Northern lights (Aurora Borealis)
Northern lights, mostly called aurora, are the fluttering lights that can be observed in the sky over the northernmost parts of the world. A similar light is located over the southernmost parts of the world; it is called southern lights. Together the northern- and southern lights are called polar lights.
The Latin word for northern lights, Aurora Borealis, was first used in the early seventeenth century. Aurora was the goddess of dawn, i.e. red light of dawn, in ancient Roman mythology and Borealis means northerly. A translation would be the northerly light of dawn. The Latin word for southern lights is Aurora Australis, the southerly light of dawn. Even the Northern Saami (Lappish) word for auroras, guovsahas, is related to the light of dawn.
On the following pages you will be told about auroras. You can read a little bit about why there are auroras, how different auroras can look, some history and something about research on auroras.
Texts by: Jenny Jutström
See the literature list below.
The Earth is surrounded by a thin gas cover, the atmosphere, and fast charged particles, plasma, are moving in space above it. Auroras arise when some of those particles enter the Earth’s atmosphere and collide with atoms and molecules. When the particles collide the energy used to give them their velocity changes into a light, the aurora.
The particles that make auroras come from the ionosphere but have an extremely high velocity due to the energy from the solar wind. The particles are caught by the Earth’s magnetic field and are steered towards the poles. When a particle reaches the atmosphere it collides with one of the many present atoms.
When the particle collides with an atom, the atom takes over some of the energy that has given the particle its velocity. The particle keeps on moving but with less velocity, since it has lost some energy to the atom. The particle soon collides with a different atom.
The atom that has taken over some energy from the particle has now got too much energy and lets go of it. The surplus energy becomes light. The next atom that collides with the particle also takes over some of its kinetic energy, resulting in the particle losing even more velocity. The new atom also lets go of the energy. As the particle moves down through the atmosphere the atoms become more and more crowded, resulting in more collisions for the particle. Each time the particle collides it moves a bit slower and more light is emitted.
When the particle has collided a number of times it has lost so much of its kinetic energy that it stops moving. This occurs when the particle is approximately 100 kilometres from the Earth’s surface. When a lot of particles collide with atoms, releasing light, an aurora occurs.
For auroras to arise on a planet five things are required.
First of all the planet has to have an atmosphere. The atmosphere is the screen upon which the aurora is shown. If there was no atmosphere the particles from space would find no atoms to collide with and no light would be visible.
Second, there must be charged particles, plasma, that can collide with the atmosphere. If there were no particles the situation would be the same as above.
Third, something that can steer the plasma particles down to the atmosphere is needed. That is a magnetic field. If the magnetic field was not present most of the particles would miss the earth and keep moving through space.
Fourth, an energy source that can give the plasma particles all the energy they need to create auroras is required. On Earth that energy source is the Sun. If the particles were not provided with all the necessary energy auroras would not occur.
Lastly, something to carry the energy from the Sun to the particles is needed. This is the solar wind.
The Earth is a huge magnet surrounded by magnetic fields, similar to the ones surrounding a rod magnet.
The Earth’s magnetic field steers the charged particles from space in a way that makes them interact with the atmosphere way up north or down south, over the poles.
If you want to make sure that the magnetic field is shaped like this it is easily done. You need a rod magnet and some iron filings or a compass. Put the rod magnet on a plane surface and powder the iron filings around it, or move the compass back and forth around the magnet. Now it is possible to see the magnetic fields.
For the charged particles in space to gain the high velocity required to make auroras, an energy source is needed. That energy source is the Sun. The part of the Sun’s energy that makes auroras is transported to the Earth in a special way, with the so-called solar wind. The solar wind mostly consists of protons, positive charged particles, and electrons, negative charged particles. These charged particles have a common name, plasma. The plasma is hurled from the Sun with an enormous velocity, approximately 400 kilometres per second.
