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The aurora borealis is one of the most beautiful natural phenomena visible to human eye. On a bright winter night, the northern sky can be seen glowing with dim light and bright colours. The aurora borealis is caused by solar wind blowing electric particles that collide with the particles in the atmosphere approximately 80 to 300 kilometres above the ground. Many kinds of tales and beliefs are linked to this event. The phenomenon itself is real, but the glimmer of beautiful colours has captivated people’s imagination. Generally, aurora borealis flame as slow and steady light but sometimes they “dance” cheerfully as the colours change from green-yellow to red. The picture below was taken at midnight in Munavuoma in Lapland. Due to the long exposure time, the stars have moved as the Earth has rotated. The routes of the stars can be seen as parallel lines. (Picture IH, Finland.)
The aim: Getting to know aurora borealis as the phenomenon of starry sky, learning the secrets behind their colours, forms and movements and the connection to the physical world.
Explorations: Aurorae are the most visible on northern and southern Polar Regions due to the form of the Earth’s magnetic field. In Finland, aurora borealis can be most often viewed in the northernmost Lapland, approximately on 200 nights a year, whereas in southern Finland the phenomenon is rare with about two dozen sightings annually. In 2000 and 2001, a very powerful aurora borealis storm occurred, when bright bands of aurora borealis were seen on the northern hemisphere all the way down to southern Europe. Most commonly, aurora borealis can be seen as steady arch but many kinds of forms can be seen from time to time.
Electric particles from the sun, such as protons and electrons, are carried by the solar wind to the Earth’s magnetic field that reaches as far as 100 000 km above the atmosphere. Due to the influence of the solar wind, this field, i.e. magnetosphere, stretches forming a tale of millions of kilometres on the night side of the Earth. Part of the electric particles drift to the Earth’s magnetic field to form the so-called Van Allen zone. Part of the particles are stored in the tale of the magnetic field and carried from there along force currents to the proximity of the poles of the Earth. These particles are pushed into the atmosphere of the Earth, colliding with the molecules of different matter 80 to 300 km above the ground and exciting the atoms. When the excitation is released, the extra energy is released as light. Depending on the variety of the excited atom, the released light can have different wavelengths that are visible to the eye in different colours.
Aurora borealis cannot be seen at all times. The best time of the day is during the magnetic midnight, which occurs approximately an hour before the actual midnight. The difference is caused by the 11º-difference between the North Pole on top of the axis of the Earth and the magnetic North Pole. According to observers, the aurora borealis is the most commonly seen during Equinoxes, but also on other bright winter nights viewing is possible. It has been noted that there is a connection between the visibility of aurora borealis and appearance of sunspots. Scientists have noted cyclical variation in the amount of sunspots every 11 years. During the maximum amount of sunspots, aurora borealis occurs more than usually.
1) Exploring the northern sky by observing it in the environment, when possible. During mid-winter, it is dark already at early afternoon and a short walk with the pupils can be easily arranged. On clear weather, the walk can be carried out after the lessons. The Polaris, and the most probable polar area for aurora borealis, is searched using a star chart. While waiting for the aurora borealis the circumpolar constellation can be revised and targets invisible to bare eye can be searched for with binoculars.
2) After successful aurora borealis trip, the observations can be discussed with the pupils. The observations are gathered under most common criteria, such as colour, form, and movement.
The basic forms include arch, band, blotch, and corona. These can be divided into more precise subcategories. An arch can be glowing low above the horizon, or straight, homogenous arch with clear bottom line, which can also include clear vertical rays. The folding bottom and upward rays are seen clearly in the aurora bands. Aurora blotch is indefinable light area that glows steadily or pulsates slightly. Corona is rarer form of aurora borealis. During powerful aurora storm, a torrent of rays seems to come from one spot in the sky and spread all over forming the most imaginary patterns.
The most common colour is light green low above the horizon. The dim gloom or arch is commonly seen as light colourless area. The more powerful bands and coronas can be seen in the shades of even red or red-green higher in the sky. During the aurora borealis storm in 2000, the corona spot was visible almost above head-level in southern Finland (Fig. S.Ekko, Finland).
Like it was mentioned earlier, the colours are caused by the different wavelengths of colour released from excited atoms. In the lower ionosphere, 100 km above the ground, light of yellowish green (oxygen) and reddish blue (nitrogen) can be seen. Light yellowish green light can be seen at approximately 200 km and ruby-coloured light (oxygen) at almost 300 kilometres.
