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INTERNATIONAL YEAR OF ASTRONOMY Article THE TURBULENT SUN Pankaj Agarwal JIL Information Technology Ltd., Sahibabad Industrial Area, Sahibabad-201010 (U.P.) (Photographs on inside back cover) ABSTRACT Sun is not quiet. The region on the solar atmosphere, where excess of magnetic flux is occurring, is referred as active regions. Sunspots in the photosphere; prominences, plages and flares in the chromosphere tend to occur together and collectively referred as active regions. All of these active regions may grow, change their shapes and disappear in hours or days or weeks or months. This article highlights some features of the active regions of solar atmosphere. 1. Introduction The sun is an average middle age star. The sun is very important for all living beings. Life would not have sustained on Earth without the existence of sun. Without its heat all oceans would be frozen and without its light all plants would die. Being a nearest star, sun is always an object of study and can be used as a model to study the other stars. Sun is not quiet; it has lots of turbulence in its atmosphere. Solar atmosphere is made up of three main layers, namely the photosphere, the chromosphere and the corona. Photosphere The only visible layer of the sun is called Photosphere, which looks like sphere of light (Fig. 1a). This is that layer of the sun from where the light energy escapes into the space. The photosphere is about 400 km deep. Temperature in the photosphere is about 5600 K, and the gas densities are quite small. Hence, the gaseous atoms are unable to block the radiative energy flow. As the hot atoms cool, they release their excess energy as radiation, which streams out unrestrictedly into the space. Chromosphere The next layer to photosphere is the Chromosphere, which looks as a coloured sphere (Fig. 1a). It is only momentarily visible during a total solar eclipse as a reddish-pink strip, which encircles the silhouette of the moon, when the moon blocks the light from the Photosphere. It is very thick layer, roughly about 2500 km, and its temperature rises to IAPT Bulletin, August 2009 30,000K (Fig. 1b). Visually, the chromosphere is more transparent than the photosphere. Corona Most mysterious layer of the sun is its upper part of the atmosphere, which is known as Corona. Similar to the chromosphere, it is only visible during a total solar eclipse as a luminous white halo surrounding the glowing disk of the sun (Fig. 1a). The corona glows during a total solar eclipse because light from the photosphere bounces off free electrons in the coronal plasma. The shape of the corona is synchronized with the solar activity cycle; its shape is just like a serrated ring around the sun, during the peak of the solar cycle, whereas it is like delicate plumes, during the dip of the solar cycle. The temperature of this layer rises up to 1.7 x 106 K that makes the corona the second hottest part of the sun, after the central core (Fig. 1b). Solar corona is extending millions of kilometers into space. An interesting feature of the corona is the fact that it is much hotter than the photosphere. The corona is much less dense than the photosphere and hence produces less light. The possible mechanism for the coronal heating is the induction by the suns magnetic field and sonic pressure waves from below. 2. Sources of solar activity The core of the sun is the only part of the sun that generates energy. Though the solar energy from the core is radiated all around uniformly, but from certain areas, such as sunspots surrounded by plages and prominences, energy SILVER JUBILEE YEAR 2009 196 is not released uniformly. Changes of different descriptions follow each other to make an endless series, are referred as solar activates. Steady state and spherical symmetry of the sun are continuously disturbed because of the solar magnetic field and nonuniform solar rotation. The sun is a gigantic ball of superheated gas; therefore it does not rotate like a rigid body. The sun rotates faster at equator than the poles, similarly its interior rotates faster than the surface. Different solar rotations twist the magnetic field lines, running from pole to pole and frozen into the surface, i.e. the magnetic field lines and plasma behave as if they are frozen together. Due to the shearing action on the field, the density of magnetic field lines increase, which is supposed to be the reason for solar activity. 