Güneş yüzeyindeki granülasyon
Güneş yüzeyindeki granülasyon
Dosyalar
Tarih
1997
Yazarlar
Kaya, Cennet
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
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.
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 ix size. 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
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 ix size. 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
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1997
Anahtar kelimeler
Granül,
Güneş,
Sun,
Granule