Güneş yüzeyindeki granülasyon
<thr granulation over the solar surface
- Tez No: 66515
- Danışmanlar: PROF. DR. GÜLÇİN KANDEMİR
- Tez Türü: Yüksek Lisans
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Granül, Güneş, Granule, Solar
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Fizik Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Granüller güneş yüzeyi olan fotosferin her tarafından ve her zaman gözlenirler. Güneş ve diğer yıldızlan anlamamız için granüllerin fiziksel yapısını oluşum mekanizmasını incelememiz ve matematiksel model yapabilmemiz gerekmektedir. Güneş yüzeyinde çokgen biçimli, parlak bölgeler olarak gözlenen hücresel yapılara granül denir. Bu parlak bölgeleri birbirinden ayıran koyu renkli dar bölgelere ise granüllerarası yol adı verilir. Granüllerin fotosferin hemen altındaki konveksiyon bölgesinde kaynaklandığı düşünülmektedir. Burada güneş merkezinden radyasyonla taşınan enerji yaklaşık 2x1 05 km kalınlığındaki konveksiyon bölgesine gelince maddenin gaz kabarcıkları biçiminde yükselmesine neden olur. Yukarı doğru genişleyerek yükselen gaz kütlesi halinde daha sonra soğuyarak parçalanır ve yeni granülleri oluşturur. Bu maddenin bir kısmı ise aşağı doğru inerek granüller arası bölgeyi oluşturur. Gözlemciye yaklaşma ve uzaklaşma durumuna göre maviye veya kırmızıya kayma biçiminde Doppler olayına neden olur. Güneşin merkez bölgesinden bize ulaşan ışıkta toplam etkisi spektrum çizgilerinde asimetri kaymaları olarak ortaya çıkar. Granüller konveksiyon bölgesi için oluşturdukları ölçeğe (boyutlar) göre; fotosferik granüller, mezogranüller, süpergranüller ve dev hücreler olmak üzere dört gruba ayrılırlar. Ayrıca güneş yüzeyindeki bulundukları konuma ve oluşum evresine göre de sınıflandırılır. Bu çalışmada granüllerle ilgili gözlemsel ve kuramsal bilgilerle ilgili yapılmış çizgi asimetri çalışmaları özetlenmektedir. Güneşi ve öteki yıldızlan anlayabilmemiz için granüllerin matematiksel modelinin yapılması gerekmektedir. Fakat bu konudaki gözlemsel kuramsal çalışmalar henüz yetersiz kalmaktadır. vuı
Özet (Çeviri)
SUMMARY THE GRANULATION OVER THE SOLAR SURFACE Our star, the sun, wears a mysterious gown: The upper layers, the chromosphere and the corona are visible only during the solar eclipses while the visible layer below them, the photosphere, looks bright and featureless at first glance. But a telescope reveals some features on this surface layer. From time to time this gown gets spotted, i.e., the surface layer, the photosphere has more or less sunspots during the 1 1 years' periods. The photospheric layer is woven from a fine fabric only visible through high- resolution telescopes and this fine fabric is made of granules. On the contrary to the more popular feature, the sunspots, the granules prevail the whole solar surface at all times. Therefore they may be more important than the sunspots in understanding the physical principles governing the stars. In this study, the observed properties of the solar granules such as their lifetimes, velocities, sizes, evolution and positions on the solar surface are reviewed. Theoretical aspects such as the convection zone where the granules are thought to originate and the simulations made to explain the solar granules are also reviewed. The granules display an ever-changing nature. The materials of these polygonal- shaped grains or granules are brighter and therefore, hotter than the plasma surrounding them. In between these granules, darker and therefore cooler lanes take place. These lanes are called the intergranular lanes. Tn section 1, the general structure of the sun is reviewed. Then, the generation mechanism of the granules have been discussed. A radiative zone surrounds the solar core where the radiation originates from the nuclear reactions. The kind of nuclear reaction in the solar core is the fusion of hydrogen into helium. The radiative zone is believed to be surrounded by a convection zone where the granules originate. This layer extends up to about 2 x 105 km below the solar surface according to the mixing length theory. Granules are thought to be the top layer features or bubbles of the convection zone. Due to the lack of direct observation, theoretical models have been constructed to explain the convection zone. Thermodynamical and magnetohydrodynamical equations are employed for this purpose. Considering ionization, the plasma properties of the sun should be taken into account. The observations indicate granulation at different scales. The normal granules are about 1 03 km in diameter (more precisely, they are between a few hundred km and 2X101 km), live for about 8 minutes and their plasma is transported at the velocity of 1-3 km/s. The next larger features are the mesogranules. They are 5-10xl03 km in ixsize. Their plasma is transported at slower velocities; i.e., at about 0. 