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Northern Light- Aurora Borealis
The solar wind can bring fast-moving particles towards the Earth. Some of them are then guided by the Earth’s magnetic field towards the north and south magnetic poles. As they approach the Earth, the particles interact with the atmosphere making it glow in many different colours and causing the effect we call an aurora.
There are many different forms of the northern-light or aurorae, but the most well-known look like curtains of light, with ripples and curls moving along them.
The aurora are caused by particles in the solar wind brushing past the Earth’s magnetic field which captures some of them and sends them spiralling down through the atmosphere.
Nothing else in the sky looks like the northern lights. The sun and the moon, the stars and the planets are an eternal, regular and predictable part of human life. The northern lights, on the other hand, are transient, variable and unpredictable. Here the cosmos parades its electrical and magnetic forces, and produses colours and movements that are unique in Nature. Photograps are pale, static shadow of the real thing. The northern lights must be seen andexperienced outside, under a still and cold winter sky.
The northern lights-the aurora borealis-can be seen, on rare occasions, almost anywhere on earth. In February 1872 , for example, the north lights were seen in Bombay and in Egypt, and in September 1909 they were observed in Singapore and Jakarta! However they belong primarily to the polar regions of the world, occurring most often in a belt around the magnetic pole at a distance of 2,500 km from it. This so-called auroal zone passes over northern Skandinavia, over Iceland and the southern tip of Greenland, through northern Canada, over Alaska and along the northern coast of Siberia. The coast from Troms and Finnmark is where the northern lights appear with the greatest frequency fo all. So it is obvious that northern Norway, with its easy accessibility and mild winter climate, is attractiv to people who wants to see this phenomenon of the heavens.
There is an exactly equivalent zone around the south magnetc pole. These “southern lights”-the aurora australis-can generally be seen only in the Antarctic and the surrounding waters. Of the inhabited aeras in the southern hemisphere, it is only in Tasmania and in the southern part of New Zealand that people get a glimps of them with any frequency. Insidentally, the northern lights and “the southern lights” occur simultaneously and are almost mirror images of each other.
In the auroal zone the northern lights are an everyday phenomenon. From a scientific point of view, it can well be clamed that there is an aurora every night, but some of the occurrences are so faint that most people hardly notice them. From the tourists` point of view, however, it is hardly an exaggeration to say that we can see the northern lights at least every other clear night in the counties of Troms and Finnmark. If we go to southern Norway, they will occur a few times a month, and in central Europe they will perhaps be seen once a year. In the Mediterranean area are among the very rare events which occur perhaps a few times each country. In the polar regions, within the auroal zone itself. Thus there are fewer auroras in Svalbard than in northern Norway. We associate the northern lights with winter, but in fact they are there all the year. It is just that we cannot see them on light nights; the sun has to be belowe the horizon. In practice, in northern Norway the northern lights are limited to the period from the beginning of September to the middle of April. Intens auroras can, however, be seen against a very light sky. In northern Norway it is, for example, not uncommon to see the northern lights in an evening sky in August.
The northern lights we see in northern Norway are usually called the night-time aurora because they are on the night side of earth. The display usually begins in the late afternoon or in the evening and continues with varying intensity often far into the night. This is the common form of aurora, but on Svalbard in the polar night we can see in addition the rarer day-time aurora, which occurs on the day side of the earth.
The northern lights are far above cloud cover, so we must have clear weather in order to see them. The weather is therefore the most important obstacle to the observation of the northern lights in the northern Norway. The most stable winter weather with a clear sky undoubtedly occurs in inner fjord areas and the interior of the country. So for tourists in search of the northern lights these areas are probably to be preferred to the coast.
The days around the full moon are not the best ones for observing the northern lights. The sky is so light then that the expirience is considerably paler. Finally, one ought to get away from the towns and densely populated areas with lots of lights to get full value from an evening with northern lights. We often forget this last point. People today are surrounded by so much artificial light that to a large exent they have “lost” the darkness and forgotten what the firmament looks like.