3. Active regions The region on the solar atmosphere, where excess of magnetic flux is occurring, is referred as active regions. Heating of local atmosphere exist due to this magnetic flux. Sunspots in the photosphere; prominences, plages and flares in the chromosphere tend to occur together and collectively referred as active regions. In the formation of these active regions magnetic disturbance plays an important role. All of these active regions may grow, change their shapes and disappear in hours or days or weeks or months. It has been assumed that these all phenomena are due to the sunspots. Sunspots Earlier, sunspots were considered as planetary phenomena. In 1610, Galileo Galilei, Johann Fabricius and Christopher Scheiner independently observed dark spots on the face of the sun and explained sunspots as clouds floating in the solar atmosphere. Sunspots appear as dark blemishes on the solar disk, big spots can be visible to the naked eye with the help of goggles. Galileo observed the spots regularly and found that the spots appeared to move forward across the solar disk, which suggested him that the spots were a part of the sun and these appeared to move forward due to the rotation of the sun. The number of observed sunspots is treated as a measure of solar activity. Each sunspot has two regions umbra, i.e. dark central region, and penumbra, i.e. less dark and filamentary region, surrounding the umbra around the edges (Fig.2). The diameter of penumbra is about 2.5 times of umbra. High magnetic pressure and less temperature of the gas, in the umbra region, decrease the density of the gas and IAPT Bulletin, August 2009 make the sunspots more transparent than the photosphere. Therefore we can see to a large depth in a sunspot, as there is a hole in the umbra region, when observing near the limb. Sunspots are very bright, but they appear dark due to the high temperature of surrounding bright solar disk. Sunspots are cooler than photosphere as they get less energy from the solar interior than that of photosphere. The photosphere gets energy by convection, but in the sunspots there is no convection to provide sufficient energy, because the sunspots are strongly magnetized. Generally sunspots occur in groups and the total number of sunspots varies from no spot condition to about 400 spots. Single sunspot may appear very tiny and then grow and split. Day to day photographs of the small sunspots show many changes in them. Previously, a sunspot observed as a tiny dark spot like a small sunspots umbra, i.e. without penumbra, which also known as Pore. Some of the pores become darker and larger, but without any distinguishable internal features, and other spots split into the sunspot groups which contain some simple spots but with the time they increase in size, become more complex and split into more spots and then dissolve and disappear after some weeks. In an average, lifetimes of sunspots are of one day, only the extraordinary big one lives few months. Observations show that the sunspot groups start from eastern limb of solar disc move westward and finally vanish at the western limb. Pair of sunspots can be considered as gaseous horseshoe electromagnet, where hot solar gases act as electrical conductors and carry a huge amount of electrical current of about trillion amperes. We can consider the conducting gases as a wire coil, which ends at a pair of spots. Magnetic field in the umbra region is vertical, while in the penumbra region it is approximately horizontal. Plages Sunspots are surrounded by bright clouds, which appear before them. These bright clouds are known as chromosphere faculae or Plages, i.e. beaches. Presence of plages is also a symbol for solar magnetic activities and used to define the degree of active region. Plages appear as a brighter area than the surrounding chromosphere (Fig.3). Plages are the regions of gas of higher density than the surrounding atmosphere, which are covered by hot corona and glow with heat. Sunspots are darker and plages are brighter than their surrounding mediums, because sunspots have stronger magnetic field SILVER JUBILEE YEAR 2009 197 while plages do not, in other words plages get large convection energy from the solar interior. Therefore the plage fields increase the relatively smaller flow of mechanical energy, which originates as noise in the convection zone and dissipate in the solar atmosphere. Solar flares After the sunspots, another significant and most dramatic phenomenon of solar activity is solar flares, which can be defined as a sudden, rapid, and intense variation in brightness, which influence the earth strongly. On Sept.1, 1859, Richard Carrington and Richard Hodgson, were observing sunspots, independently, and they suddenly viewed a pair of brilliant white ribbons flash, only for 15 minutes, across the umbra and mentioned that as solar flares. Solar flare is the final major out-burst of solar activity and has importance in solar physics and