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Tuesday, 24 October 2017 05:43

Irma Hannula

"EAAE Summerschools" Working Group

University of Helsinki (Finland)

Abstract

On the Earth you can see the Northern lights especially around the North and South poles. Here in Finland it means the Lapland. In the Finnish literature there are many kinds of beautiful legends and fabulous myths of this phenomenon.

The storms of Northern lights (Aurora Borealis) are connected with the very active areas on the surface of the Sun. On the other hand, the active area is connected with the sunspots. Sometimes you can see those spots on the surface for a long time. The activity of the Sun is usually expressed by a sunspot number. The observations have shown that the activity of the Sun has a period of about 11 years.

You can make some physical experiments for measuring the strength of the magnetic field and find out the existence of Northern lights. You can also forecast them by observing the Sun and sunspots, listening the space with radio or checking the situation in the web pages. In this workshop you learn three simple ways to measure the strength of the magnetic field.

Aurora Borealis - The enchanting phenomenon in the sky

Aurora Borealis (The Northern lights) is a very beautiful phenomenon of light seen in the northern and southern polar region. The electrical particles (protons and electrons) come from the Sun to the magnetic field of the Earth. The particles will collide with the molecules in the upper atmosphere (ionosphere), at a height of 80-300 kilometres. The molecules become excited in those collisions and shall radiate light; we call it the Northern lights in English or Aurora Borealis in Latin.

Fig.1 - The magnetosphere of the Earth.
Fig.1 - The magnetosphere of the Earth.

The magnetic field of the Earth, called the magnetosphere, reaches outside the atmosphere, even for 100 000 kilometres in the direction to the Sun (Fig. 1). There the particles blown by the solar wind will collide to the shock front. In the opposite direction the solar wind will blow the magnetosphere to the tail of millions of kilometres in length. The magnetic field of the Earth has captured electrons and protons blown by the solar wind to form the van Allen's belt (Fig. 2). Some of those particles come to the atmosphere and cause the Northern lights. If the Earth had no magnetosphere, the solar wind would blow the particles straight to the atmosphere, and this could cause many problems in the circumstances on the Earth.

Fig.2 - The aurora and the magnetosphere.
Fig.2 - The aurora and the magnetosphere.

The best location on the Earth for seeing the Northern lights is Fennoscandia. In the northernmost Lapland the Northern lights are seen in over 200 nights in the year. Even in the southern part of Finland you can see ten to twenty aurora shows during one year.

The best time for seeing aurorae is in the magnetic midnight. Because of the magnetic pole of the Earth is not the same as the North pole (the angle between the earth's axis and the magnetic axis is about 11°), those two midnights are not at the same time. The magnetic midnight comes about one hour earlier than the 'normal' midnight.

You can find some annual variations in seeing aurorae. At the time of equinoxies you can see the Northern lights much more often than in the other periods. There is a very strong connection between the existence of the Northern lights and the sunspots. The scientists have found a period of eleven years in the existence of sunspots, and the same period has been seen with the aurora storms. The latest strong storms of aurorae happened in 6. -7.4. 2000 and in 22.-23.10.2001. They were seen very well in the middle part of Europe, even as south as in Italy and Spain.

It is a very interesting thing that you can also 'hear' aurorae! The radio amateurs may listen such kinds of radio frequencies which usually are not been monitored. It is possible because of the ionization caused by the aurorae.

The form of aurorae is mostly like an arc which is usually motionless and dim. The band is more active. The lower edge of the band uses to be folded with vertical, moving pillar-shaped rays. Sometimes the form is like a patch or corona (Fig. 3).

Fig.3a - Forms of aurorae: Arc - Rayed band, Patch - Corona of rays.
Fig.3a - Forms of aurorae: Arc - Rayed band, Patch - Corona of rays.
Fig.3b - Forms of aurorae: Rayed arc over the fell Saana.
Fig.3b - Forms of aurorae: Rayed arc over the fell Saana.
Fig.3c - Forms of aurorae: Examples of auroral forms (drawings).
Fig.3c - Forms of aurorae: Examples of auroral forms (drawings).