1 km/s. These are at the size of exploding granules; therefore, their existence is sometimes questioned. The next larger structures are the supcrgranules. They are about 3x10 km in size, live for about 1 day and have a velocity of 0.5 km/s. Giant cells are the fourth largest in range. These are 3x10' km in size, live for about 1 year and access a velocity of 0.05 km/s. Their observations have been verified several times; but the existence of such giant features is hard to explain (SCHMELZ and BROWN, 1994). Circulation, due to hydrogen and helium, occurs in the above mentioned 3 types of granulation. The depths where hydrogen and helium are ionized are at the order of their sizes. Hydrogen is thought to be highly ionized at a depth of 1000 km. He becomes %90 singly ionized at depth of 5- 104 km and doubly ionized at a depth of 3xl04 km These depths correspond to the sizes of the granules, the mcsogranules and of the supergranules as seen figure 1. When the material at these depths gets ionized, it absorbs energy by ionization and excitation. This decreases the value of the adiabatic index, y. Therefore, the adiabatic temperature gradient gets lower and causes convection (PRIEST, 1984).. 3 x 10* km- Ûûö'.!'<MöOOOOOÖOOOCCöCCOÖCÖÖ02^^.i Supergranules Figurel. A comparison of the sizes of granules and supergranules (FOUKAL, 1990). Granules may also be classified according to their positions over the solar surface: The photospheric granules have a lifetime of 7-10 minutes. The facular granules live for 2 hours. Granules observed in the umbral regions live for 15-30 minutes (BRAY and LOUGHHEAD, 1967). According to their generation mechanisms, the granules may be categorized into three types: The active granules, the quiet granules and the declining granules. The intensity contrast of the solar granules are considered important since they represent the solar temperature fluctuations. The maximum relative contrast is given by,^ ^-V *max iminj ' V -Mnax ^i mmj Here, The Imax is the intensity in the bright sections of the granules while the Imin is the intensity in the dark intergranular lanes. The velocity field of the granules is hard to obtain from the granular photographs; because the lifetimes of the granules are too short for such an investigation. In order to construct a mathematical model of the granules, Alfven was the first to suggest a convection zone (ALFVEN, 1950). Mixing Length Formalism is used to calculate the depth of the convection zone. This theory is developed to configure turbulent convective motions such as the blobs or eddies observed on the solar surface. / = a Hp is the mixing length, where Hp the pressure scale height. The solar material contained in the granules are assumed to be a mixture of parcels which move vertically over the distance t (SCHÜSSLER 1992). On the top of the convection zone and just below it two edditional thin layers namely, the overshoot layers are considered. The depth of the overshoot layer is calculated as 2x10 km (STIX,1989). The mixing length formalism is also used in the prediction of granular velocities from the observed sizes of the granules. The turbulent element sizes are found proportional to the granular velocities (FOUKAL, 1 990). In the Doppler effect investigations, the granules manifest themselves with blue-shifts since they may be considered as hot columns of gas approaching us. But the intergranular lanes shift towards the red since they are falling back to the solar surface as they cool down. Due to the total effect of the granules and intergranular lanes, the asymmetry of the solar spectral lines is about 0.9 % in the center of the observed photosphere, but it drops to zero at the solar limb. Böhm was able to explain this with the three-stream model (BOHM,1952). A similar investigation using the nickel lines by Voight verified this interpretation (VOIGHT,1959). Investigation of the