| Read more about sunspots and magnetic fields |
|
 |
The size of the sunspots is given in relation to how large a part of the Sun’s disc they cover. Often the unit “millionth” of the Sun’s disc is used. A sunspot measuring a millionth has a surface area like 0.000001 times the visible surface of the Sun.
|
|
|
| |
| The figures of sunspots |
Usually a large sunspot measures 300 to 500 millionths. In September 2000 a sunspot measuring 2140 millionth emerged. The largest spot we know appeared in 1947 and measured 61130 millionths. In comparison the surface of the Earth is only 169 millionths of the Sun’s visible hemisphere.
The number of sunspots has been registered continually by astronomers for more than 250 years. Gradually it turned out that the spots varied in a cycle with a period of 11 years. The cycle of the sunspots was described for the first time in 1863. On average the periods last 11.3 years, but this varies from 7 to 17 years. |
| |
|
| Butterfly diagram. Illustration: NASA. |
|
The diagram to the right shows how the magnetic field of the Sun develops over time. There are strong magnetic fields in the spots, so we see the spot areas as coloured ribbons in the middle of the Sun’s disc. We see how the spots start at high latitudes and gradually work their way closer towards the Equator. Each single spot lives far too short a time to be able to move much closer to the Equator, but when a spot has perished the next spots occur usually somewhat closer to the Equator.
The diagram is somewhat similar to the wings of a butterfly and is therefore called a butterfly diagram.
Also the forms of activity on and round the Sun vary with the same cycle as the spots. |
| |
| How to measure the magnetic field? |
We cannot travel to the Sun and measure how strong the magnetic field is. How then can we find out anything about it?
The magnetic fields affect the light from the Sun. This is perhaps not so strange if we keep in mind that light is a form of electromagnetic radiation. Magnets affect other magnets. The magnetic field of the Sun affects the radiation, namely the light. By splitting the light into a spectrum, researchers observe that the magnetic fields split certain spectral lines into two. Consequently we get two lines instead of one.
By measuring the distance between these lines, they measure how strong the magnetic field is. In the sunspots the magnetic field may be up to 1000 times stronger than the magnetic field of the Earth. Since the magnetic field on the Sun and the spots turn every 11th years, a full Sun cycle lasts in fact 22 years. Only after 22 years will the Sun again be in the same part of the cycle and with the magnetic field pointing the same way. |
| |
| The largest spots |
|
Image of a large spot in March 2001.
Photo: ESA/NASA. |
|
Some few spots grow so large that they with sufficient protective equipment can be seen with the bare eye.
Especially round the Sun maximum, such as in 2000 and 2001, some such spots occur. These spots are in size far larger than the Earth!
NEVER look at the Sun without protection! This applies both to the naked eye and in particular when using binoculars. Your sight may very quickly suffer permanent injury from it and the sunlight is also too strong to make us able to see anything of interest without using special methods. |
| |
| Solar eclipse spectacles |
In connection with solar eclipses, special, very dark spectacles often become available. Many people have almost certainly still got similar from the eclipse in August 1999.
It is safe to use these glasses, and by using them the largest sunspots can be seen. This, however, presupposes that the glasses are undamaged. There must NOT be scratches or similar things in the “glasses”. Then they will let through so much light that your sight may be damaged. |
| |
| Other safe equipment |
Never look at the Sun directly with the bare eye or with binoculars or a telescope!! If you want to study the Sun you must always be careful. Serious and lasting injury to the eye may occur within a short period of time. The inventor of the astronomical telescope, Galileo Galilei, studied the sun through a telescope 400 years ago and got permanent injuries to his eyes. If this could happen to Galileo, it may also happen to you!
A safe way of studying sunspots is to image the Sun through a telescope or ordinary prism binoculars on to a white screen (for instance white carton). If you use prism binoculars, cover up one of the lenses. |
| |
On the screen you will see a strong circle of light. This is the Sun’s disc. Adjust the distance between the screen and the binoculars in order to get a suitable size of the Sun’s disc (10-15 cm). You will perhaps have to adjust the focus of the binoculars in order to get a clear picture.
By means of this method you should be able to study sunspots and details around these very well. It may be worthwhile fastening the binoculars to a stick or a tripod, as it is otherwise here difficult to keep the binoculars steady.
The method described here is called the projection method.
If you took care of the special sunglasses that were sold for the eclipse in 1999, these may also be used. Be careful to check that they are undamaged and without cracks or scratches. |
| |
| The Wilson effect |
As the Sun rotates, the sunspots move towards the brim of the Sun and disappear. With a telescope (NB! Read the safety regulations above!) we can see something strange happen as the same time as a large sunspot is lying on the limb of the Sun. Apparently it is a dip – a depression – just in the spot.
The phenomenon is named after the astronomer who discovered this strange phenomenon.
There is a strong magnetic field in the sunspot. As the gas is charged, the magnetic field works on each individual atom and tries to remove them. This resembles what happens if you move a strong magnet towards another magnet. In a way the magnetic field pushes much of the gas away so that we see further into the Sun just in the spot. It is only possible to observe this when the sunspot is seen on the limb of the Sun. |
|
|
|
|
| |
|
|
| |
|
