How Things Work: Auroras
isplay observed in the night sky. During an aurora, the sky lights up on its own in a multitude of colors, ranging from red to green to purple. Glowing colors in the sky pulsate gently and grow bigger and bigger until they fill the entire sky.
Auroras occur mostly in the regions around the north and south poles.
Auroras observed in the northern hemisphere are commonly called “northern lights,” but their scientific name is “Aurora Borealis.” This name is derived from the Roman goddess of dawn, Aurora, and the Greek term for north wind, Borealis.
The auroras observed in the southern hemisphere are called “Aurora Australis.”
If one were to compare auroras to fireworks, one could say that the “ammunition” for the auroras is provided by the sun. To understand how the sun comes into the picture, one must first go back to Earth’s structure.
Earth can be viewed as a huge magnet. Like all magnets, Earth has a magnetic field that extends from it. In particular, magnetic field lines surround the earth, entering at the south pole and exiting at the north pole.
The north and south poles are the magnetic poles of Earth. The region around Earth to which Earth’s magnetic field extends is called Earth’s magnetosphere.
Another magnetic layer around Earth is called the ionosphere. The ionosphere forms the inner edge of the magnetosphere. The ionosphere is ionized by solar radiations and contains a large number of free electrons and protons.
Such a mixture of protons and electrons in space is called plasma. Plasma can conduct electricity, which is essential to the occurrence of auroras.
The outermost part of the atmosphere of the sun is called the corona, which is the ring that is visible during a solar eclipse.
The sun’s corona is associated with a magnetic field, and because magnetic fields are known to be formed due to moving charges, one can infer that the corona consists of plasma.
Furthermore, the sun has a high gravitational force that pulls the corona inward. However, because of the corona’s high temperature, this gravitational force cannot hold the entire corona.
Consequently, jets of corona stream out from the sun. Such a jet of corona coming from the sun is called solar wind.
This solar wind speeds toward Earth, but Earth has a shield — its magnetic field — against the solar wind. As a result, a huge cavity is formed between Earth and the wind.
Though the magnetosphere blocks the solar wind, the solar wind still pushes its way around the magnetosphere. In the process, it squeezes the magnetosphere.
The magnetosphere then acquires a contorted structure which is compressed on the day side of Earth (toward the sun), and it has a tail-like portion on the other side of Earth, which is called the magnetotail.
Although the Earth’s shield keeps out a certain amount of solar energy, solar particles always manage to enter the magnetosphere through the magnetotail. These solar particles then move toward the sun-side of the earth.
Now and then, this squeezing and extra charge due to solar particles gives rise to a buildup of pressure, and this pressure gives rise to an electric voltage between the magnetotail and the poles of Earth. This voltage can be as high as 100,000 volts.
This voltage pushes some of the lighter charged particles of the plasma that surrounds Earth along Earth’s magnetic field lines at a very high velocity.
These charged particles then travel to Earth’s poles (where the field lines converge) and collide with atoms of gases in the ionosphere.
This collision imparts energy to the gas atoms, and the atoms become excited. The electrons in the atoms move up to higher energy levels because of the extra energy. They soon jump back to their original energy level (ground state), however, and release the energy that they absorbed.
The atoms release the energy in the form of light. Each light photon has a specific wavelength depending on the nature of the atom. The specific wavelength of the light determines the color of the light.
Thus, the colors that are produced depend on the gases that are present in the atmosphere.
Oxygen, for instance, is found in a high proportion in the higher atmosphere, and excitation of oxygen atoms releases green and red light.
This is why most auroras are colored red or green in the atmosphere.