Auroras: Mysterious Lights in the Sky

16 Mar 2007 - Articles, Atmosphere & Space, Arctic, Antarctic

Aurora Borealis at US Air Force Base Eielson in Bear Lake, Alaska

Auroras are wondrous and mysterious phenomena occurring primarily in the polar regions of the Earth. During an aurora, coloured bands of light appear in the sky and seem to dance around creating an eerie yet spectacular light show in the sky. They are a constantly occurring phenomenon, although they are much easier to see when the sky is dark. Many parts of the world where auroras occur frequently have developed thriving tourism industries because of them.

In the Northern Hemisphere, the phenomenon of the aurora is commonly known as the Northern Lights. Italian astronomer and mathematician Galileo Galilei gave the Northern Lights their Latin name, Aurora Borealis, which literally means "northern red dawn." Auroras occurring in the Southern Hemisphere are called the Aurora Australis, which translates to their more common name, the Southern Lights.

Humans have been fascinated by auroras for centuries. The ancient folklore of far northern cultures came up with very creative and interesting stories to explain the strange lights in the sky. The stories are as diverse as the cultures inhabiting this part of the planet. It has since been discovered that auroras are, in fact, the result of charged particles from the solar wind interacting with the Earth's magnetic field and the various gasses in the Earth's atmosphere.

The Role of the Sun and the Magnetosphere

The sun constantly emits streams of ionised (highly charged) particles from its corona, creating a solar wind. The solar wind travels out into the solar system, attaining an average speed of 400 kilometres per second. When the solar wind reaches the magnetic field of a planet such as the Earth, they interact with the magnetosphere of the planet. When the solar wind reaches the Earth's magnetosphere, the magnetic flow distorts the wind.

Charged particles (electrons and positively charged ions) enter the Earth's magnetosphere and are guided along the Earth's magnetic field lines towards the poles of the Earth's magnetic field, located in the Earth's Polar Regions. While the Earth's magnetic poles are not in the same location as the Earth's geographic poles, they are not too far away. It is here that the charged particles from the solar wind are drawn closer inside the part of Earth's atmosphere known as the ionosphere.

At between 80 and 300 kilometres above the Earth's surface the charged particles collide with the various gas molecules in the ionosphere and "excite" them by transferring enormous amounts of energy to them. When the gas molecules become "excited", they release this excess energy in the form of visible light, as well as non-visible infrared, ultraviolet and X-ray radiation, creating the phenomenon we know as an aurora.

Heightened solar activity caused by sunspots and solar flares usually creates strong gusts in the solar wind. It takes about 40 hours for a solar gust to reach the Earth from the sun. These gusts create geomagnetic storms which, in turn, make for spectacular auroras if they are strong enough. Geomagnetic storms tend to occur most frequently around the equinoxes, due to the particular angle of inclination the Earth's magnetic field is in compared to the sun's magnetic field.

Auroral oval over Antarctica seen from space

Appearance

The contour of the Earth's magnetic field is responsible for the location and appearance of auroras. As the ionised particles from the solar wind follow the path of the Earth's magnetic field lines, this gives auroras a curtain-like or arc-like appearance stretching from east to west. The aurora is usually strongest at a radius of about 2500 kilometres from the Earth's magnetic poles, because it is here that the majority of the charged particles from the solar wind enter the Earth's atmosphere along the magnetic field lines and react with the gasses in the Earths atmosphere. Observed from space, an aurora looks like a glowing oval encircling the polar regions of the planet. This is known as the auroral oval. However, it is also possible to see an aurora much farther away from the poles during a severe geomagnetic storm, even well into middle latitudes.

One can see many different colours in the sky during an aurora, including hues of red, blue, purple, green and white. This is due to the fact that there are many different gasses present in the Earth's atmosphere, each of which emits a different wavelength of light when it is "excited" by charged particles from the solar wind. For example, atmospheric oxygen produces greenish-white light, while atmospheric nitrogen tends to produce blue and purple light.

Studying the Power of an Aurora

Studying auroras is definitely a worthwhile pursuit. It can help scientists better understand how the solar wind and the Earth's magnetosphere function. On 17 February 2007 NASA launched five probes into space as part of its two-year THEMIS Mission to study the Aurora Borealis. Named after the goddess of justice, wisdom and good counsel in Greek Mythology, the goal of the THEMIS mission is to learn more about the phenomenon of auroral substorms. For many years, there has been much contention in the scientific community as to where and when auroral substorms are triggered in the Earth's magnetosphere, and exactly which processes trigger them. The THEMIS mission hopes to resolve this contention by studying as many as 30 substorms over the mission's two-year period.

Auroral substorms occur when the auroral oval brightens in a localised area and then suddenly breaks into different streams of light that spread out towards the poles and the equator. These events can be very intense and beautiful, however the electromagnetic energy associated with them can be quite destructive to satellites and communication systems and create surges in power lines. The charged particles from the solar wind are associated with an enormous amount of energy. As much as one terawatt (one trillion watts) of electricity can be generated during an auroral substorm. Understanding the causes of auroral substorms will help scientists forecast when they will occur, which will help ensure the safety of astronauts and valuable equipment.

By: Joseph Cheek