The INTER-SOL Programme (ISP)

Basic Information on the Sun


The Sun

Our sun is a mainstream star with a diameter of 1.4 million km and an absolute magnitude of 4.73. Its core temperature is approximately 14.6 million kelvin. The average distance from the earth to the sun is approximately 149.6 million km and the suns light takes about 8 minutes to reach us. This light and warmth is the source of all life on earth. Because of its proximity to us the sun is the only star that astronomers can observe and research in detail. The study of events on the surface of the sun has made possible advances in atomic, nuclear and plasma physics. Observations of the sun's surface, also called the photosphere, with special filters show an ever changing surface punctuated by the appearances of sunspots, faculae, flares and protuberances. The sun exhibits differing rotational periods of between 24 and 34 days depending on the solar latitude. All surface phenomena rotate accordingly.


What are sunspots?

Sunspots are cooler areas in the photosphere produced by local changes in the magnetic field. Sunspots tend to occur in groups and are combined with other phenomena of solar activity such as faculae, filaments and flares. Although sunspots have an average temperature of 3700K they appear dark compared to the brighter surrounding regions which have an approx. temperature of 5400K. Sunspots vary in size from 2,000 km to over 100,000 km (an angle of one arcsecond corresponds to a length of approx. 700 km on the surface of the sun as seen from earth.) The development of the different phenomena proceed at varing rates. Larger sunspot groups tend to consist of two larger magnetically linked spots surrounded by smaller spots. This is a typical 'bipolar' group. Larger spots consist typically of an umbra (dark core) surrounded by a penumbra (brighter grey area). Sunspots have a lifetime from as little as a few hours to many weeks. Sunspot activity has a life cycle of approximately 11 years. At the start of the cycle the sunspots appear around the 35° solar latitude north and south and, as the sunspot cycle progresses, move more and more towards the suns equator. At the end of the 11 year cycle the magnetic polarity of the spots changes and a further cycle starts. This represents a 22 year magnetic cycle.


How sunspots form

The occurance of sunspots is related to other solar phenomena such as faculae, flares and protuberances which in turn are all influenced by the solar magnetic field. The following example shows a typical process of development of sunspot groups:
The formation begins when field lines emerge on the surface. If the magnetic flux in the stressed regions exceed a value of 0.1 tesla faculae may appear. Soon after this, a first small sunspot forms on the westerly edge of these faculae. As the magnet flux continues to increase this process continues with the formation of spots on the easterly side of the faculae but with magnetic polarity opposite to that of the first one. While smaller spots fuse and form larger spots the westerly main spot (preceding, or p) forms a penumbra. In the following days the easterly main spot (following, or f) may also form a penumbra. Between these two main spots many other smaller spots may form until the entire group reaches its maximum size. After a time all spots will begin to disappear except for the westerly main spot which will disappear later, too. The magnetic field becomes weaker and irregular. This total process of development is described in the Waldmeier Schedule.

Note:  The two main spots in a group are characterised as p- or f- spots. As the sun rotates from east to west the westerly spots are therefore called the preceding (p-) spots, and the easterly spots are known as the following (f-) spots.


Observation methodes

Observing the sun requires the utmost care as the sensitive eye can be irreparably damaged in one careless moment. Never look at the sun with the naked eye and NEVER VIEW THE SUN DIRECTLY WITH A TELESCOPE OR BINOCULARS. Two safe methods to observe the sun with a telescope are described below. Keep the covers on the finderscope at all times to avoid inadvertantly looking through it to find the sun.

  1. The image of the sun can be projected through the telescope onto a suitable white flat surface. As the temperature at the focal point of the telescope can get extremly hot it is advisable when using this projection method not to use eyepieces with cemented lenses as they may be damaged from the heat.

  2. Using an appropriate full aperature sun filter on the telescope reduces the intensity of the sunlight to a level suitable for visual observation. We cannot stress the importance of using a full aperature sun filter of the highest quality. Saving a few dollars is no compensation for the loss of eyesight. With a suitable full aperature sun filter the sun can be viewed directly with the telescope and will yield more contrast and details than the projection method.

We strongly advise against the use of eyepiece sun filters which in most cases cannot reliably withstand the very high temperatures created at the eyepiece and may shatter with disastrous results.

We also advise studying the appropriate literature before commencing with your solar observations.


Special tips for observing

The classification of spot groups according to the Waldmeier principle may be difficult in times of high activity, especially if many groups are grouped together or form complex structures. Some simple rules can be applied here:

  • Spots within an area of 5°x5° on the sun can be classified as a group if no bipolar structure is visible. Bipolar groups on the other hand can reach lengths of 20° and more.

  • Two single spots with a separation of up to 15° solar longitude can be classified as one group if they are the remains of a previous larger group.

  • A bipolar accumulation of spots can be classified as a group if the westerly part appears at the same or lower latitude than its easterly part. The inclination of the longitudinal axis of a group at or around +/- 10° solar latitude can amount to 1 - 2° whereby the inclination of a group at or around +/- 30° latitude can be up to 4°. [H. Künzel, Astronomie und Raumfahrt 14, 121 (1976)]

Please note:  A single spot is, contrary to the common counting methods, not (!) regarded as a group in the INTER-SOL Programme.


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