Photoglossary

Aurora   Among the most spectacular sights in nature are the shimmering curtains of light visible in the night sky near Earth's northern and southern magnetic poles. These lights are called the aurora borealis (northern lights) and the aurora australis (southern lights). Although they vary in intensity, these lights are permanent fixtures in the Earth's upper atmosphere. Their range extends from about 80 km (50 miles) above ground to about 960 km (600 miles) into space. In satellite images, they look like giant halos encircling the magnetic poles. The ever-present auroras form from the interaction of the Earth's magnetic field and the solar wind. The solar wind is a continuous stream of hot, ionized gas (plasma) that blows continually away from the Sun, carrying with it charged particles and magnetic fields. When the solar wind encounters the magnetosphere of Earth, most of it is deflected, but some particles do penetrate at the north and south magnetic poles. These particles accelerate as they travel down the magnetic field lines, and they enter Earth's upper atmosphere at tremendous speed.     Upon reaching the upper atmosphere, these high-energy particles collide with molecules of atmospheric gases (primarily nitrogen and oxygen), and some of the energy produced from these collisions is turned into a burst of colored light. Oxygen emits both green and red light depending on its altitude and electrical state. At low altitudes, [about 100 km (60 miles)] oxygen atoms glow a brilliant green, which is the most common auroral color. At higher altitudes, between 200 and 400 km (125 to 250 miles), oxygen atoms emit a red color. High altitude nitrogen glows blue and violet when electrically charged, and red when it is neutral.   Photo Credit: NASA Illustration Credit: NASA/Goddard Space Flight Center     Io Home Page Photoglossary Home Page