Physics 335: Astronomy
Instructor : M. Azad Islam
WEB SITE: NASA Solar Obseravatory
7-1 The Solar Atmosphere
Below the surface
The photosphere
The chromosphere
The corona
7-2 Solar Activity
The magnetic cycle
Prominences and flares
Coronal activity
This chapter talks mostly about the structure and phenomena of the observable layers of the sun.
Atmosphere of the Sun consists of three layers, photosphere, chromosphere and corona.
Photosphere: Visible surface of the sun is a thin layer 500 km of low-density gas at 6000 K from which visible photons most easily escape. This thin layer consists of gas currents produced from below its surface.
Chromosphere: Easily visible during total solar eclipse. It is a thin hot later of gas just above the photosphere. Its color is brilliantly pink made by emission lines from gas atoms. There are large jets called spicules extending upto the corona.
Corona: It is the outer layer of Sun's atmosphere. It is made up of very low density hot gas extending at least 30 times the radius of the sun measured from the center. Its high temperature is about 2 million degrees.
The outer parts of the Corona merge with solar wind, a breeze of low density ionized gas streaming away from the Sun. The earth is frequently hit by the Solar wind.
Sunspots that are relatively darker (because of cooler temperature) circular patches of areas on the sun. Sunspots are about twice the size of the earth and contain 1000 times stronger magnetic field that its average value. Solar activity are related to the sunspots that may pop up at any time. Solar prominences and flares are common events as part of the solar activity. These produce auroras and communication blackouts on earth.

Sun is 109 times larger in diameter than the earth. It has 300,000 times more mass than the earth. Observation of the sun is restricted to its atmosphere only.
The surface phenomena are directly related to what goes on in the depth of the sun. The interior of the sun is undergoing the Nuclear Fusion reactions that produce huge amount of energy flowing out to the surface and beyond. Evidences of energy flow from the interior are,
the presence of hot and cool regions, gas motions, magnetic fields and solar flares.
It is the visible surface of the sun. Sun is gaseous from its outward atmosphere right down to its center. Most of the sunlight we receive comes from the thin layer (500 km thick) of solar gas at a temperature of about 6000 K, called Photosphere.
Below Photosphere the solar gas is very dense so that radiations coming from it cannot escape outward. Such light from under the Photosphere cannot be detected here on earth.
The gas on top of the Photosphere is much less dense and is unable to radiate much light.
The thin layer of Photosphere is just dense enough (about 3000 times denser than air) to emit plenty of light. Density of gas comparable to the air can be found below Photosphere, at a distance of 10% of the way towards the center of the sun.
The higher density gases in the deeper layers of Photosphere produce continuous spectrum like liquids and solids. The lower density gases in the upper layer of the Photosphere produce dark Absorption lines in the background of the continuous spectrum.
Photosphere is not uniformly homogeneous, contains cell like structures called Granulation, about the size of Texas having lifetime of 10-20 minutes. Such a Granule is a few hundred degrees hotter in the middle than its edge. The center of Granule rises and its edge sinks at a speed of 1 km/sec, providing a Convection of gas current in its structure like the movement of warm masses of air in the earth's atmosphere and of water in the earth's oceans
The layer (about 10,000 km thick) on top of the Photosphere is called CHROMOSPHERE. It is 1000 times fainter than the Photosphere. Hence, it can be observed only during the total Solar Eclipse. It is a sheer luck that the moon can exactly cover up the sun upto the Photosphere during total eclipse for us to observe it for a few seconds. When viewed, it is a thin line of pink just above the Photosphere. The pink spectrum tells about the conditions in the Chromosphere. It is an emission spectrum of low density gas. The pink color results from the hue of red, blue and violet mixture of chiefly Balmer lines emitted by hydrogen, plus emission of other elements.
NOTE THAT THE TEMPERATURE OF THE CHROMOSPHERE IS AT 10,000 K TO 50,000 K, much higher than Photosphere.
Gases in the Chromosphere is mostly transparent to the visible light. Even so, the hydrogen atoms in it are very good absorbers of Balmer lines. A tiny fraction of Balmer lines that can escape from the upper atmosphere of Chromosphere, is detected as Filtergrams recorded through the light filters at specific wavelengths. The Filtergrams reveal flame like structure ( 100-1000 km diameter) called Spicules, extending upto 10,000 km above the Photosphere and lasting for 5-15 minutes. These Spicules appear to be cooler region of the Chromosphere at 10,000 K, seen as flames of burning prairie here on earth. Near center of the solar disk, these Spicules spring up around the edge of much large Granule structure of the Photosphere, called Supergranules.
