Aurora is a natural display of light in the sky, usually limited to Earth’s polar regions. Auroras can be seen with the unaided eye only at night. They commonly appear as arcs, clouds, and bands of moving light that can extend across the sky for thousands of miles. An auroral display in the Northern Hemisphere is called the aurora borealis or northern lights. An auroral display in the Southern Hemisphere is called the aurora australis.
Most auroras occur about 60 to 250 miles (100 to 400 kilometers) above Earth. The most common color in an aurora is green, but displays that occur extremely high in the sky may be red or purple.
Auroras result in part from Earth’s magnetic field, the area of magnetic influence that surrounds the planet. The magnetic field dominates a region of space called the magnetosphere. Above the atmosphere, the magnetosphere contains neutral and charged particles. The charged particles are produced when the atmosphere absorbs radiation from the sun.
Loading the player...Auroral substorm
The magnetosphere is continuously buffeted by a flow of electrically charged particles from the sun, called the solar wind. This buffeting squeezes the magnetosphere and causes the charged particles already in the magnetosphere to move along the magnetic field toward the north and south poles. There, the energetic particles collide with particles in the atmosphere, releasing energy in the form of light.
Auroras are most intense during solar maximum, the most active phase in the 11-year sunspot cycle. During this period, violent eruptions on the sun’s surface occur most frequently. These eruptions are known as solar flares and coronal mass ejections. They release additional material into the solar wind. The added material helps compress Earth’s magnetosphere, producing extremely bright auroras. It also creates sharp variations in Earth’s magnetic field, called magnetic storms. During these storms, auroras may expand toward the equator, making them visible beyond the polar regions.
Most other planets in our solar system also have auroras. The giant planets—Jupiter, Saturn, Uranus, and Neptune—have powerful magnetic fields and thick atmospheres. These planets’ auroras are similar to Earth’s. Scientists have also detected auroras on Venus and Mars, even though these planets do not produce magnetic fields. On Venus, the solar wind affects electric currents in the atmosphere, causing the electric currents to act as a magnetosphere. On Mars, weak patches of the planet’s ancient magnetic field remain. Because there is no strong magnetic field to direct particles to the poles on Venus and Mars, auroras can occur over a wide area on both planets. Scientists think auroras also occur on exoplanets (planets outside the solar system).
See also Magnetic storm; Seven natural wonders of the world (The aurora borealis); Sun; Sunspot.