The movements of aurora borealis and reasons behind them can be discussed with the pupils. The aurora borealis flaming steadily are the most common, but the best and most beautiful sighting involves “dancing” aurora borealis. That is a sight that the pupils won’t easily get tired of looking at (IH, Finland, 2013)
3) Getting to know the Earth’s magnetosphere more thoroughly. Illustrative images can be found in the literature. The pupils can draw individually or in groups sketches of the Earth and the surrounding magnetosphere and its tail on their notebooks or on blackboard. Different areas, such as the poles, and Van Allen zone, bow shock, plasma sphere, tail, and solar wind, can be marked in the pictures. More advanced pupils can study these concepts more thoroughly in pairs and present them to the class.
4) The aurora borealis cause changes in the Earth’s magnetic field. The changes in the magnetic field can be studied with simple magnetometer. It can be built with the pupils in pairs or together with the entire class. A bar magnet with a piece of mirror glued on the other side is attached to fine string that is attached on the centre of the lid of a glass jar. The lid is closed so that the movements of the magnet are not interfered with the airflow. The jar is placed in a room with as few iron constructions or electronic devices as possible. Then the magnet is let to settle, the movement should be from north to south. The ray of laser pointer is targeted on the mirror and the reflected red dot on the wall of the classroom or big shade is examined. Changes in the magnetic field interfere with the balance of the bar magnet and this rotation can be seen in the movement of the red dot. The image demonstrates the structure of the simple magnetometer (illustration VH, Finland).
5) * The occurrence of aurora borealis can be studied through Earth currents. The alterations in the magnetic field cause changes in the underground electric currents of, e.g. veins of gold and water, and mineral-rich wetlands. The potential difference, i.e. current, can be measured with sensitive meters from metallic poles stuck in the ground 100 to 300 meters apart. With a computer connected to the meter, the time-current chart of a broken line with interference peaks can be calibrated on the screen. It is a good idea to discuss the reliability of the results in forecasting aurora borealis. The best way to confirm the results is to take the equipment outside and at the same time observe aurora borealis. In the picture, equipment suitable for measuring Earth currents from metal poles stuck on the bog terrain of Lapland (illustration VH, Finland).
6) * Potential difference can be measured vertically in the air by hammering two nails on dry wood, e.g. dead tree or high pole. However, there shouldn’t be any strong electric devices or iron constructions nearby. The current between the nails can be measured with a similar device as in the previous assignment. Because the nails are quite close to each other, the changes in the current are surely quite weak and the results difficult to observe. These difficulties should be discussed with the pupils. The results can be compared with the results from the Earth current measurement if the assignment in question has been previously completed.
7) * Pupils who are interested in photographing can challenge themselves and try and photograph aurora borealis. Steadily flaming aurora borealis can be photographed using longer exposure times but with dancing aurora borealis flaring with colours, the exposure time can be only few seconds. When using mechanic camera, the film sensitivity should be e.g. 1600. While photographing with digital camera, photographing requires familiarization with photographing and functions of the camera. Interested pupils and teachers will find the ways and settings to take good pictures based on information in the literature, as well as trial and error.
8) * More special way to observe aurora borealis is to listen to them on the radio. There might be some amateur radio enthusiasts among the pupils, who ”listen” to the environment and space on different frequencies. The changes in the magnetic field interfere with the connection. The interested pupils can search more information online, and share the information with others. Perhaps even a club for listening to the sounds of the space can be set up to the school.
Methods: Observing in nature, making and testing observing equipment, electric measurements and analysis, drawing figures and charts, discussion.
Materials: Binoculars, glass jars, mirrors, magnets, laser pointers, current and voltage meters, metallic pole, computer with suitable software, radio, craft materials.
Discussion: Discussing observations and measurement methods is very important. It is a good idea to thoroughly contemplate the reliability and predictability of the results. To support this, the pupils and teacher can together search old research and measurement data.
Examining the results: Discussing the results together with the class. Comparing observations and figures. Making sure that the concepts are understood and revision, if needed. The pupils’ presentations on the research and discoveries. Compiling the presentations on the classroom wall.
Tips: Try to find those with special skills among the pupils; they can perform as experts. If possible, visit nearby research centre.
Key words: Solar wind, magnetic field, magnetosphere, Van Allen zone, plasma zone, bow shock.