The height of the northern lights above ground level was a controversial subject for a long time. Around 1900 there were few people who claimed that the northern lights could reach right down to the ground, but whether they were located at a height of a few kilometers or many hundered kilometers up was unclear. The problem was solved by photographing them simultaneously from two places. If the appropiate distance between the two places was selected, it was possible to discover the displacement of the northern lights in relation to the stars in each photographs and thus calculate their height. Thousands and thousands of such triangulations were preformed from 1910 up to the 1950s. They showed that the northern lights were generally between 90 and 130 km above ground level, but many auroras – particulary the rayed aurora – extend to a height of several hundred kilometers. By way of comparison, the usual flight level of a jet is ca 10 km, and the ozone layer is located 20-30km up. We have to go almost as hight as satellites to find the northern lights. A consequence of the great height is that the northern lights are visible over distances of several hundred kilometers. Thus an aurora over Bear Island could be seen in both northern Norway and Svalbard, and an aurora over Finnmark could be seen in the northern sky in the county of Troendelag.
Sunlights contains in all the colours of the rainbow in an unbroken series from blue through green and yellow to red. In an aurora, on the other hand, the light is collected in a selcetion of narrow bands of colour, so-called spectral lines. An aurora occurs when large quantities of electric particles (electrons) approach the earth at height speed along the magnetic field and collide with the upper layers of the atmosphere. The molecules and the atoms in the gas will then be infused with energy which they subsequently emit again as shafts of light. It is a bit like what happens in a neon tube. The spectral lines reflect which gases are found up there. It turns out that the northern lights are dominated by lines which comes from common gases like nitrogen and oxygen. The oxygen molecules, however, are split by sunlight so that we have atomic oxygen (O), and some of the nitrogen molecules (N2) have lost an electron and become a positiv ion (N2+). The northern lights are characteristically a greenish-yellow colour, but also have a considerable element of blue. The greenish-yellow is due to a strong line from O, whereas the blue comes from N2+. When the northern lights acquire an element of redish-violet on the lower border, the red lines from the N2+ are making an apperance. If they turn red in the upper parts, we are seeing the light from O again. During large-scale auroral displays, this red oxygen may be very prominent and colour large parts of the northern sky deep red when seen in southern Scandinavia and even central Europe. It was this red aurora which spread alarm and terror in central Europe in previous centuries.
The light from a very intensiv aurora can be compared to the light from a full moon, but usually it is some what fainter. Normaly, however, photographing the norhtern light is not a problem. You need a tripod-mounted camera with a high-speed lens (at least f:1,8) and fast film (400 ASA). Then you can reduce your exposure to about one second and capture the finer details which are often lost with long exposures.
There are many reports of sound in connection with the northern lights. On still nights with unusually intese auroras people say that they hear a crackling sound. Scientific instruments have not yet succeeded in recording this phenomenon, but it must nevertheless be accepted as genuine on the basis of many reliable testimonies. It is quite inconceivable that the sound reach us from a height of 100 km, but we know that the northern lights are associated with a power full electric field which can be recorded at ground level. Presumably the sound is the result, eighter in the form of electrical discharges or directly influenced by our auditory nerves.
The northern lights orginate in a complicated interplay between the so-called solar wind and the earth`s magnetic field. The solar wind is a constant stream of electric particles from the sun. It varies in intensity and therefore links the northern lights with the solar activity. The solar winds rushes along the earth`s magnetic field, compresses it on the day side, draws it out into a tail on the night side and generates electric currents and fields in the areas around the earth. A number of solar wind particles are trapped in the earth`s magnetic field and, together with particles which orginate in the earth`s atmosphere, end up in the tail on the magnetic field on the night side. As a result of mechanisms we still do not really understand, they receive extra energy there, stream toward the polar regions at great speed and give us the night-time aurora. These unpredictable showers of electric particles controlled by the earth`s magnetic field give the northern lights their forms and movements. The showers tear along like impetuous squalls, creating arcs, draperies and rays.
