Aurora borealis: the science behind the dazzling northern lights
Auroras or polar lights are one of the most breathtaking shows of nature.
An aurora can be defined as an atmospheric phenomenon that takes place when electrically charged particles from the Sun hit gas atoms in the Earth’s atmosphere. The energy released during this collision ultimately creates a colorful light show that is visible in the Earth’s polar regions.
The auroras occur most often in areas south of the poles because the Earth’s tear-shaped magnetic field, which acts as a shield that protects us from solar wind and solar storms, traps the charged particles in the magnetosphere, and diverts them toward the geomagnetic poles, which are located near the geographic poles.
As the particles collide with atoms and molecules in Earth’s upper atmosphere, electrons in Earth's magnetosphere transfer their energy to oxygen and nitrogen atoms and molecules. As the gases return to their normal state, they emit small bursts of energy in the form of light. The glowing aurora is made up of a billion individual collisions, lighting up the magnetic field of Earth.
Who discovered the northern lights?
Auroras may have been depicted in some prehistoric cave paintings. It has been suggested that the Cro-Magnon drawings dubbed "Macaronis," created around 30,000 B.C., might be depictions of the aurora.
The oldest written record of an aurora is cited in a Chinese work from 2600 B.C. that explains the birth of Huangdi, the Yellow Emperor — one of the five mythological emperors of ancient China. Huangdi was portrayed as the originator of the centralized state and as a cosmic ruler.
The name “aurora borealis” was coined in 1619 A.D. by Italian astronomer and scientist Galileo Galilei. The term “aurora” refers to Aurora, the Roman goddess of the dawn, and “Borealis” comes from the name of the Ancient Greek god of the north wind, Boreas.
The name of the southern aurora is Aurora australis. “Australis” is derived from the Latin word “Austra”, meaning South. The term “aurora australis” was coined by British explorer James Cook after an expedition to the Antarctic Circle between 1772 and 1775.
What causes the different colors in the aurora?
The charged particles from the Sun collide mostly with atoms of nitrogen and oxygen in the Earth’s atmosphere at different heights, ranging from 50 miles to 400 miles. The colors of the aurora depend on which gas molecules are interacting with the solar wind, how much energy the electrons in the solar wind have at the time of their collisions, and the altitude at which the interaction is taking place.
High energy electrons colliding with oxygen higher in the ionosphere (above 180 miles) emit red light, while low energy electrons collide with oxygen lower in the ionosphere (between around 62 to 186 miles) causing oxygen to emit green or greenish-yellow light. Collisions with nitrogen generally result in red or blue light. Colors such as purples, pinks, and whites can result from a blending of these colors.
The collisions also result in the emission of ultraviolet light, which is invisible to the naked eye but can be detected by special cameras on satellites.
Green auroras are the most common because the Earth’s atmosphere contains a lot of atomic oxygen and human eyes are more sensitive to the green color. In addition, the blue and purple colors are not very visible in the night sky.
Blue and purple lights may be more common at lower altitudes, this is why they are typically found at the lower edges of the “curtains” of the auroras. Blue and purple lights are related to molecular nitrogen in the lowest part of the Earth’s atmosphere, although the impact of sunlight can also create blue lights on rare occasions.
Where can you see the northern lights?
Auroras usually occur in the aurora oval, a ring-shaped belt about 2,500 miles in diameter near the magnetic poles of the Earth. The aurora oval is asymmetric and can be displaced by the solar wind; it also expands and contracts somewhat with the level of auroral activity.
The northern lights are more likely to be visible in high northern latitudes, especially those that are near the center of the Arctic Circle.
The northern lights are commonly spotted in Alaska, especially in the area of Fairbanks, which is located under the aurora oval.
Northern lights can also be seen in Canada, Russia, Iceland, some places in the USA, the high arctic tundra ecoregion in Greenland, Sweden, Finnish Lapland, and Norway, especially around Tromsø, a city in the north of the country that is at the center of the aurora oval at night — and because, due to its location, the Polar Night in Tromsø can last for six weeks or more — making the lights more visible.
Aurora borealis and aurora australis
Auroral activity is not only present in the Arctic but also in the Antarctic. Auroras in the Earth’s south pole are called aurora australis or southern lights. It is the same exact phenomenon except for the fact that it occurs in high southern latitudes.
In the southern hemisphere, the auroral oval is mostly over the oceans around Antarctica, but it sometimes reaches the far edges of New Zealand, Chile, Australia, and sometimes even Argentina and South Africa.
Do other planets have aurorae?
Auroras have also been observed on other planets in the Solar System.
Not all of these planets have magnetospheres, but scientists have found that the interaction of the solar wind with the planet’s atmosphere can be enough to produce an aurora in planets with weak magnetic fields.
Given that the atmospheric composition of each planet is different, auroras look different on other planets. For example, Venus does not have a strong magnetic field, but it has a lot of carbon dioxide and only traces of oxygen and nitrogen atoms in its atmosphere. Because of this, auroras on Venus are not bright enough to be seen from the planet’s surface but only from space.
On the contrary, Jupiter’s magnetic field is 20,000 times stronger than Earth’s, so the giant planet’s auroras are much brighter than ours. Jupiter’s magnetic field, however, is constantly bombarded by particles from its close-orbiting moon Io —apart from the solar wind. X-ray flares have been found in Jupiter’s auroras due to the interaction of Io’s charged sulfur and oxygen ions and Jupiter’s magnetic field.
Saturn’s atmosphere is filled with exciting forms of hydrogen, so its auroras are red/pink and purple, as captured by NASA's Cassini spacecraft. It also has auroras in the ultraviolet and infrared wavelengths (not visible to the human eye).
On Mars, the auroral activity is restricted to areas covered by remnants of the global magnetic field that the red planet is believed to have had in the past.
Auroras have also been detected in Neptune, Uranus, on the Jupiter-family comet 67P/Churyumov–Gerasimenko at far-ultraviolet wavelengths (by ESA's Rosetta spacecraft), and on the brown dwarf ultracool star LSR J1835+3259.
With many scientists still unhappy with the IAU's definition of "planet," it's possible the debate will never be resolved!