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Northern and Southern lights

An Aurora is a natural electric phenomenon that creates bright and colorful light displays in the skies. Auroral light displays are common at higher latitudes, and most auroral activity generally occurs within the Arctic and the Antarctic Circles.

Illustration image
Aurora Borealis as seen in Michigan, US.
©iStockphoto.com/Constance McGuire

Auroras that occur in the Arctic Circle are known as aurora borealis or northern lights, while auroras in the antarctic circle are known as aurora australis or southern lights.

Auroras are caused when electrically charged particles from solar winds enter the Earth’s atmosphere and interact with the gases in the atmosphere.

Solar wind

The Sun continuously emits electromagnetic radiation and highly energized particles into space. These emissions produce space weather. Solar wind is part of space weather, and is a continuous stream of highly energized particles - mostly electrons and protons - that flow out from the Sun through space at very high speeds and high temperature. Solar winds can reach speeds of one million miles per hour.

Earth’s magnetic field

The Earth is a giant magnet, with its magnetic field extending from the Earth’s core to the area in space where it meets solar winds. The region of this field where the Earth’s magnetic influence dominates over solar winds is known as the magnetosphere. The shape and size of Earth’s magnetosphere is affected by solar winds and is continuously changing.

The Earth’s magnetosphere shields the Earth from solar winds and other harmful cosmic rays. It deflects most of the highly charged particles from solar winds, and stops them from entering the Earth’s atmosphere.

What causes an aurora?

While the Earth’s magnetosphere is responsible for protecting it from the highly charged particles in solar wind, sometimes, when the conditions are right, these particles enter the Earth’s atmosphere at the two poles, where they collide and interact with gas molecules and atoms.

When such collisions occur, the energy from the electrons in the solar winds is transferred to electrons in the atoms of different atmospheric gases. Any excess energy is then released by these excited atoms in the form of light.

Colors of the aurora

The color of the light released depends on the kind of gas molecules, their electrical state at the time of collision, and the type of the solar wind particles they collide with. Oxygen atoms emit yellow-green or red colored light, while nitrogen atoms generate blue or purplish red colored light. A mix of gases in the Earth’s atmosphere, therefore, creates multi-colored auroras.

Because particles from solar winds continually enter the Earth’s atmosphere and interact with gas atoms, aurora displays can be static as well as dynamic – they can change shape and colors, and pulsate in the skies.

Auroral light displays tend to occur at between 50 miles (80.46 kilometers) and 200 miles (321.87 kilometers) above the surface of the Earth.

Auroral shapes tend to fall in six categories – curtains, bands, veils, coronas, patches, and rays.

When and where to see the aurora?

If one were to look from space, they would see a ring shaped aurora spanning around 2500 miles (4000 kms) around both poles. This auroral zone covers Central and northern Alaska and Canada, Greenland, northern Scandinavia and Russia in the Northern Hemisphere, and Antarctica in the Southern Hemisphere. In the south, auroras can sometimes be seen from southern Australia, New Zealand, and Chile.

Sometimes high solar activity can lead to strong and violent gusts of solar winds interacting with the Earth’s magnetosphere, causing a geomagnetic storm. This can expand the region around the poles where auroral activity can be observed from, increasing the chances of seeing auroras at lower latitudes.

On very rare occasions, auroral displays can be observed from locations close to the Equator. For instance, in 1909, because of a very strong geomagnetic storm, people in Singapore were able to observe auroral displays.

Solar activity

Auroras are directly connected to solar activity, which is measured by the number sunspots - dark spots on the surface of the sun caused by high magnetic activity on the Sun. A larger number of sunspots means that a larger number of highly charged particles are being pushed out by the sun. This in turn can lead to more auroral activity on Earth.

Solar astronomers have found that the Sun goes through cycles of solar activity. This cycle, also called the solar cycle, comes around every 11 years.

Scientists have observed 24 solar cycles since 1755, when solar activity started being recorded by humans. The 24th solar cycle is said to reach its peak sometime in mid 2013.

While auroral activity and auroras can occur throughout the year, day and night, the best time to view them is at night during the winter months. This is because during the winter, areas around the North and the South Poles have longer periods of darkness.

Auroras are best observed around midnight – when it is darkest – on a clear night, and at a location that is away from the city. Light sources – artificial or natural, like from a full moon – can make it very hard to view the aurora.

Did you know?

Auroras have been observed on some other planets as well. Any planet that has a magnetic field and an atmosphere will have auroral activity.

Topics: Astronomy, Sun, Atmospheric Phenomena

In this Article

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Atmospheric phenomena

  1. Types of atmospheric phenomena
  2. What is Earthshine?
  3. What is zodiacal light or false dawn?
  4. Northern and Southern lights

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