Plasma is a kind of gas, but there is an important difference. A gas has no electric charge, all of the protons and electrons are stuck into its atoms. In an atom there is the same amount of negative charged electrons, and positive charged protons. There are also neutrons that have no charge at all. When there is the same amount of positive and negative charged particles the total sum of the atom is no charge.
In a plasma the electrons and protons move freely; because of this the plasma can be electrically charged.
The Earth is an impediment to the solar wind. Actually it is the Earth’s magnetic field that stops the solar wind. The solar wind is pushing, the Earth’s magnetic field is pushing back and a bow shock is created. The bow shock is similar to the one created by an aeroplane flying faster than sound. The main part of the solar wind plasma is diverted from its path and does not interact with the Earth. You can compare it to a rock in a stream. An essential difference is that the Earth’s bow shock arises without any particles colliding.
The solar wind encloses the Earth’s magnetic field in a bubble. The bubble is the Earth’s magnetosphere. The magnetosphere acts as a shelter to the Earth: it makes sure that no particles from space hit the Earth. The magnetosphere is a limited area in space where forces from magnetic fields control the movements of the plasma in the area. Magnetospheres are found around planets with magnetic fields, but even other astronomical objects that are pulsating, and whole galaxies, can have magnetospheres.
In a period of one second, the outer limit of the magnetosphere, the magnetopause, is hit with the same amount of energy that is produced by all Swedish power plants in three years. Just a small part of the energy reaches the magnetosphere and a small amount of that energy becomes auroras.
A magnetic substorm is a local disturbance in the magnetic field acting in the auroral zone. The storm lasts about 10- 30 minutes and is followed by strong auroras. The substorms are due to changes in magnetic fields and charged particles in the magnetotail, which is why they are also called magnetospheric storms.
Many consider the aurora to be something of the most beautiful things that can be seen. The different colours in green, blue and red that swiftly move across the sky are really fascinating. If it is the first time you see a sparkling aurora it is easy to lose one’s breath. Regardless of the fact that I have seen quite many beautiful auroras, I cannot just walk by when I see a new one. There are no two auroras that are alike, so if you have seen one aurora it does not mean that you have seen them all.
Auroras appear in many different shapes. The aurora mostly seen early in the evening is shaped as an arc and stretches all across the sky in an east-west direction. The arc has indistinct edges and is green.
Sometimes arcs can become active and start to look like pieces of drapery with distinct rays that show the direction of the Earth’s magnetic field. The length of an aurora arc can be quite large, maybe 1000 kilometres or more, but the width can be as small as 100 metres.
If you stand directly beneath an aurora in the direction of the magnetic field the rays look like they are all coming from one point and radiating in all directions. This is called a corona. If you stand further north or south and look at the same aurora you will see it as an arc.
During the most active auroras, which occur during so-called sub storms, the whole sky may be filled with the most incredible colours. The shapes and colours can change from one second to the next. The most intensive phase of an aurora normally lasts only for about 10 minutes.
When the magnetosphere gets rid of most of the surplus energy related to a sub storm you can often see a different type of aurora, pulsating auroras. The sky will be filled with pale light spots that are switched on and off independently of each other and at different speed; the spots are lit for a few seconds. This type of aurora is common after midnight.
The picture shows a series of photographs taken during a pulsating aurora. They are taken with a special camera adjusted to faint light. The dot indicates the position of a sounding rocket. At 9:54:58 p.m. the rocket was surrounded by the aurora, but four seconds later it was almost gone, reappearing two seconds later.
The most commonly occurring auroras are not so easily noticed, since they have no shape. This is the indistinct aurora that lies like a faint glow across the sky.
Very rarely red aurora (630 nm wavelength) appears quite a long way south of the ordinary green (558 nm wavelength from atomic Oxygen) and purple (428 nm wavelength from ionized Nitrogen molecule) aurora. This happens only a few times during solar maximum, i.e., large red aurora like this photo appears only a few times in a decade.
