Solar Activity

Sunspots. The clearest visible sign of solar activity are the sunspots. The existence of sunspots has been known for long (Fig. 12.9), since the largest ones can be seen with the naked eye by looking at the Sun through a suitably dense layer of fog. More precise observations became available beginning in the 17th century, when Galilei started to use the telescope for astronomical observations.

A sunspot looks like a ragged hole in the solar surface. In the interior of the spot there is a dark umbra and around it, a less dark penumbra. By looking at spots near the edge of the solar disc, it can be seen that the spots are slightly depressed with respect to the rest of the surface. The surface temperature in a sunspot is about 1500 K below that of its surroundings, which explains the dark colour of the spots.

The diameter of a typical sunspot is about 10,000 km and its lifetime is from a few days to several months, depending on its size. The larger spots are more likely to be long-lived. Sunspots often occur in pairs or in larger groups. By following the motions of the spots, the period of rotation of the Sun can be determined.

The variations in the number of sunspots have been followed for almost 250 years. The frequency of spots is described by the Zürich sunspot number Z:

where S is the number of spots and G the number of spot groups visible at a particular time. C is a constant depending on the observer and the conditions of observation.

In Fig. 12.10, the variations in the Zürich sunspot number between the 18 th century and the present are shown. Evidently the number of spots varies with an average period of about 11 years. The actual period may be between 7 and 17 years. In the past decades, it has been about 10.5 years. Usually the activity rises to its maximum in about 3-4 years, and then falls off slightly more slowly. The period was first noted by Samuel Heinrich Schwabe in 1843.

The variations in the number of sunspots have been fairly regular since the beginning of the 18th century. However, in the 17 th century there were long intervals when there were essentially no spots at all. This quiescent period is called the Maunder minimum. The similar Spörer minimum occurred in the 15th century, and other quiet intervals have been inferred at earlier epochs. The mechanism behind these irregular variations in solar activity is not yet understood.

The magnetic fields in sunspots are measured on the basis of the Zeeman effect, and may be as large as 0.45 tesla. (The magnetic field of the Earth is 0.03 mT.) The strong magnetic field inhibits convective energy transport, which explains the lower temperature of the spots.

Sunspots often occur in pairs where the components have opposite polarity. The structure of such bipolar groups can be understood if the field rises into a loop above the solar surface, connecting the components of the pair. If gas is streaming along such a loop, it becomes visible as a loop prominence (Fig. 12.11).

The periodic variation in the number of sunspots reflects a variation in the general solar magnetic field. At the beginning of a new activity cycle spots first begin to appear at latitudes of about ±40°. As the cycle advances, the spots move closer to the equator. The characteristic pattern in which spots appear, shown in

Fig. 12.9. The sunspots are the form of solar activity that has been known for the longest time. The photograph was taken with the Swedish 1-meter Solar Telescope in July 2002. (Photograph Royal Swedish Academy of Sciences)
Fig. 12.10. The Zürich sunspot number from 1700 to 2001. number of sunspots and spot groups varies with a period of Prior to 1700 there are only occasional observations. The about 11 years

Fig. 12.11. In pairs of sunspots the magnetic field lines form a loop outside the solar surface. Material streaming along the field lines may form loop prominences. Loops of different size can be seen in this image, which the Trace satellite took in 1999. (Photo Trace)

Fig. 12.11. In pairs of sunspots the magnetic field lines form a loop outside the solar surface. Material streaming along the field lines may form loop prominences. Loops of different size can be seen in this image, which the Trace satellite took in 1999. (Photo Trace)

Fig. 12.12, is known as the butterfly diagram. Spots of the next cycle begin to appear while those of the old one are still present near the equator. Spots belonging to the new cycle have a polarity opposite to that of the old ones. (Spots in opposite hemispheres also have opposite polarity.) Since the field is thus reversed between

Fig. 12.12. At the beginning of an activity cycle, sunspots appear at high latitudes. As the cycle advances the spots move towards the equator. (Diagram by H. Virtanen, based on Greenwich Observatory observations)

Fig. 12.12. At the beginning of an activity cycle, sunspots appear at high latitudes. As the cycle advances the spots move towards the equator. (Diagram by H. Virtanen, based on Greenwich Observatory observations)

Fig. 12.13. Because the Sun rotates faster at the equator than at the poles, the field lines of the solar magnetic field are drawn out into a tight spiral

consecutive 11 year cycles the complete period of solar magnetic activity is 22 years.

The following general qualitative description of the mechanism of the solar cycle was proposed by Horace W. Babcock. Starting at a solar minimum, the field will be of a generally dipolar character. Because a conducting medium, such as the outer layers of the Sun, cannot move across the field lines, these will be frozen into the plasma and carried along by it. Thus the differential rotation will draw the field into a tight spiral (Fig. 12.13). In the process the field becomes

Solar Magnetic Period
Fig. 12.14. A quiet Sun in August 2006 around the last sunspot minimum. Both pictures were taken by the Michelson Doppler Imager on the SOHO satellite. On the left the Sun in visible

stronger, and this amplification will be a function of latitude.