granules through the solar line asymmetries is an important tool in astronomy. Convection manifests itself by assymmetries and wavelength shifts. The assymmetry of the profile of a solar spectral line may be given in terms of its“bisector”. Bisector is a line breaking the profile in two parts, such that the equivalent widths of these parts are equal in size. Figure 1. illustrates how the granules cause the spectral line assymmetries and wavelength shifts. The bisector is also shown on a profile (DRAVINS, ET. AL, 1981). XIFigure 2: How the granules cause the spectral line assymmetries and wavelength shifts. Stathopoulou and Alissandrakis (1993) studied a large number of Fe I spectral lines, which were observed simultaneously. Using a model by Holweger and Miiller (1974), they computed the effective depth of line formation along the line profile and studied the variation of the asymmetry with the optical depth for each line. These calculations are made using the conventional bisector method. Kuli-Zade proposed a quantitative method for the determination of the differential and total asymmetries of weak and medium strength solar spectrum lines (KULI-ZADE, 1992). In addition to the method of assymmetries simulation is the other new investigation tool used to reveal the nature of the granules. These two new methods since Alfven's time, are used reveal times. Dravins (1982) and Nordlund (1982) made the most important simulations for granulation using the magnctohydrodynamic equations and the fluid approximations. Recently (1994), Kaya and Kandemir considered the plasma properties of the granules as well and applied a simulation to judge the micro instabilities in the convection layer and tried to display the known granular properties as a result of these microinstabilities. They used Maxwell and Vlasov equations in a one dimensional electrostatic plasma code, ESI (BIRDSALL and LANGDON, 1985). Electrostatic instability occurs as like charges bunch up at same position. The resulting instability is the fastest growing and the most destructive type among all plasma instabilities (KRALL and TRIEVELPJECE, 1973). Since ionization is important at the depths where the granules form, such strong instability should not be neglected. Two-stream instability arises where two plasma streams flow along opposite directions. The bunching causes strong electric fields and acceleration of the charges. Thus, a density perturbation starts and leads to exponentially growing forces. As the thermal velocity, vr grows, the drift velocity, vD reduces and the growth ceases. It has been shown that as vr approaches v01.3, the exponential growth ceases in a cold plasma (STRINGER, 1964). But, the growth may continue although it will not be exponential anymore. XIIThe number density of particles in the solar atmosphere is 10 (KRALL and TRIEVELPIECE, 1973). Using this value, the thermal velocity, the drill velocity and the initial perturbation arc calculated. The temperature is taken approximately as 104 K where the thermal velocity for the electrons become vT- 5x 109 m/s. the drift velocity vn=7xl09 m/s and the perturbation in velocity arc 2vT=7x 10' m/s. These values arc approximately 40 times lower for the protons. In the simulation the electron streams arc taken over a static background. The drift velocity lias increased up to T 300km/s in the phase space. The non-linear effect of the grouping electrons appeared as vortices in the phase space. Since ^/m, /m8 =42.8, the drift velocity is slower for the protons and much slower for heavier ions. These simulation results agree with the observational values of a few km/s. The Japanese satellite Yohkoh has been launched in 1992 and revealed the existence of rigidly rotating dark coronal channels while the photosphcric features such as the sunspots arc known to rotate differentially. The new solar-terrestrial satellites scheduled for 1997 may clarify our views on the granules. The normal granules belong to the differentially rotating sun; but we arc not sure about the giant cells. In this study, the granules that arc most ordinary and ever prevailing feature on the sun, arc reviewed. Our understanding of such a vital solar feature has been found surprisingly insufficient and more serious emphasize on the subject should be encouraged. XIII
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