The magnetic field is the key to why the northern lights prefer the polar regions. The electric particles travel most easily along the magnetic field and reach farthest down into the atmosphere in the polar regions, because here in the field is almost perpendicular to the earth`s sureface. Most such particle precipitation is found in a ring around the magnetic poles, and in this ring the northern lights are situated like an unbroken halo around both poles. We usually call this snapshot of the northern lights the auroral oval. On the night side of the earth the particles from the tail of the magnetic field from the powerful and active night-time northern lights. On the day side they come more directly from the solar wind and from the fainter and more subdued day-time northern lights which we can see in Svalbard, where it is dark 24 hours a day in the middle of winter. The auroral oval is fixed in a relation to the sun, while the earth revolves below. In northern Norway we enter it in the afternoon and pass through it in course of the evening and night. During the day the oval is to the north of us, over Svalbard.
When solar activity is intens, the oval may expand towards the south so that we in Troms and Finnmark are on the inside, while we get large auroral displays over southern Norway and perhaps even farther down into Europe. During periods of modest activity it contracts and we get the northern lights to the north of us. Then they are usully also faint. The auroral oval must not be confused whit the auroal zone. The former is the aurora as it is found above the earth at the given time; the auroral zone tells us where the night side of the oval is generally to be found.
The magnetic poles move slowly and take the auroral zone and auroral oval with them. Over periods of several hundred years the changes can be great. Five hundred years ago the auroral zone was probably over southern Norway, and presumably it will move northwards from Norway in centuries to come. Thus we are now in a favourable era in norther Norway with regards to the northern lights. If the magnetic field were to disappear, we would lose the northern ligths and be left with a pale souvenir in form of a diffuse light over the whole of the night sky.
Kilde: Professor Truls Lynne Hansen
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What is aurora
Aurora is a luminous glow of the upper atmosphere which is caused by energetic particles that enter the atmosphere from above.
This definition differentiates aurora from other forms of airglow, and from sky brightness that is due to reflected or scattered sunlight. Airglow features that have “internal” energy sources are more common than aurora, for example lightening and all associated optical emissions like sprites should not be considered aurora.
On Earth, the energetic particles that make aurora come from the geospace environment, the magnetosphere. These energetic particles are mostly electrons, but protons also make aurora. The electrons travel along magnetic field lines. The Earth’s magnetic field looks like that of a dipole magnet where the field lines are coming out and going into the Earth near the poles. The auroral electrons are thus guided to the high latitude atmosphere. As they penetrate into the upper atmosphere, the chance of colliding with an atom or molecule increases the deeper they go. Once a collision takes place, the atom or molecule takes some of the energy of the energetic particle and stores it as internal energy while the electron goes on with a reduced speed. The process of storing energy in a molecule or atom is called “exciting” the atom. An excited atom or molecule can return to the non-excited state (ground state) by sending off a photon, i.e. by making light.
What makes the color of the aurora
The composition and density of the atmosphere and the altitude of the aurora determine the possible light emissions.
When an excited atom or molecule returns to the ground state, it sends out a photon with a specific energy. This energy depends on the type of atom and on the level of excitement, and we perceive the energy of a photon as color. The upper atmosphere consists of air just like the air we breathe. At very high altitudes there is atomic oxygen in addition to normal air, which is made up of molecular nitrogen and molecular oxygen. The energetic electrons in aurora are strong enough to occasionally split the molecules of the air into nitrogen and oxygen atoms. The photons that come out of aurora have therefore the signature colors of nitrogen and oxygen molecules and atoms. Oxygen atoms, for example, strongly emit photons in two typical colors: green and red. The red is a brownish red that is at the limit of what the human eye can see, and although the red auroral emission is often very bright, we can barely see it.