When the subsurface field becomes strong enough, it gives rise to a "magnetic buoyancy" that lifts ropes of magnetic flux above the surface. This happens first at a latitude about 40°, and later at lower latitudes. These protruding flux ropes expand into loops forming bipolar groups of spots. As the loops continue expanding they make contact with the general dipolar field, which still remains in the polar regions. This leads to a rapid reconnection of the field lines neutralising the general

light, on the right a magnetogram, which shows the opposite polarities of the magnetic fields as black and white. (Photo SOHO/NASA/ESA)

12. The Sun

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Fig. 12.15. (a) Quiescent "hedgerow" prominence (Photograph Sacramento Peak Observatory). (b) Larger eruptive prominence (Photograph Big Bear Solar Observatory)

field. The final result when activity subsides is a dipolar field with a polarity opposite the initial one.

Thus the Babcock model accounts for the butterfly diagram, the formation of bipolar magnetic regions and the general field reversal between activity maxima. Nevertheless, it remains an essentially phenomenological model, and alternative scenarios have been proposed. In dynamo theory quantitative models for the origin of magnetic fields in the Sun and other celestial bodies are studied. In these models the field is produced by convection and differential rotation of the gas. A completely satisfactory dynamo model for the solar magnetic cycle has not yet been found. For example, it is not yet known whether the field is produced everywhere in the convection zone, or just in the boundary layer between the convective and radiative regions, as some indications suggest.

Other Activity. The Sun shows several other types of surface activity: faculae and plages; prominences; flares.

The faculae and plages are local bright regions in the photosphere and chromosphere, respectively. Observations of the plages are made in the hydrogen Ha or the calcium K lines (Fig. 12.14). The plages usually occur where new sunspots are forming, and disappear

Fig. 12.16. A violent flare near some small sunspots. (Photograph Sacramento Peak Observatory)

when the spots disappear. Apparently they are caused by the enhanced heating of the chromosphere in strong magnetic fields.

The prominences are among the most spectacular solar phenomena. They are glowing gas masses in the corona, easily observed near the edge of the Sun. There are several types of prominences (Fig. 12.15): the quiescent prominences, where the gas is slowly sinking along the magnetic field lines; loop prominences, connected with magnetic field loops in sunspots; and the rarer eruptive prominences, where gas is violently thrown outwards.

The temperature of prominences is about 10,000-20,000 K. In Ha photographs of the chromosphere, the prominences appear as dark filaments against the solar surface.

The flare outbursts are among the most violent forms of solar activity (Fig. 12.16). They appear as bright flashes, lasting from one second to just under an hour. In the flares a large amount of energy stored in the magnetic field is suddenly released. The detailed mechanism is not yet known.

Flares can be observed at all wavelengths. The hard X-ray emission of the Sun may increase hundredfold during a flare. Several different types of flares are observed at radio wavelengths (Fig. 12.17). The emission of solar cosmic ray particles also rises.

The flares give rise to disturbances on the Earth. The X-rays cause changes in the ionosphere, which affect short-wave radio communications. The flare particles give rise to strong auroras when they enter the Earth's magnetic field a few days after the outburst.

Solar Radio Emission. The Sun is the strongest radio source in the sky and has been observed since the 1940's. In contrast to optical emission the radio picture of the Sun shows a strong limb brightening. This is because the radio radiation comes from the upper layers of the atmosphere. Since the propagation of radio waves is obstructed by free electrons, the high electron density near the surface prevents radio radiation from getting out. Shorter wavelengths can propagate more easily, and thus millimetre-wavelength observations give a picture of deeper layers in the atmosphere, whereas the long wavelengths show the upper layers. (The 10 cm emission originates in the upper layers of the chromosphere and the 1 m emission, in the corona.)

Fig. 12.17. An X-ray picture of the active Sun, taken by the Japanese Yohkoh satellite in 1999, around the last maximum of sunspot activity. (Photo JAXA)

The Sun looks different at different wavelengths. At long wavelengths the radiation is coming from the largest area, and its electron temperature is about 106 K, since it originates in the corona.

The radio emission of the Sun is constantly changing according to solar activity. During large storms the total emission may be 100,000 times higher than normal.

X-ray and UV Radiation. The X-ray emission of the Sun is also related to active regions. Signs of activity are bright X-ray regions and smaller X-ray bright points, which last for around ten hours. The inner solar corona also emits X-rays. Near the solar poles there are coronal holes, where the X-ray emission is weak.

Ultraviolet pictures of the solar surface show it as much more irregular than it appears in visible light. Most of the surface does not emit much UV radiation, but there are large active regions that are very bright in the ultraviolet.

Several satellites have made observations of the Sun at UV and X-ray wavelengths, for example Soho (Solar and Heliospheric Observatory, 1995—). These observations have made possible detailed studies of the outer layers of the Sun. Observations of other stars have re-

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Fig. 12.18. The SOHO (Solar and Heliospheric Observatory) satellite keeps a constant watch on the Sun and its surroundings in many wavelengths. Here the LASCO (Large Angle and Spectrometric Coronagraph) instrument sees a large Coronal Mass Ejection erupting from the Sun. The surface of the Sun is covered by a disk, and the size and position of the Sun is indicated by the white circle. (Photo SOHO/NASA/ESA)

Fig. 12.18. The SOHO (Solar and Heliospheric Observatory) satellite keeps a constant watch on the Sun and its surroundings in many wavelengths. Here the LASCO (Large Angle and Spectrometric Coronagraph) instrument sees a large Coronal Mass Ejection erupting from the Sun. The surface of the Sun is covered by a disk, and the size and position of the Sun is indicated by the white circle. (Photo SOHO/NASA/ESA)

vealed coronae, chromospheres and magnetic variations similar to those in the Sun. Thus the new observational techniques have brought the physics of the Sun and the stars nearer to each other.

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