Do auroras make noise? That is a question that has been discussed for a long time. Some people are convinced that the aurora has a sound, while others say that it is impossible. One thing that makes some scientists doubt it is that people who have heard the sound say that it happens at the same time as the aurora is fluttering the most. Since the sound is made of waves that move slower than the speed of light, the sound could not possibly be heard at the same time as the light is observed. The aurora is located approximately 100 kilometres from the Earth’s surface and because of the great distance the sound would be almost five minutes slower than the light.
Despite all these doubts, a lot of people around the auroral zone have heard sounds related to very strong auroras.
The sounds seem to be of two different kinds, a sparkling sound and a whiz. The sounds have been noticed during calm weather while the aurora has moved rapidly directly above the listening person’s head. Auroral sounds are extremely rare and a lot of people who have seen many auroras have never heard them.
In Finland, Prof. em.Unto Laine, Aalto University, has done extensive studies and attempts to record and explain auroral sounds. According to his hypothesis, the sounds originate from altitudes of only about 70 m.
Before people knew what they know today they tried, in their own way, to explain why there were auroras. People knew that there were auroras but they did not know why. At that time people were very superstitious so their explanations sometimes became very imaginative.
Three old Nordic explanations are mentioned in the book “Kongespeilet” from the thirteenth century. At that time people thought the Earth was flat and surrounded by oceans. One explanation was that the oceans were surrounded by fire and that auroras were the light from those fires, reflected in the sky.
One other possibility was that the sun threw its beams high in the sky although the sun itself was located beneath the edge of the earth plate.
A third possibility was that glaciers could absorb so much power that they began to shine.
One of the ancient Swedish names for aurora is sillblixt (herring flash). The name comes from people who thought that the aurora was a reflection of large herring shoals in the ocean. This is preserved in documents from Närke and different parts of Norway.
From Närke comes yet another explanation, the aurora was believed to arise from the Laplanders’ torches when they were looking for their reindeer in the mountains.
In Småland some people believed that the swans had a competition about flying furthest to the north. Those who got furthest froze in the sky. The aurora arises when the swans try to come loose by flapping their wings.
The Finnish name for aurora is revontulet, fox fires. According to the legend there were fire foxes in Lapland and the auroras were sparks given off by their fur as they ran in the mountains.
The natives, for example Indians and Laplanders, who live in the aurora zones today think that the aurora is something to be respected. This opinion is still active in our century. A lot of elderly people living in the north of Sweden can remember as children being told to act nice and silent when there were auroras in the sky. To misbehave at that time was very serious.
In both Scandinavia and North America some people believe that you can call the aurora by whistling, but to do so can be dangerous.
There are a lot of stories describing how dangerous the aurora could be. There is a story about a young man from Norway, who despite his big brother’s warnings, was killed by the aurora because he was teasing it.
Some people would not let their children outside to play while there were auroras, since they could get killed. Others thought it was all right as long as the children had hats on, so that the aurora would not burn their hair off.
A lot of people thought that auroras, especially the red ones, foretold bad times, such as plague, war or great fires. The reddish auroras often made people believe that a city close by was on fire and they rushed there only to find that it was not the case. It is easy to see how people who were not used to auroras could mistake them for fires, especially since most of the houses were made of wood and easily caught fire.
For the Laplanders, as for other people in northern Europe, Asia and America, the aurora was a place for the dead. Above all it were people who had died a violent or too early death who came to live in the aurora. It could be people who were murdered, killed in war, took their own life, died in child birth or unborn children.
In some areas the spirits of the dead seemed to have quite a good time in the aurora. The Inuits in Greenland and northern Canada thought that the spirits were playing soccer with a walrus skull. Their name for the aurora is aqsalijaat, the trail of those who play soccer. From Baffin Island it is told that the walrus skull found it all so amusing that it clattered its jaws. Those who looked at their ancestors’s games had to look out so they did not get their heads knocked off by the skull.
