Aurora Borealis

February 19, 2013, 12:56 pm
Source: NOAA
Content Cover Image

By United States Air Force photo by Senior Airman Joshua Strang [Public domain], via Wikimedia Commons

Introduction

Also known as the Northern Lights, an Aurora is a beautiful natural phenomenon that often occurs in the polar regions of Earth. It appears as colorful clouds and rays of green and red (and sometimes blue) light that dance across the sky. The aurora borealis and aurora australis (Latin for "northern" and "southern" dawn, respectively) occur in symmetric ovals centered on the northern and southern magnetic poles of Earth.

The aurora is formed when charged particles (electrons and protons) are guided by the Earth's magnetic field into the atmosphere near the poles. When these particles collide with atoms and molecules of the upper atmosphere, primarily oxygen and nitrogen, some of the energy in these collisions is transformed into the visible light that characterizes the aurora.
caption Aurora bands over Quartz Lake State Park, Alaska 6 September, 1996. (Source: Jan Curtis)
Aurora over Quartz Lake State Park (Photo by Jan Curtis)

What causes the Aurora?

The energy source for the aurora is 149 million kilometers (km) (93 million miles) from Earth at the sun. The sun continuously emits charged particles (mostly protons and electrons), which are the byproducts of thermonuclear reactions occurring inside the sun. These charged particles make up the solar wind, which travels away from the sun through space at speeds ranging from 300 to 1,000 km/sec.—about a million miles per hour. Traveling at this high speed, the solar particles can reach the Earth in two to three days.

At Earth, the steady solar wind is deflected by Earth's magnetic field, or magnetosphere. The solar wind flows around the magnetosphere much like a river flows around a stone. It also pushes on the magnetosphere and distorts it so that instead of a symmetric set of magnetic field lines—like one might have around a bar magnet—the magnetosphere is stretched and elongated into a comet shape with a long tail trailing away from Earth on the side away from the sun.

When there is a disturbance on the sun, such as a solar flare or coronal mass ejection, it can produce a disturbance in the solar wind. This in turn will cause a disturbance in the balance between the solar wind and Earth's magnetic field. As a result, electrons and protons are accelerated within the magnetosphere. These charged particles are constrained to the magnetic field lines much like beads on a wire. The accelerated particles will travel down the magnetic field lines of Earth and collide with the atoms and molecules of the upper atmosphere where the magnetic field lines reach down to surface of the Earth near the north and south magnetic poles.

When the particles from the magnetosphere collide with the atoms and molecules of the atmosphere, the particle's energy can be transferred to the atoms and molecules (typically O, N, and N2) of the atmosphere forming excited states of O, N and N2. When these finally release their energy and return to their normal ground state, they give up energy in the form of light. This is the light that we see from the ground as an aurora.

What do the Northern Lights look like?

The aurora has a variety of shapes, colors, and structures, and can also change rapidly in time. During a typical night, the aurora often starts as a single long arc that stretches from horizon to horizon in a generally east-west direction. Near midnight, the arc may begin to brighten. Then curls or waves may start to form along the arc. It may also start to have vertical structures that look like thin tall rays of light. Then, just about midnight, the whole sky may become filled with bands and rays that move rapidly from horizon to horizon. This heightened activity can last anywhere from a few minutes to several hours. As dawn approaches the aurora will typically quiet down and form wispy quiet patches that can last until morning. While this may be a typical night of aurora, any number of variations on this theme may occur.

The physics of Auroral light formation

The high-energy electrons and protons traveling down Earth's magnetic field lines collide with the atmosphere (i.e., oxygen and nitrogen atoms and molecules). The collisions can excite the atmospheric atom or molecule or they can strip the atmospheric species of its own electron and create an ion. The result is that the atmospheric atoms and molecules are excited to higher energy states. They relinquish this energy in the form of light upon returning to their initial, lower energy state. The particular colors we see in an auroral display depend on the specific atmospheric gas struck by energetic particles, and the energy level to which it is excited. The two main atmospheric gases involved in the production of auroral lights are oxygen and nitrogen:

  • Oxygen is responsible for two primary auroral colors: green-yellow wavelength of 557.7 nanometers (nm) is most common, while the deep red 630.0 nm light is seen less frequently.
  • Nitrogen in an ionized state will produce blue light, while neutral nitrogen molecules create purplish-red auroral colors. For example, nitrogen is often responsible for the purplish-red lower borders and rippled edges of the aurora.

