Polar aurorae are optical phenomena characterized by colorful displays of light in the night sky. An auroral display in the Northern Hemisphere is called the aurora borealis, or the northern lights; in the Southern Hemisphere it is called the aurora australis. Auroras are the most visible effect of the solar wind upon the Earth's atmosphere. The aurora occur when the Van Allen radiation belts become "overloaded" with energetic particles, which cascade down magnetic field lines and collide with the earth's upper atmosphere. The most powerful aurora tend to occur after coronal mass ejections. Aurora in Latin means dawn and Borealis comes from Boreas, the name of the Greek god of the northern wind.
Origin and appearance
The origin of the aurora is 93 million miles (149 million km) from Earth at the Sun. Energetic particles from the Sun are carried out into space along with the ever present hot solar wind. This wind sweeps supersonically toward Earth through interplanetary space at speeds ranging from 300 to over 1000 km per second, carrying with it the solar magnetic field. The solar wind distorts the Earth's magnetic field to create the comet-shaped , plasma-filled magnetosphere. The terrestrial magnetic shield acts as a barrier, protecting the Earth from energetic particles and radiation in the hot solar wind. Most of these energetic particles are deflected around the Earth by the magnetosphere, but some get trapped. Electrons trapped in the Earth's magnetic field (the magnetic mirror effect) are accelerated along the magnetic field toward the polar regions and then strike the atmosphere to form the aurora. Auroras are most intense at times of intense magnetic storms caused by sunspot activity. The distribution of auroral intensity with altitude shows a pronounced maximum near 100 km above the Earth.
The particles, which stream down the magnetic field of the Earth, reach the neutral atmosphere in a rough circle called the auroral oval. This circle, or annulus, is centered over the magnetic pole and is around 3000 km in diameter during quiet times. The annulus grows larger when the magnetosphere is disturbed. The location of the auroral oval is generally found between 60 and 70 degrees north and south latitude. During intense solar activity, the auroral oval expands, and aurorae have been seen from latitudes as low as 25-30 degrees north and south on extreme occasions. For example, on November 7, 2004, following a Coronal Mass Ejection, they were seen as far south as Arizona. At 45 degrees, aurorae are visible approximately five times per year, while above 55 they are visible almost nightly.
Photograph of the aurora australis, taken from the space shuttle in orbit in May 1991, at a geomagnetic maximum.
Auroral features come in many shapes and sizes. Tall arcs and rays start brightly 100 km above the Earth's surface and extend upward along its magnetic field for hundreds of kilometers. These arcs or curtains can be as thin as 100 meters while extending from horizon to horizon. Auroral arcs can nearly stand still and then, as though a hand has been run along a tall curtain, the aurora will begin to dance and turn. After magnetic midnight, the aurora can take on a patchy appearance and the patches often blink on and off once every 10 seconds or so until dawn. Most of the auroral features are greenish yellow but sometimes the tall rays will turn red at their tops and along their lower edge. On rare occasions, sunlight will hit the top part of the auroral rays creating a faint blue color. On very rare occasions (once every 10 years or so) the aurora can be a deep blood red color from top to bottom. In addition to producing light, the energetic auroral particles deposit heat. The heat is dissipated by infrared radiation or transported away by strong winds in the upper atmosphere.
The physics of the aurora
The aurora is caused by the interaction of high energy particles (usually electrons) with neutral atoms in the Earth's upper atmosphere. These high energy particles can excite (by collisions) valence electrons that are bound to the neutral atom. The excited electrons can then return to their initial, lower energy state, and in the process release photons (light particles). This process is similar to the plasma discharge in a neon lamp.
Any particular color of the aurora depends on a specific atmospheric gas and its electrical state, and on the energy of the particle that hits the atmospheric gas. Atomic oxygen is responsible for the two main colors of green (wavelength of 557.7 nm) and molecular oxygen for red (630.0 nm). Nitrogen causes the color blue to appear.
Variations on the Sun
The Sun is a star with some features that are highly variable on time scales of hours to hundreds of years. The interplanetary magnetic field direction and solar wind speed and density are driven by the activity on the Sun. They can change drastically and influence the geomagnetic activity. As geomagnetic activity increases, the southern edge of the aurora borealis usually moves to lower latitudes. Similarly, solar mass ejections coincide with larger auroral ovals. If the interplanetary magnetic field is in the opposite direction of the Earth's magnetic field, there can be increased energy flow into the magnetosphere and thus, increased energy flow into the polar regions of the Earth. This will result in an intensification of the auroral displays.
Disturbances in the Earth's magnetosphere are called geomagnetic storms. These, in turn, can produce sudden changes in the brightness and motion of the aurora called auroral substorms. The magnetic fluctuations of these storms and substorms may cause surges in electric power lines and occasional equipment failures in the power grid, resulting in widespread power outages. They can also impact the performance of satellite-to-ground radio communications and navigation systems. Magnetospheric storms can last several hours or even days, and auroral substorms can occur several times a day. Each substorm can deliver several hundred terajoules of energy, as much as the electrical energy consumed in the entire United States over 10 hours.
Measuring the geomagnetic field
The geomagnetic field can be measured with instruments called magnetometers. Data from many magnetometers allow observers to track the current state of the geomagnetic conditions. The magnetometer data are often given in the form of 3-hourly indices that give a quantitative measure of the level of geomagnetic activity. One such index is called the K-index. The K-index value ranges from 0 to 9 and is directly related to the amount of fluctuation (relative to a quiet day) in the geomagnetic field over a 3-hour interval. The higher the K-index value, the more likely it is that an aurora will occur. The K-index is also, necessarily, tied to a specific observatory location. For locations where there are no observatories, one can only estimate what the local K-index would be by looking at data from the nearest observatory. A global average of auroral activity is converted to the Kp index.
It is frequently claimed that sightings of aurorae are accompanied by humming and/or crackling sounds. The existence of these sounds is questionable - aurorae occur around 100km above the earth in extremely rarefied conditions which certainly could not transmit audible sounds well enough for them to reach ground level. Most claims of auroral sound report simultaneous auroral movement and noise, impossible given the sound travel time of many minutes between the upper atmosphere and ground level.
One possibility is that electromagnetic waves are transduced into sound waves by objects in the vicinity of the observer. However, the few detailed studies of auroral sounds that have been carried out have come to no definite conclusions.
Helsinki University of Technology has made examinations and recordings of these sounds and according to the newspaper Kaleva, says, that during bright high level polar aurora, can be registered hum, rumble and pops.
In Norse mythology the polar aurora represents the Ride of the Valkyries to War.
Pop Culture References
On the Simpsons television show Principal Skinner claimed that the aurora borealis was occurring in his kitchen in an attempt to cover up the fact that his stove was on fire.
An episode of the television show Northern Exposure featured the aurora and the Japanese fascination with them. This show also originated the idea that to copulate under the northern lights and conceive a child will bring good luck to that child.
In the United Kingdom, the first volume of Philip Pullman's His Dark Materials trilogy is called Northern Lights, named after the equivalent phenomenon in Lyra's world.
Welsh Band Super Furry Animals recorded a song titled "Northern Lites" on their Guerilla album. Rumor says the song is about marijuana smoking, but singer Gruff Rhys says it's all about the spectacle of this natural phenomenon.
"Northern Lights" is a song by the band Phish.
- For current information on magnetospheric activity from satellites see - http://www.sel.noaa.gov/pmap/index.html
- Northern Lights Pictures in Spitsbergen - Norway (http://www.svalbard-images.com/spitsbergen/northern-lights-e.php)