What causes auroras?

Auroras are indicators of the connection between the Earth and the sun. The frequency of auroras correlates to the frequency of solar activity and the sun's 11-year cycle of activity.

solar winds
Image courtesy NASA
Illustration of how solar winds impact the Earth's magnetosphere

As the process of fusion occurs inside the sun, it spews high-energy particles (ions, electrons, protons, neutrinos) and radiation in the solar wind. When the sun's activity is high, you'll also see large eruptions called solar flares and coronal mass ejections. These high-energy particles and radiations get released into space and travel throughout the solar system. When they hit the Earth, they encounter its magnetic field.

The poles of the Earth's magnetic field lie near, but not exactly on, its geographic poles (where the planet spins on its axis). Scientists believe that the Earth's liquid iron outer core spins and makes the magnetic field. The field is distorted by the solar wind, getting compressed on the side facing the sun (bow shock) and drawn out on the opposite side (magnetotail). The solar winds create an opening in the magnetic field at the polar cusps. Polar cusps are found on the solar side of the magnetosphere (the area around the Earth that's influenced by the magnetic field). Let's look at how this leads to an aurora.

  1. As the charged particles of solar winds and flares hit the Earth's magnetic field, they travel along the field lines.
  2. Some particles get deflected around the Earth, while others interact with the magnetic field lines, causing currents of charged particles within the magnetic fields to travel toward both poles -- this is why there are simultaneous auroras in both hemispheres. (These currents are called Birkeland currents after Kristian Birkeland, the Norwegian physicist who discovered them -- see sidebar.)
  3. When an electric charge cuts across a magnetic field it generates an electric current (see How Electricity Works). As these currents descend into the atmosphere along the field lines, they pick up more energy.
  4. When they hit the ionosphere region of the Earth's upper atmosphere, they collide with ions of oxygen and nitrogen.
  5. The particles impact the oxygen and nitrogen ions and transfer their energy to these ions.
  6. The absorption of energy by oxygen and nitrogen ions causes electrons within them to become "excited" and move from low-energy to high-energy orbitals (see How Atoms Work).
  7. When the excited ions relax, the electrons in the oxygen and nitrogen atoms return to their original orbitals. In the process, they re-radiate the energy in the form of light. This light makes up the aurora, and the different colors come from light radiated from different ions.

Note: The particles that interact with the oxygen and nitrogen ions in the atmosphere don't come from the sun, but rather were already trapped by the Earth's magnetic field. The solar winds and flares perturb the magnetic field and set these particles within the magnetosphere in motion.

For more information on auroras, take a look at the links on the next page.

How do we know what causes an aurora?

In 1895, a Norwegian physicist named Kristian Birkeland addressed the queston of what causes auroras. Birkeland believed that auroras were caused by electrons from the sun that interacted with the Earth's magnetic field. To test this, he placed a spherical magnet called a terrella inside a vacuum chamber. He also had an electron gun inside the chamber. When he turned on the gun, electrons interacted with the magnet's field and produced an artificial aurora, supporting his hypothesis.

Birkeland's artificial aurora didn't show the characteristic oval ring. The auroral ring was actually predicted by a Japanese graduate student named Shun-ichi Akasofu in 1964. He examined photographs of auroras and concluded that auroras were rings. So, why weren't Birkeland's auroras oval? Birkeland thought the electrons that excited the oxygen and nitrogen ions came directly from the sun. Only when satellites began to study auroras and measure the magnetosphere did scientists figure out that the electrons came from the magnetosphere itself. When this idea was placed in mathematical models, auroral rings could be explained.