• An aurora is a natural light display that shimmers in the sky. Colorful blue, red, yellow, green, and orange lights shift gently and change shape like softly blowing curtains. Auroras are only visible at night, and usually only appear in lower polar regions.

    Auroras are visible almost every night near the Arctic and Antarctic Circles, which are about 66.5 degrees north and south of the Equator. In the north, the display is called aurora borealis, or northern lights. In the south, it is called aurora australis, or southern lights.

    Auroras and the Solar Wind

    The activity that creates auroras begins on the sun. The sun is a ball of superhot gases made up of electrically charged particles called ions. The ions, which continuously stream from the sun’s surface, are called the solar wind.

    As solar wind approaches the Earth, it meets the Earth’s magnetic field. Without this magnetic field protecting the planet, the solar wind would blow away Earth’s fragile atmosphere, preventing all life. Most of the solar wind is blocked by the magnetosphere, and the ions, forced around the planet, continue to travel farther into the solar system.

    Although most of the solar wind is blocked by the magnetosphere, some of the ions become briefly trapped in ring-shaped holding areas around the planet. These areas, in a region of the atmosphere called the ionosphere, are centered around the Earth’s geomagnetic poles. The geomagnetic poles mark the tilted axis of the Earth’s magnetic field. They lie about 1,300 kilometers (800 miles) from the geographic poles, but are slowly moving.

    In the ionosphere, the ions of the solar wind collide with atoms of oxygen and nitrogen from the Earth’s atmosphere. The energy released during these collisions causes a colorful glowing halo around the poles—an aurora. Most auroras happen about 97-1,000 kilometers (60-620 miles) above the Earth’s surface.

    The most active auroras happen when the solar wind is the strongest. The solar wind is usually fairly constant, but solar weather—the heating and cooling of different parts of the sun—can change daily.

    Solar weather is often measured in sunspots. Sunspots are the coldest part of the sun and appear as dark blobs on its white-hot surface. Solar flares and coronal mass ejections are associated with sunspots. Solar flares and coronal mass ejections are sudden, extra bursts of energy in the solar wind. Sunspot activity is tracked over an 11-year cycle. Bright, consistent auroras are most visible during the height of sunspot activity.

    Some increased activity in the solar wind happens during every equinox. These regular fluctuations are known as magnetic storms. Magnetic storms can lead to auroras being seen in the midlatitudes during the time around the spring and autumnal equinoxes. Auroras have been visible as far south as the Yucatan Peninsula in Mexico.

    Magnetic storms and active auroras can sometimes interfere with communications. They can disrupt radio and radar signals. Intense magnetic storms can even disable communication satellites.

    Coloring an Aurora

    The colors of the aurora vary, depending on altitude and the kind of atoms involved. If ions strike oxygen atoms high in the atmosphere, the interaction produces a red glow. This is an unusual aurora—the most familiar display, a green-yellow hue, occurs as ions strike oxygen at lower altitudes. Reddish and bluish light that often appears in the lower fringes of auroras is produced by ions striking atoms of nitrogen. Ions striking hydrogen and helium atoms can produce blue and purple auroras, although our eyes can rarely detect this part of the electromagnetic spectrum.

    To find out more about the mysterious light displays, scientists have launched satellites specially designed to study auroras. Until 2005, NASA’s IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellite used ultraviolet and radio waves to study auroras and how they are formed.

    Green auroras display an oxygen atom returning from an "excited" state to a "ground" state.

    Dawn Wind
    The aurora borealis, or northern lights, was studied by ancient Roman and Greek astronomers. The phenomenon was named for the Roman goddess of the dawn, Aurora, and the Greek god of the north wind, Boreas.

    Fox Fire
    In Finland, the aurora borealis is called revontulet, which literally translates to fox fires. According to one Finnish folk tale, the lights are caused by a magical fox sweeping his tail across the snow and sending sparks up into the sky.

  • Term Part of Speech Definition Encyclopedic Entry
    altitude Noun

    the distance above sea level.