Photographic film has a different sensitivity to colors than the eye, therefore you often see more red aurora on photos than with the unaided eye. Since there is more atomic oxygen at high altitudes, the red aurora tends to be on top of the regular green aurora. The colors that we see are a mixture of all the auroral emissions. Just like the white sunlight is a mixture of the colors of the rainbow, the aurora is a mixture of colors. The overall impression is a greenish-whitish glow. Very intense aurora gets a purple edge at the bottom. The purple is a mixture of blue and red emissions from nitrogen molecules.
The green emission from oxygen atoms has a peculiar thing about it: usually an excited atom or molecule returns to the ground state right away, and the emission of a photon is a matter of microseconds or less. The oxygen atom, however, takes its time. Only after about a 3/4 second does the excited atom return to the ground state to emit the green photon. For the red photon it takes almost 2 minutes! If the atom happens to collide with another air particle during this time, it might just turn its excitation energy over to the collision partner, and thus never radiate the photon. Collisions are more likely when the atmospheric gas is dense, so they happen more often the lower down we go. This is why the red color of oxygen only appears at the very top of an aurora, where collisions between air molecules and atoms are rare. Below about 100 km (60 miles) altitude even the green color doesn’t get a chance. This happens when we see a purple lower border: the green emission gets quenched by collisions, and all that is left is the blue/red mixture of the molecular nitrogen emission.
What is the altitude of aurora
The bottom edge is typically at 100km (60 miles) altitude.
The aurora extends over a very large altitude range. The altitude where the emission comes from depends on the energy of the energetic electrons that make the aurora. The more energy the bigger the punch, and the deeper the electron gets into the atmosphere. Very intense aurora from high energy electrons can be as low as 80 km (50 miles). The top of the visible aurora peters out at about 2-300 km (120-200 miles), but sometimes high altitude aurora can be seen as high as 600 km (350 miles). This is about the altitude at which the space shuttle usually flies.
What causes the aurora
Energetic charged particles from the magnetosphere.
The immediate cause of aurora are precipitating energetic particles. These particles are electrons and protons that are energized in the near geospace environment. This energization process draws its energy from the interaction of the Earth’s magnetosphere with the solar wind.
The magnetosphere is a volume of space that surrounds the Earth. We have this magnetosphere because of Earth’s internal magnetic field. This field extends to space until it is balanced by the solar wind.
The solar wind is the outermost atmosphere of our sun. The sun is so hot that it boils off its outer layers, and the result is a constant outward expanding very thin gas. This solar wind consists not of atoms and molecules but of protons and electrons (this is called a plasma). Embedded in this solar wind is the magnetic field of the sun. The density is so low that we may well call it a vacuum. However tenuous it is, when this solar wind encounters a planet, it has to flow around it. When this planet has a magnetic field, the solar wind sees this magnetic field as an obstacle, as protons and electrons cannot move freely across a magnetic field. These charged particles are constrained to move almost always only along the magnetic field. Likewise, when they are forced to move in a specific direction, a magnetic field will move with them or will be bent into the direction of the flow. Whether the magnetic field forces the plasma motion or whether the plasma motion bends the magnetic field depends on the strength of the field and the force of the motion. When the solar wind encounters Earth’s magnetic field, it will thus bend the field unless the field gets too strong. The strength of the magnetic field falls off with distance from Earth. The distance at which the solar wind and the magnetic field of the Earth balance each other is about 10-12 Earth Radii (1 RE is 6371 km). For comparison, the moon is at about 60 RE, geostationary satellites are at about 6 RE. A plot that shows the actual distance in real-time can be found at this website. The inside of this volume that is bounded by the solar wind is called the magnetosphere.
