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The activity of the sun

The Sun doesn't raditate stable. In contrast there are a lot of changes which repeat with a period of about 11 years. The reason is the change of polarity of the solar magnetic field. This perid is called activity cycle, Sun spot cycle or Sun cycle.

Solare magnetic field and plasma

You could consider the Sun as a huge magnet which magnetic sphere of influence is called magnetic field. A magnet has a south- and northpol but at the Sun they are not constant at their position but changes their position after about 11 years. That change of polarity has a lot of consquences for surface phenomena.
On the other hand you could consider the Sun as a huge gas ball. Exacty it isn't gas but plasma, ionized gas. An ion is an atom which hasn't the same amount of electrons and protons.
The magnetic field and the plasma are in interaction with each another. That means motion of the plasma causes motion of the magnetic field and the other way round. The physical description is based on the magnetohydrodynamic. Information of the magnetic field and the plasma you can get in observations of surface phenomena.
Now we will desribe some of these objects.

Sun Spots

The two figures show Sun spots with umbra and penumbra. In the left figure you can also see the granulation.(Vacuum Tower telescope NSO, NASA, 2005). Right iamge Observatorium Kanzelhöhe October 26, 2014.

Sun spots are cooler and darker regions on the surface of the sun. They have about 2000 K less temperature than its surrounding, but nevertheless 4000 K. In most cases they consist of a dark and cool umbra and a surrounding penumbra. Its not rare that their area is bigger than size of earth. Since the invention of the telescope Sun spots are observed regularly, among other things also at the observatory Kanzelhöhe. Sun spots are the most outstanding phenomenon of solar activity. Because of that the activity cycle is also called Sun spot cycle. The amount of visible Sun spots determinates the Wolfs relativ Sun spot number:


In the equation f is the number of spots, g the number of spot groups and k is a constant. There are a lot of spots when the solar activity is high (maximum) and there are less when solar activity is low (minimum).
Sun spots evolve by magnetic fields. Magnetic fields prevent the ascent of hot plasma and so the over leying regions cool down.

photospheric plages

Plages are also more often when the solar activity is high. Plages are anregularly formed regions which look brighter than its surrounding. In most cases they are found near Sun spots but they exist bevor Sun spots appear and still exist when spots have disappered.

Furthermore the emitted radiation and the solar constant are higher when the solar activiy is higher. Perhaps that is astonishing because when we have a maximum there are also more cooler spots. But there are more plages too, which have bigger areas. In single cases also an interaction of the number of spots and the temperature on earth was found.

Plages and sunspots in the Chromosphere

With increasing solar activity more sunspots nad plages can be observed in the chromosphere. The sunspots are better visible in the photosphere, plages are more prominent in the chromosphere.


As a result of magnetic reconfigurations sudden energy releases (up to 3*1025J) can occur. The energy is released in a broad band of wavelength, reaching from X-Ray, ultraviolett up to radiowaves.

Sequence of images taken in the Hα spectral line on Apr. 11th, 2013, showing a so-called chromospheric flare. The field of view is ca. 290.000 x 290.000 km2.


CME is the abbreviation for Coronal Mass Ejection, they are often observed in combination with with flares, but both phenomena can also occur without the other one.

The chromosphere of the Sun with a plane passing, observed at the Observatory Kanzelhöhe in Hα on January, 15 2005. In the background a CME observed by LASCO C2 is superimposed (image editing W. Pötzi).


Prominences are phenomena in the solar corona, they are “magnetic tubes ” where solar plasma is trapped within. As it cannot interact with its surroundings it is cooler and denser. At the solar limb they are called prominences (bright in comparison to background) and on the solar disc they are called filaments (dark).

The figure shows a large prominence with the Earth as comprison. This prominence had an extend of about 35 Earth diameters (one third of the Sun). The image was observed by the Extreme-UV Imaging telescope (EIT), on board of the » SOHO satellite, on July 24, 1999 in Helium II - at 30.4 nm.

Coronal holes

When the Sun is observed in extremely short wavelengths, like UV or X-Ray, dark regions become visible, so called coronal holes. From these regions solar plasma can escape into space. These coronal holes have a lower density as their sourroundings and appear therefore darker. During solar minimum these coronal holes are more concentrated near the solar poles, during high solar activity they can be observed all over the Sun.

The image shows the Sun observed in X-Ray by the SDO satellite on June 3, 2012. The dark region is a coronal hole. (Copyright NASA)

Solar wind

The solar wind is a stream of electrical positive charged protons and negative chrged electrons comming from the Sun. CMEs are a form of solar wind. Solar wind has a velocity between 350 und 750 km/s. When the particles rreach the magnetic field of the Earth, they are deflected, nonetheless some particles reach the Earth's atmosphere and can induce radio disturbances or they become visible as auroras.

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