interplanetary physics. Solar flares release a huge amount of plasma and radiate it in a very short interval of time because of annihilation of magnetic energy, which is very high near the sunspots, into the kinetic energy. Flares are highly concentrated release of energy in the solar corona and observed as a small, star-like bright spots emerging suddenly within the plages area, i.e. near a large sunspot or a group of sunspots (Fig. 4). These bright spots expand explosively for few minutes and cool down quickly. Brightening of such spots rapidly spreads along preferred direction and converse a large area and gradually decays in about 1/3 to 3 hours. Generally, flares are very small in comparison with all other structures of an active region. Flares can be classified on the basis of their brightness, area, and the intensity. Large flares or great flares are of much importance as they cover most of the active region with lifetime of very few hours. These great flares extend into ribbons, which bend along the boundaries of opposite polarity regions and light up like a flash only for one or two minutes. This duration is referred as flash phase or impulsive phase. After this flash phase, a great flare may expand and some times covers up one percent area of the solar surface within few minutes and then the flare shrinks gradually and finally disappears within few hours. This phase is referred as gradual phase. Due to the rapid changes and disappearance of the flares within few minutes, observation of flare is a big problem. Prominences During the peak of solar activity, most of the plasma enters the corona through chromosphere, this ejected plasma IAPT Bulletin, August 2009 condenses into the regions of higher density and lower temperature, and flow gradually downward towards the chromosphere along the magnetic field lines, in a very graceful shape and this activity is referred as Prominence and appears as dark filaments on the solar disc (Fig.4). This high-density material is supported against the solar gravitational attraction by the magnetic field lines, where these lines are parallel to the surface. Prominences can be classified as highly dense and cool gas suspended above the surface, in the corona. It is considered that the different linear and filamentary structures of the prominences have strong electric current running within their gases. The temperature of the prominences is less than the chromosphere and the shape of prominences indicates that they are well insulated against the surrounding heat and coronal radiation. Reason for this insulation may be due to magnetic and electric field within the prominences. Prominences have a large variety of shapes, sizes, and life times due to the magnetic field structures and the sources of condensed matter. Types of prominences in which upward motion take place are spicules, surges, sprays and eruptive filaments (all are short-lived with lifetime of few minutes to few hours). Types of prominences in which downward motion take place are coronal rains, loop prominences (both are short-lived with life time of about an hour or so) and long-lived filaments. Long-lived prominences, also known as quiescent prominences, are important for the coronal structure. These quiescent prominences are gradually changing prominences and always seen away from active regions with sunspots. The lifetimes of such type of prominences are up to 6 to 10 months and remain after the disappearing of other activities of an active region. Coronal holes In x-ray photographs, distinct regions of low x-ray emission are referred as coronal holes (Fig. 5). One of the largest coronal holes, extended from north to south is known as coronal hole number one, which looks like the map of Italy. Coronal holes are visible in the regions without magnetic field. Because of the presence of very low-density gases in coronal holes there is lack of radiation and this may be the reason for their dark appearance. It has been suggested that the magnetic field lines are open over coronal holes, which emerge from photosphere to somewhere in interplanetary space, while magnetic field lines are closed over active regions e.g. loops. Open field lines of coronal SILVER JUBILEE YEAR 2009 198 holes permit the gases to flow from the sun towards the interplanetary space. Hence, the coronal holes are empty. Solar wind Resulting out going gas, from the coronal holes is referred as solar wind. Solar wind arrived at earth at least three or four days later the coronal hole faced the earth because of the wind transits from the sun to the earth at least in three or four days. 4. Why to study the sun Study the sun is important, as it drives the weather on the Earth. Any variations