At the interface of the solar wind and the magnetosphere, energy can be transfered into the magnetosphere by a number of processes. Most effective is a process called reconnection. When the magnetic field in the solar wind and the magnetic field of the magnetosphere are anti-parallel, the fields can melt together, and the solar wind can drag the magnetospheric field and plasma along. This is very efficient in energizing magnetospheric plasma. Eventually, the magnetosphere responds by dumping electrons and protons into the high latitude upper atmosphere where the energy of the plasma can be dissipated. This then results in aurora
Why does aurora have the shape of curtains
The magnetic field confines the motion of auroral electrons. Think of it as painted magnetic field lines.
The electrons that make the aurora are charged particles, and they are not free to move in just any direction. Magnetic fields impede motion of charged particles when they try to cross the magnetic field. Charged particles can move freely only parallel to the magnetic field (either in the direction of the field or against it). When the solar wind encounters the outer reaches of Earth’s magnetic field, the field gets distorted by the motion of the plasma (see the previous question). Near the Earth the magnetic field is too strong and the motion of the electrons is guided by Earth’s magnetic field. When an electron spirals along the magnetic field into the atmosphere, it stays on or near this field line even when it makes a collision. Therefore the aurora looks like rays or curtains.
How often is there aurora
There is always some aurora at some place on Earth.
Weak aurora, with a small, barely visible auroral oval in this image from the POLAR VIS instrument. The bright crescant shape light on the left is from the sun illuminating the Earth.
When the solar wind is calm, the aurora might only be at high latitudes and might be faint, but there is still aurora. In order to see aurora, however, the sky must be dark and clear. Sunlight and clouds are the biggest obstacle to auroral observations. If you have a camera on a satellite you can look down on the aurora, and you’ll find an oval shaped ring of brightness crowning Earth at all times. When the solar wind is perturbed from a recent flare or other event on the sun, we might get very strong aurora. After the solar wind has transferred a lot of energy into the magnetosphere, a sudden release of this built-up tension can cause an explosive auroral display. These large events are called substorms. A substorm usually starts with a slow expansion of the auroral oval followed by a sudden brightening of a small spot, called the auroral breakup. This spot usually is near that place of the auroral oval that is on the opposite side of the sun, which means near the place where midnight is. This brightening rapidly grows until the entire auroral oval is affected. An observer on the ground where this breakup occurs will see a sudden brightening of the aurora which may fill almost the entire sky within tens of seconds. This aurora will be in the shape of rapidly moving curtains. If you are under the auroral oval west of this breakup, you will see a bright aurora moving toward you from the east that might cover almost the entire sky and move from the eastern to western horizon within minutes. This aurora will often look like a huge spiral of curtains, with many smaller curls within the curtains. After these auroral curtains subside, the sky might be filled with diffuse patches of aurora that turn on and off. The whole substorm typically lasts between 30 and 90 minutes. During periods of high solar activity, we might have several substorms per night, here is a movie of 4 substorms following each other (3.8 Mb) following each other, observed from the IMAGE satellite. On average, there are about 1500 substorms per year, but often there can be several days between substorms.
Where is the best place to see aurora. And what time is best
The best places are high northern latitudes during the winter, Alaska, Canada, and Skandinavia.
To see aurora you need clear and dark sky. During very large auroral events, the aurora may be seen throughout the US and Europe, but these events are rare. During an extreme event in 1958, aurora was reported to be seen from Mexico City. During average activity levels, auroral displays will be overhead at high northern or southern latitudes. Places like Fairbanks, Alaska, Dawson City, Yukon, Yellowknife, NWT, Gillam, Manitoba, the southern tip of Greenland, Reykjavik, Iceland, Tromso, Norway, and the northern coast of Siberia have a good chance to have the aurora overhead. In North Dakota, Michigan, Quebec, and central Scandinavia, you might be able to see aurora on the northern horizon when activity picks up a little. On the southern hemisphere the aurora has to be fairly active before it can be seen from places other than Antarctica. Hobart, Tasmania, and the southern tip of New Zealand have about the same chance of seeing aurora as Vancouver, BC, South Dakota, Michigan, Scotland, or St Petersburg. Fairly strong auroral activity is required for that. The best time to watch for aurora is around midnight, but aurora occurs throughout the night. There are very few places on Earth where one can see aurora during the day. Svalbard (Spitzbergen) is ideally located for this. For a 10 week period around winter solstice it is dark enough during the day to see aurora, and the latitude is such that near local noon the auroral oval is usually overhead.
