If you're camping near the United States/Canada border or points farther north, you might see an eerie glow in the night sky. Sometimes it can look like twilight. At other times it can look like a glowing, dancing ribbon of light. The light may be green, red, blue or a combination of these colors. What you are seeing is called the aurora borealis, or simply an aurora.
Auroras have signified different things to different cultures. The Vikings thought that auroras were reflections off the armor of the mythical Valkyries. To the native Eskimos of Greenland and nearby Canada, auroras were communications from the dead. To American Indians, they were lights from huge campfires far to the north. In medieval times, auroras were omens of war or disasters, such as plague. Today, we know that they're a light phenomenon caused by high-energy particles from the sun's solar winds interacting with the Earth's magnetic field. However, knowing the physical reason for auroras certainly doesn't detract from these beautiful natural light shows.
Because auroras are caused by the interaction of solar winds with the Earth's magnetic field, you can see them most often near the poles, both north and south. In the north, they're called aurora borealis, or Northern Lights. Aurora is the name of the Roman goddess of the dawn, and "boreal" means "north" in Latin. In the southern hemisphere, auroras are called aurora australis (Latin for "south").
Auroras follow solar cycles and tend to be more frequent in the late autumn and early spring (October, February and March are the best months for seeing them). Around the Arctic Circle in northern Norway and Alaska, you can see them almost nightly. As you travel south, their frequency diminishes. Around southern Alaska, southern Norway, Scotland and the United Kingdom, they might show up about one to 10 times per month. Near the United States/Canada border, you may see them two to four times a year. Once or twice a century, they might pop up in the southern United States, Mexico and the equatorial regions.
Let's take a closer look at auroras and what causes them.
What do auroras look like?
As we mentioned, auroras take on different appearances. They can look like an orange or red glow on the horizon -- like a sunrise or sunset. Sometimes they may be mistaken for fires in the distance, like the American Indians thought. They can look like curtains or ribbons and move and undulate during the night.
Auroras can be green, red or blue. Often they will be a combination of colors, with each color visible at a different altitude in the atmosphere.
- Blue and violet: less than 120 kilometers (72 miles)
- Green: 120 to 180 km (72 to 108 miles)
- Red: more than 180 km (108 miles)
After a particularly active solar maximum in the sun's cycle, the red color may appear at altitudes between 90 and 100 km (54 to 60 miles).
Oxygen ions radiate red and yellow light. Nitrogen ions radiate red, blue and violet light. We see green in regions of the atmosphere where both oxygen and nitrogen are present. We see different colors at different altitudes because the relative concentration of oxygen to nitrogen in the atmosphere changes with altitude.
Auroras can vary in brightness. People who regularly observe auroras and report on them generally use a rating scale from zero (faint) to four (very bright). They'll note the aurora's time, date, latitude and colors and make quick sketches of the aurora against the sky. Such reports help astronomers, astrophysicists and Earth scientists monitor auroral activities. Auroras can help us understand the Earth's magnetic field and how it changes over time.
Because the Earth's magnetic field is three-dimensional, the aurora appears as an oval ring around the pole. This has been observed from satellites, the International Space Station and the space shuttle. It isn't a perfect circle because the Earth's magnetic field is distorted by the solar winds.
The auroral ring can vary in diameter. Auroras can be seen as far south as the southern United States, but not frequently. In general, they stay near the polar regions. They also occur in pairs -- when we see an aurora borealis, there is a corresponding aurora australis in the southern hemisphere (learn why on the next page).
What causes auroras?
As the process of fusion occurs inside the sun, it spews high-energy particles (ions, electrons, protons, neutrinos) and radiation in the solar wind. When the sun's activity is high, you'll also see large eruptions called solar flares and coronal mass ejections. These high-energy particles and radiations get released into space and travel throughout the solar system. When they hit the Earth, they encounter its magnetic field.