The process is similar to the lights that illuminate a neon light or computer and TV screens. In a neon light, neon gas is excited by electrical currents. Likewise, in a picture or computer screen, a beam of electrons controlled by electric and magnetic fields strike the screen, making it glow in different colors, according to the type of chemicals (phosphors) that coat the screen.

Auroras typically occur between 95 and 1,000 km. Auroras stay above 95 km because at that altitude the atmosphere is so dense (and the auroral particles collide so often) that they finally come to rest at this altitude. On the other hand, auroras typically do not reach higher than 500-1,000 km because at that altitude the atmosphere is too thin to cause a significant number of collisions with the incoming particles.

When do they occur?

The aurora is a near daily occurrence somewhere on Earth and there is almost always an aurora in the sky (both day and night, but in the daytime it is out-shined by sunlight). However, the following factors can increase your chance of seeing them:

  • Time of Day: Because the intensity of the light in an aurora is low, it can only be seen at night. Furthermore, the most active and brilliant displays usually occur near midnight. Therefore, the best time to observe the aurora is, on average, between 11 p.m. and 2 a.m.
  • Season: In the northern hemisphere, the best time to view an aurora is during the winter. At latitudes where auroras are common, it is typically light all night in the summer—so you rarely have warm weather and a good aurora. Furthermore, in most polar regions, the weather tends to be clear during the middle of winter—so often the best time to see an aurora is also the coldest.
  • Sun Rotation: It takes the sun 27 days to rotate one time around its axis, so 27 days after an aurora display, the active region on the sun that caused the aurora will face Earth again. Although solar activity in that region on the sun might have decreased in the mean time, there is still a greater chance of aurora 27 days after the last period of increased auroral activity.
  • Solar Activity: Auroral activity also correlates with the activity of the sun, which changes according to an 11-year solar cycle. In general, the more active the sun, the greater the number of auroras. Thus, auroral displays are more likely around the time of the solar maximum (when solar activity is high). Aurora displays remain frequent and strong for several years around solar maximum. During solar maximum, the auroras are not only more frequent and more active, but they also can come further south away from the poles (it should be noted, however, that bright and active auroras can be observed at any time during the solar cycle).

Weather, the full moon, and light pollution also affect your ability to see aurora. Your best bet for seeing aurora is to get as close as you can to the position of the auroral oval, and as far away as you can from sources of artificial light and overcast skies. Tips on viewing the aurora can be found at NOAA's Space Environment Center.

Where do they occur?

Auroras form in an oval band centered at each magnetic pole. The width of the band ranges from 10 to 1,000 kilometers (km) and it is approximately 3,000 km (1,900 miles) from the magnetic pole during quieter solar periods. If you live near this oval, you will see the aurora on most clear, dark nights. In the northern hemisphere, for example, prime viewing locations include Fairbanks, Alaska, many locations in northern and middle Canada, and in the northern parts of Russia and Scandinavia.

As auroral activity increases, the aurora not only increases in brightness, but it also tends to move further towards the equator. Auroral activity is directly linked to disturbances in Earth's magnetic and electrical current system. These increases in activity are known as geomagnetic storms. To determine how high the geomagnetic activity needs to be for aurora to occur in your area, see the table at NOAA's Space Environment Center. People in the northern United States and northern Europe may see the aurora a few times in a decade, while people in southern Europe, the southern United States, and even Mexico, may see the aurora only once-in-a-lifetime. It should be noted that the auroral oval does not follow lines of equal latitude, so people on the East Coast of the United States have a higher likelihood of seeing aurora than those at the same latitude on the West Coast.

Geomagnetic storms and the resulting auroral activity, vary unpredictably throughout the year. Because geomagnetic activity often results from events on the sun, it can be predicted by looking at the sun and solar flares. For this reason, auroral forecasts can only be made two or three days in advance. NOAA's Space Environment Center issues forecasts of geomagnetic activity. Since 1979, NOAA's polar-orbiting satellites have been measuring the energy flux of particles into the auroral zones.

Further Reading

Glossary

Citation

(2013). Aurora Borealis. Retrieved from http://www.eoearth.org/view/article/51cbecff7896bb431f68f0b5

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