Since clear sky and darkness are essential to see aurora, the best time is dictated by the weather, and by the sun rise and set times. The moon is also very bright, and should be taken into account when deciding on a period to travel for the purpose of auroral observation. You might see aurora from dusk to dawn throughout the night. The chances are higher for the 3 or 4 hours around midnight.
Do auroras occur on other planets
Almost all planets in the solar system have aurora of some sort.
If a planet has an atmosphere and is bombarded by energetic particles, it will have an aurora. Since all planets are embedded in the solar wind, all planets are subjected to the energetic particle bombardment, and thus all planets that have a dense enough atmosphere will have some sort of aurora. Planets like Venus, which has no magnetic field, have very irregular aurora, while planets like Earth, Jupiter, or Saturn, which have an intrinsic magnetic dipole field, have aurora in the shape of oval shaped crowns of light on both hemispheres. When the magnetic field of a planet is not aligned with the rotational axis, we get a very distorted auroral oval which might be near the equator, like on Uranus and Neptune. Some of the larger moons of the outer planets are also big enough to have an atmosphere, and some have a magnetic field. They are usually protected from the solar wind by the magnetosphere of the planet that they orbit, but since that magnetosphere also contains energetic particles, some of these moons also have aurorae.
Does the aurora have any effect on the environment
Yes, but limited to the high altitude atmosphere.
Since the aurora takes place at about 90-100 km altitude, only the atmosphere at or above that height is affected by aurora. Some ionization may occur a few tens of kilometers further down, and can have effects on radio wave propagation. Ham radio operators may find that at some frequencies, radio waves will not propagate far. The major effect of the aurora is, however, at the altitude range of 100-200 km. The precipitating particles that cause the light also cause ionization and heating of the ambient atmosphere. The ionization has the consequence that the electric properties of the atmosphere change, and currents can flow more easily. Aside from the charged particles that cause the light of the aurora, there are currents flowing between the magnetosphere and the ionosphere inside and in the vicinity of the aurora. These currents also contribute to the heating of the atmospheric gas at auroral altitudes. The heating from these currents is usually much more than by the particle precipitation itself. Once the gas in the aurora is heated, it wants to rise, so that convection can be driven by the aurora.
The currents in aurora not only flow vertically. A current has to be a closed loop, so there are currents flowing to and from the magnetosphere and horizontally in the vicinity of aurora as well. The currents in and around aurora are actually charged particles that move; positive charges in one direction, negative in the other. These moving particles can collide with the neutral gas of the upper atmosphere and drag the gas along. This means that not only vertical convection will be caused by the aurora, but also horizontal winds.
Although the change in temperature and wind inside and near the aurora can be very large, at some altitudes the temperature can rise to its tenfold value, and the wind can blow at several hundred meters per second (more than 1000 mph), none of these disturbances reach down to where the weather takes place. There is some speculation that long term changes in space weather, i.e. long-term effects of aurora and similar phenomena, may influence the long-term variation of the climate on Earth. This is the subject of ongoing research.
Other phenomena associated with aurora are perturbations in the magnetic field of the Earth. When we have a strong substorm, the magnetic field under the aurora can be decreased by as much as a few percent of its value. That, by the way, is the reason that these strong auroral events are called “substorms”: Earth experiences occasional magnetic storms, which are global changes in the magnetic field. The auroral substorm is a similar change in the magnetic field, but only happens on a smaller scale limited to the polar regions, thus they are “sub”-storms.
Kilde: Dirk Lummerzheim
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