The poles of the Earth's magnetic field lie near, but not exactly on, its geographic poles (where the planet spins on its axis). Scientists believe that the Earth's liquid iron outer core spins and makes the magnetic field. The field is distorted by the solar wind, getting compressed on the side facing the sun (bow shock) and drawn out on the opposite side (magnetotail). The solar winds create an opening in the magnetic field at the polar cusps. Polar cusps are found on the solar side of the magnetosphere (the area around the Earth that's influenced by the magnetic field). Let's look at how this leads to an aurora.
- As the charged particles of solar winds and flares hit the Earth's magnetic field, they travel along the field lines.
- Some particles get deflected around the Earth, while others interact with the magnetic field lines, causing currents of charged particles within the magnetic fields to travel toward both poles -- this is why there are simultaneous auroras in both hemispheres. (These currents are called Birkeland currents after Kristian Birkeland, the Norwegian physicist who discovered them -- see sidebar.)
- When an electric charge cuts across a magnetic field it generates an electric current (see How Electricity Works). As these currents descend into the atmosphere along the field lines, they pick up more energy.
- When they hit the ionosphere region of the Earth's upper atmosphere, they collide with ions of oxygen and nitrogen.
- The particles impact the oxygen and nitrogen ions and transfer their energy to these ions.
- The absorption of energy by oxygen and nitrogen ions causes electrons within them to become "excited" and move from low-energy to high-energy orbitals (see How Atoms Work).
- When the excited ions relax, the electrons in the oxygen and nitrogen atoms return to their original orbitals. In the process, they re-radiate the energy in the form of light. This light makes up the aurora, and the different colors come from light radiated from different ions.
Note: The particles that interact with the oxygen and nitrogen ions in the atmosphere don't come from the sun, but rather were already trapped by the Earth's magnetic field. The solar winds and flares perturb the magnetic field and set these particles within the magnetosphere in motion.
For more information on auroras, take a look at the links on the next page.
More Great Links
- Discoverak.com, What is the Aurora Borealis or Northern Lights?http://www.discoverak.com/what_is_the_aurora_borealis_or_northern_lights.pdf
- Exploratorium: Auroras. http://www.exploratorium.edu/auroras/
- Exploratorium: Auroras: Paintings in the Sky. http://www.exploratorium.edu/learning_studio/auroras/
- NASA GSFC POLAR Brochure. "What Causes the Northern Lights?"http://pwg.gsfc.nasa.gov/polar/EPO/northern_lights/aurora_broch.pdf
- NASA IMAGE Web site.http://image.gsfc.nasa.gov/
- NASA IMAGE, Exploring the Earth's Magnetic Field.http://ds9.ssl.berkeley.edu/themis/pdf/odenwald_mag_lessons.pdf
- NASA IMAGE, The Northern Lights.http://ds9.ssl.berkeley.edu/themis/pdf/nlbook_col.pdf
- NASA MSFC Space Plasma Physics, What is the Magnetosphere?http://science.nasa.gov/ssl/pad/sppb/edu/magnetosphere/bullets.html
- NASA STEREO Home, Space Weather.http://stereo.gsfc.nasa.gov/spaceweather/spaceweather.shtml
- NASA THEMIS Mission, Auroras.http://www.nasa.gov/mission_pages/themis/auroras/index.html
- Nordlys, Northern Lights.http://www.northern-lights.no/english/what/index.shtml
- Petersen, F. "Aurora Borealis - The Northern Lights."http://www.imv.uit.no/english/science/publicat/waynorth/wn1/contents.htm
- Stern, David P. "NASA GSFC, A Teacher's Introduction to the Earth's Magnetosphere."http://www-istp.gsfc.nasa.gov/Education/Cluster1.htm
- Stern, David P. "NASA GSFC, Exploration of the Earth's Magnetosphere."http://www-istp.gsfc.nasa.gov/Education/index.html
- The Viking Answer Lady, The Aurora Borealis and the Vikings.http://www.vikinganswerlady.com/njordrljos.shtml
- Windows to the Universe, Basic Facts About Space Weather.http://www.windows.ucar.edu/spaceweather/basic_facts.html