroras seen on other planets such as Jupiter and Saturn.
The aurora that colours the night sky opens onto a grand subject, where electromagnetism and space physics meet and research continues today.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: how magnets work | Basic terms: solar wind, field lines, magnetosphere |
| High school | Basic Physics: distance and time | Calculating the solar wind's travel time to Earth |
| High school+ | Physics: currents and magnetic fields (advanced) | The Lorentz force and the spiral motion of charged particles |
| University | Magnetospheric physics | Structure of the magnetosphere and the cusp regions |
| Research | Magnetospheric physics and space weather forecasting (under research) | Details of the light-emission mechanism, space weather prediction |
- Explanations of the solar wind and Earth's magnetic field in earth science and physics textbooks.
- Research reviews on how the aurora is produced, in the field of magnetospheric physics.
- Explanations of solar wind speed and travel time to Earth in space physics materials.
- Explanations of the Lorentz force and charged-particle motion in physics textbooks.
- Explanations of solar flares and their effects on the magnetosphere in space weather forecasting materials.
* Solar wind speed and travel time are rough values that change with solar activity.
* This article is a general-audience science explainer. If you go to see the aurora, dress warmly, follow the weather and local rules, and enjoy it safely.