of the suns output affect the Earths climate. The winds and circulation of ocean patterns are all affected by the suns energy output. The differential heating of the Earth generates the winds and major ocean currents as well as causing different seasons. However, the solar cycle has a deep impact on our climate. Studies also show a close correlation between Earths ice ages and the solar activities. In our technology-based society we depend extensively on satellites and various forms of high frequency communication systems. We know that most of the solar activities are driven from the dominant magnetic force in the solar atmosphere. In contrast to the dense plasmas in the suns lower regions, the plasmas of the solar atmosphere are very dilute and unable to restrain the gigantic magnetic field of the sun, which may result in geomagnetic storms. Geomagnetic storms have a direct effect on the Earths atmosphere, impacts satellites, perturbing their orbits, scoring their surfaces, and disrupting communication system. Geomagnetic storms consist of very high-energy protons, which ionize the Earths upper atmosphere and expand the ionosphere and disrupt long distance radio signals. Satellites orbit around the Earth can be disturbed by the enhanced drag on the satellites from the expanded atmosphere. Energetic protons escape into interplanetary space are dangerous for the electronic instruments in space as well as for the electronic components of satellites. Furthermore, after one or two days, the Earth is slammed with a magnetic shock wave travelling at more than 1000 kilometers per second that may totally disrupt power grids everywhere on Earth. and b) studies of the inside of the sun. Studies of the solar interior reveal for the motions and thickness of the various internal zones as predicted by various models of stellar interiors such as the nuclear core, the radiative zone, and the convective zone. The interface between the radiative and convective zones is just like a shell where the sun generates the magnetic fields ultimately seen on the surface in sunspots and other structures associated with the 11 year solar cycle. Therefore, understanding of the interior of the sun is important to understand solar activities due to the effects of intense magnetic fields. Studies of solar atmosphere also provide an opportunity to study some natural physical processes, e.g., study of solar flares is similar to the study of controlled thermonuclear fusion in a laboratory. Still, there are mysteries about the structure of the sun. There are so many unsolved problems about the sun, as how the suns surface and corona interact, how energy is transported from the solar surface into the outer atmosphere? Author is thankful to the learned referee for his invaluable suggestions to improve the manuscript. He is also thankful to Dr. Rakesh Sharma for his help in modifying the manuscript. However all images are edited but by courtesy of NASA. BIBLIOGRAPHY 1. Athey R.G., The Solar Chromosphere and Corona: Quiet Sun, D. Reidel Publishing Company, Dordrecht, 1976. 2. Dwivedi B.N., Current Science, 75, 10, 1006-1011, 1998. 3. Lang K.R., The Cambridge Encyclopedia of the Sun, Cambridge University Press, Cambridge, 2001. 4. Stix M., The Sun: An Introduction, Springer-Verlag, Berlin, 1991. 5. Sturrock P.A., Holzer T.E., Mihalas D.M., Ulrich R.K. (eds), Physics of the Sun, D. Reidel Publishing Company, Dordrecht, 1986. 6. Taylor R.J., The Sun as a Star, Cambridge University Press, Cambridge, 1996. 7. Wentzel D., The Restless Sun, Smithsonian Institute Press, Washington D.C., 1989. 8. Zirin H., Astrophysics of the Sun, Cambridge University Press, Cambridge, 1988. 5. Areas of solar research Main thrust areas of research in solar physics are: a) studies of the outer solar atmosphere and its variation, IAPT Bulletin, August 2009 SILVER JUBILEE YEAR 2009 199 PHOTOGRAPHS ACCOMPANING THE ARTICLE THE TURBULENT SUN Fig. 1. (a) Layers of solar atmosphere, where white halo surrounding the photosphere is the corona, (b) Variation in solar temperature above the photosphere. Fig. 2. Sunspots, along with illustrated magnetic lines for a pair of sunspots, in left side box, and zoomed view of a large sunspot, in right side box, in which central dark region is umbra and surrounding less dark region is penumbra. Fig. 3. CaK image of the sun shows the filaments and plages on the sun. Fig. 4. X-ray image shows the prominences and flares on the sun, whereas in side boxes zoomed views of the two features have also been shown. Fig. 5. X-ray image of the sun shows the coronal holes along with the illustrated magnetic lines for coronal holes, in side box. IAPT Bulletin, August 2009 SILVER JUBILEE YEAR 2009 219