Auroras borealis is a spectacular meteorological phenomenon observed in the northern Hemisphere. Observable on many cold winter nights in Alaska and eastern Canada, auroras take the appearance of an unmistakable curtain of blue or green light in the upper atmosphere. Because of their stunning beauty, many ancient civilizations around the world have long thought the aurora to be a supernatural sign from the gods. Although the theory behind how aurora borealis is formed is now relatively well understood, auroras still continues to inspire awe in anyone who gazes upon them.
Auroras originate from the interactions between high energy charged particles and the Earth’s atmosphere. These particles are the “solar wind” – a stream of ions that have gained enough kinetic energy to escape from the gravitation acttraction of the sun. When this wind approaches a planet, such as the Earth, it is deflected due to the Lorentz force created by the magnetic field of the planet. The ions are sent spiraling towards the poles, and most are diverted around the Earth. However, some of the ions enter the upper atmosphere of the Earth, the mesosphere. As the ions collide with the atoms in the mesosphere, these atoms become energized. Electrons receive energy from these collisions and are excited to a higher orbital within the atom. As these electrons fall from their excited states to their original ground state, the atom emits a photon.
The energy, and thus color of the emitted photon depends on the energy of the orbitals that the electron can occupy and thus on the atom energized. The colors of the aurora borealis therefore indicate the elements present in the Earth’s atmosphere. Blue light is produced by energized nitrogen, while green and red colors are produced by energized molecular and atomic oxygen. The colors of the aurora borealis can even give an indication of the ratio of gases at different altitudes. At higher altitudes, atomic oxygen has the highest concentration, and thus the tops of many auroras are outlined in red, becoming greenish white at lower altitudes because of the abundance molecular oxygen and nitrogen at lower latitudes.
Auroras are by no means limited to just the earth. Other planets in the solar system, and even moons, have been observed to have auroras. Images from the probe Galileo have shown that auroras occur frequently on Saturn and Jupiter (Nasa). In fact, auroras have been observed on every planet except Mercury, which may not be able to produce auroras because of the lack of an atmosphere for charged particles to interact with. Auroras have also been seen on the moon Io. However, auroras tend not to occur on moons as often as they do on planets due to the fact that moons are shielded by the magnetic field of the planet.
Along with being a stunning sight, auroras also are a treasure trove to scientists interested in environmental conditions of the interface between space and Earth’s atmosphere. Understanding the conditions of the upper atmosphere is of vital importance to maintaining satellites, which can be damaged by storms of radiation from the sun. Severe geomagnetic storms in the mesosphere can even damage transformers, causing power outages or even corroding conductive oil and gas pipelines. By studying auroras, scientists may be able to avert potential catastrophes caused by bursts of solar activity.
For the prospective aurora watcher, the best places to view an aurora are in the so called aurora oval, which includes northern Alaska, much of Iceland, and eastern Canada. As in the case of stargazing, auroras will be more visible against a dark sky, far away from major sources of artificial light. Equinox is believed to be the best time to view the aurora, as the skies tend to be relatively dark and clear. Several websites also offer aurora forecasts that show the chances of observing an aurora based off of solar activity. Although these forecasts may not be very accurate, they may be of use to those eager to observe an aurora. For more information, visit http://odin.gi.alaska.edu/FAQ/.