    Encyclopedic Entry: altitude
    Antarctic Circle Noun

    line of latitude at 66.5 degrees south that encircles the continent of Antarctica.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    atom Noun

    the basic unit of an element, composed of three major parts: electrons, protons, and neutrons.

    aurora Noun

    brightly colored bands of light, visible around Earth's geomagnetic poles, caused by solar wind interacting with particles in Earth's magnetic field.

    Encyclopedic Entry: aurora
    aurora australis Noun

    bright bands of color around the South Pole caused by solar wind and the Earth's magnetic field. Also called the southern lights.

    aurora borealis Noun

    bright bands of color around the North Pole caused by solar wind and the Earth's magnetic field. Also called the northern lights.

    autumnal equinox Noun

    autumn day, usually around September 22, when day and night are of generally equal length.

    axis Noun

    an invisible line around which an object spins.

    Encyclopedic Entry: axis
    charged particle Noun

    molecule that has a positive or negative electric charge.

    collide Verb

    to crash into.

    coronal mass ejection Noun

    huge burst of solar wind and other charged particles.

    disrupt Verb

    to interrupt.

    electromagnetic spectrum Noun

    continous band of all kinds of radiation (heat and light).

    Equator Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    Encyclopedic Entry: equator
    equinox Noun

    period in which daylight and darkness are nearly equal. There are two equinoxes a year.

    Encyclopedic Entry: equinox
    fluctuate Verb

    to constantly change back and forth.

    fragile Noun

    delicate or easily broken.

    gas Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    geomagnetic pole Noun

    point marking the tilted north and south axes of Earth's magnetic field, about 1,300 kilometers (800 miles) from the geographic poles.

    IMAGE Noun

    (2000-2005) (Imager for Magnetopause-to-Aurora Global Exploration) NASA satellite used to study the auroras and magnetosphere.

    interfere Verb

    to meddle or prevent a process from reaching completion.

    ion Noun

    electrically charged atom or group of atoms, formed by the atom having gained or lost an electron.

    ionosphere Noun

    outer layer of the Earth's atmosphere, 80-400 kilometers (50-250 miles) above the surface.

    magnetic field Noun

    area around and affected by a magnet or charged particle.

    magnetic storm Noun

    interaction between the Earth's atmosphere and charged particles from solar wind.

    magnetosphere Noun

    teardrop-shaped area, with the flat area facing the sun, around the Earth controlled by the Earth's magnetic field.

    midlatitude Noun

    area between the Tropic of Cancer and the Arctic Circle in the north, and between the Tropic of Capricorn and the Antarctic Circle in the south. Also called a temperate zone.

    NASA Noun

    (National Aeronautics and Space Administration) the U.S. space agency, whose mission statement is "To reach for new heights and reveal the unknown so that what we do and learn will benefit all humankind."

    nitrogen Noun

    chemical element with the symbol N, whose gas form is 78% of the Earth's atmosphere.

    northern lights Noun

    also known as the aurora borealis. The bright bands of color around the North Pole caused by the solar wind and the Earth's magnetic field.

    oxygen Noun

    chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

    polar Adjective

    having to do with the North and/or South Pole.

    pole Noun

    extreme north or south point of the Earth's axis.

    radar Noun

    (RAdio Detection And Ranging) method of determining the presence and location of an object using radio waves.

    radio wave Noun

    electromagnetic wave with a wavelength between 1 millimeter and 30,000 meters, or a frequency between 10 kilohertz and 300,000 megahertz.

    satellite Noun

    object that orbits around something else. Satellites can be natural, like moons, or made by people.

    solar flare Noun

    explosion in the sun's atmosphere, which releases a burst of energy and charged particles into the solar system.

    solar system Noun

    the sun and the planets, asteroids, comets, and other bodies that orbit around it.

    solar wind Noun

    flow of charged particles, mainly protons and electrons, from the sun to the edge of the solar system.

    southern lights Noun

    the bright bands of color around the South Pole caused by the solar wind and the Earth's magnetic field. Also known as the aurora australis.

    sun Noun

    star at the center of our solar system.

    sunspot Noun

    dark, cooler area on the surface of the sun that can move, change, and disappear over time.

    ultraviolet Adjective

    having to do with light of short wavelengths, invisible to the human eye.