Usually the Northern lights are green-coloured, but sometimes you can see also red-coloured or blue-coloured lights (Fig. 4). If you are very lucky, you may admire dancing aurorae. You'll never forget the experience!

Fig.4 - Colours of aurorae.
Fig.4 - Colours of aurorae.

Photographing aurorae

The brightness of aurorae may vary very much. When photographing aurorae you have to consider very carefully which kind of values of camera settings you are going to use. In the southern part of Finland you may use e.g. 400 ASA/f=2 for 10 seconds for the bright aurorae, but for the dim aurorae you have to use the double exposure time. The very bright shows in the northern part of Finland you may succeed to photograph using only the 5 seconds exposure time. Often the aurorae are so wide, that you have to use the 28 mm or even wider lens.

Finding aurorae - Measuring the strength of the magnetic field

Fig.5 - Periods of eleven years with sunspots and aurorae.
Fig.5 - Periods of eleven years with sunspots and aurorae.

We know now two possibilities for predicting aurorae: at first we can observe the sunspots (Fig. 5) and at second we can hear foreign radio stations. But in addition to these methods we can make some physical experiments.

In this workshop I will present three quite simple experiments for measuring changes in the strenght of the magnetic field and thus get an aurora forecast.

Experiment 1

First you can hang a bar magnet on a thread and fix a little mirror in the middle of the magnet (Fig.6). The magnet will turn back and forth at first, but then stop along the direction of the magnetic field. If you point a narrow beam of light to the mirror (e.g. use a laser light), you can see the movement of the magnet by the movement of the reflected light on the wall or on the shade. The movement of the magnet tells you the magnetic field lines are changing their direction, and that is an indication of possible Northern lights.

Fig.6 - Observing the magnetic storm.
Fig.6 - Observing the magnetic storm.

You have to make this experiment in such a place where you can eliminate as many disturbances as possible. The most efficient way to eliminate the disturbances (e.g. air currents) is to place the magnet in a closed glass jar.

Experiment 2

Another experiment you can make in a garden or a forest. The ground has to be damp. If you bury two half square meter metal sheets (e.g. plumb) into the ground and measure ground currents caused by the changing magnetic field, Northern lights may be found in the sky again. The scientists have found a connection between the magnetic field of the Earth, more specific the disturbances in it and the aurorae. The disturbances cause changes also in the ground currents.

You can put the metal sheets in two pots filled with salted soil, too. Then bury these pots in the damp ground at about 100-300 m distance. Measure the current between them. You can also connect the metal sheets to the computer and make some measurings at short or long intervals (Fig. 7). This data can be analyzed using the special computer program and the results can be presented as different kinds of diagrams or as a table.

Fig.7- Observing the magnetic storm.
Fig.7- Observing the magnetic storm.

Experiment 3

The most simple way to measure some changes in the air is to hit two nails into the three at the different levels from the ground. Then you can measure the voltage between these two nails.

References

Beck, R. (ed.) 1995. Solar Astronomy Handbook. Willman-Bell, Inc.

Falck-Ytter, H. 1984. Norrsken. Järna: Telleby Bokförlag.

Kaila, K. 2001. Revontulet - kansankäsityksistä tutkimukseen. Helsinki: Tähtitieteellinen yhdistys Ursa ry.

Karttunen et al. 2000. Tähtitieteen perusteet. Helsinki: Tähtitieteellinen yhdistys Ursa ry.

Karttunen, Hannu et al. 2001. Ilmakehä ja sää. Helsinki: Tähtitieteellinen yhdistys Ursa ry.

Minnaert, M. 1987. Maiseman valot ja värit. Suomentanut P. Kröger, alkuper. teos Licht en kleur in het landschap. Helsinki: Tähtitieteellinen yhdistys Ursa ry.

Oja, H., Poutanen, M. 1982. Planeetat. Helsinki: Tähtitieteellinen yhdistys Ursa ry.

Tähdet 2001. Helsinki: Tähtitieteellinen yhdistys Ursa ry.