The Laplanders thought that auroras and the weather were connected. When the aurora was flaming high in the sky the weather would be warm. By the magic influence of the aurora they thought it was possible to also influence the weather. This could be done in many different ways. In Kvikkjokk they called out a chant which started “gokseth (aurora) lipi, lipi”. Lipi is short for lihphuit that means flutter. From Vilhelmina it is told that you could make the aurora flutter by waving a white sheet.
All people did not think that a fluttering aurora meant warm weather, some thought that it was getting cold and others that there was a storm coming. Most people believed that a fluttering aurora meant a change in the weather though.
The aurora is not only beautiful to look at, there is also a lot of research going on. The scientists are among other things trying to find out why there are auroras, how often there are strong auroras and so on. I will try to explain how some of the research is done.
The particles that make auroras possible don’t reach the ground, they are stopped by the Earth’s atmosphere. This makes it difficult to study those particles. If you want to study the behaviour of the particles you have to send instruments to make measurements in the higher atmosphere.
The instruments sent into space with a rocket or satellite are called payload.
Sounding rockets are used for accurate measurements of selected auroras. They can reach heights of approximately 1000 kilometres and make measurements for about 10 minutes. A lot of sounding rockets for aurora research have been launched from Esrange 40 kilometres east of Kiruna.
If you want to reach areas far away from the Earth or measure for a longer period of time you use satellites. Due to such projects being very expensive and demanding it is common that many countries cooperate when it comes to satellites. The advantage of cooperation is that each country does not need as much money and or as many people working on the satellite.
In space, data from the different instruments are translated into radio signals that are transmitted to a receiver on Earth, where they are recorded on magnetic tapes. In most cases the ground station can only reach the satellite during a period of its orbit, therefore the data is often stored in memory banks on the satellite until the next contact with Earth.
If everything goes well now the most interesting part of the project starts for the scientists, to analyse the data. It is often possible for more than one explanation to correspond to the data. If you for example measure a lot of electrons for a short moment you don’t know if the satellite flew through an area of electrons or if it was a very short electron burst.
Measurements from the ground are also very important. Ground measurements have some advantages compared to measurements in space: all the measurements can be performed at the same location (the rocket or satellite is constantly moving), and it is possible to repair the instruments if they break down.
It has to be dark and fairly calm weather
It is very important to look often. The most intense part of an aurora often lasts only between 10-30 minutes.
It can sometimes be hard to see the difference between a faint aurora and a cloud. If it is an aurora you can see stars through it. Auroras are often greenish and change shape in a different way than a cloud.
In Scandinavia the most active auroras are often seen before midnight.
The aurora can be photographed with an ordinary camera, but a fast film is recommended (for example 400 ASA).
Here is a list of the books I have used to write the aurora pages:
- Ingrid Sandahl, Norrsken: Budbärare från rymden (Stockholm: Atlantis, 1998)
- Asgeir Brekke and Alv Egeland, Nordlyset, fra mytologi till rumforskning (Oslo: Grøndahl & Søn Forlag A.S, 1979). Also published in English (see below)
- Candance Savage, Aurora (Vancouver: Greystone Books, 1994)
Other books in English include:
- Asgeir Brekke and Alv Egeland, The Northern Lights: Their Heritage and Science (Oslo: Grøndahl og Dreyer, 1994)
- Robert H. Eather, Majestic Lights: The Aurora in Science, History and the Arts (Washington: American Geophysical Union, 1980)
- Neil Davis, The Aurora Watcher’s Handbook (Fairbanks: University of Alaska Press, 1992)
- Kenneth R. Lang, Sun, Earth and Sky (Berlin: Springer, 1995)
- William Petrie, Keoeeit &emdash; The Story of the Aurora borealis (Oxford: Pergamon Press, 1963)
- S. I. Akasofu and Y. Kamide (eds), The Solar Wind and the Earth (Tokyo: Terra Scientific; Dordrecht: D. Reidel, 1987)