CORONA OR THE CROWN: Above the Chromosphere is the Corona, the name originated from the Greek word meaning crown. During total solar eclipse, the corona shines as milky glow (fainter than full moon) on top of the Chromosphere. Corona is observed to be extended upto 12 solar radii. Gas in the corona is extremely hot at 1 million degrees K. Temperature in the Corona rises with altitude. Density of Corona is very low (only 1-10 atoms per cm cube) or else it will produce black body radiation. The extreme high temperature of the Chromosphere and Corona is not well understood by laws of physics. It is one of the mysteries to be revealed in the nature. The outer Corona is so hot, that the sun cannot hold it in place. The high velocity particles (mostly protons and electrons) in it, travel out as Solar Wind. The Solar wind at 300-800 km/sec, gusting at 1000 km/sec, travels past the earth disrupting the communications and at times shut down the power supply.
SOLAR ACTIVITY: Changes in sun's features over time due to magnetic fields and powerful flow of energy
Sunspots: Dark areas on the photosphere. Rotation of the sun moves the sunspots. Sunspots consists of dark center umbra and outer border penumbra. Its size is 40 sec of arc, about twice the diameter of the earth. Usually appear in pairs or groups. Large group may have 100 sunspots lasting about two months when in its peak level of activity. it looks dark, because it is cooler than rest of the photosphere. Actual color would e brilliantly orange at temperature of 4240 K. Large difference in brightness is due to fourth power dependence on temperature.
Zeeman in the solar spectrum provides quite a bit of information about the sunspot. Sunspot being cooler, do not emit much of hydrogen Balmer lines. Instead, heavier elements requiring less energy produce solar spectrum in sunspot. Zeeman effect produce splitting of spectral lines into three or more lines due to the presence of large magnetic field. From studies of splitted spectral lines, astronomers can infer about the strength of the magnetic field. Magnetic in sunspot is 1000 times stronger than other regions of the photosphere. The presence of strong magnetic field blocks any gas motion below the sunspot.
Sunspot activity goes in 11 year cycle. The sunspots appear near plus/minus 35 degrees latitude in the beginning of a sunspot cycle and appear near the equator in the later period of the cycle. The structure of its first appearance (between plus/minus 35 to 5 degrees) over the 11 year cycle when plotted in a graph, gives the resemblance of butterfly structure. Hence, it is named Maunder Butterfly Diagrams. It also goes through years of low activity called Maunder minimum.
Magnetic Cycle: Rotation of the sun near the equator is faster than near the pole. The period of rotation is 25 days at the equator and 27.8 days near mid latitudes. The period of rotation also decreases as we move into the interior of the solar surface. Such variations in time period at the surface and in the interior of the sun, is called Differential Rotation. This differential rotation results in changes in magnetic fields over and above the surface. Magnetic fields are produced by the rotation of the sun on its axis and by the flow of highly ionized (plasma) gas (carrying huge amount of energy) from the interior to the solar surface. This mechanism of producing magnetic field is referred to as Dynamo Effect. The earth is believed to produce its magnetic field in the same manner.
Evidence from the Zeeman Effect measurements show that sun's magnetic fields also go through a cycle of time period. Sunspots occur in pair so that the magnetic field may have north and south poles such as a bar magnet. A pair of sunspot would appear as a leading and a trailing dots. In the northern part, the leading one may become north pole and the trailing one be a south pole. This polarity may be reversed in the south of the sun's equator. Overall, the polarity of the sunspots reverse from cycle to next cycle of the sun spot.
Babcock model attempts to explain sun's magnetic cycle. This model considers progressive variation of the tangling of the magnetic fields to be the cause and effect reversing of the polarity. In this model, the highly mobile electrons are trapped (frozen) by magnetic fields. As the frozen electron gas moves, the magnetic fields move tangling with each other. When the fields are very strong, they create pair or groups of sunspots. The magnetic cycle goes through a period of 22 years, twice the sunspot's cycle.
Prominences and Flares: Large huge arch-shaped red color protrusions around the solar disk visible during solar eclipse. Red color is same as color of Chromosphere and comes from hydrogen lines. These burst out around the sunspot in reaction to the blocked flow of energy under sunspot. Prominences shoot out as high as 500,000 km (50 times higher than spicules) in a few hours.
Solar flare is much smaller scale phenomenon than prominences. It rises for a few minutes and decays in an hour or less. It emits vast amount of x-rays through visible radiations, along with streams of high energy electrons and protons. it releases about 10 billion megatons of TNT (a hydrogen bomb releases 100 megatons of energy).
Coronal Activity: The structure of the corona is small and flattened during sunspot minimum and it is large and blazing at sunspot maximum. Corona is not quite uniform halo. It is actually made of streamers controlled by the solar magnetic fields. Long thin streamers allow charged particle to escape at 2 to 3 solar radii. In some regions of the corona, the magnetic fields do not bend into a loop. This allows large chunk of charged particvles to escape producing cooler regions of corona, called Coronal Holes seen in x-ray images of the solar atmosphere. These emission of charged particles is the cause of frequent solar winds reaching earth's atmosphere, producing Auroras and disrupting communications.
Coronal holes: In some regions of solar surface, magetic fields do not loop back, the particles freely leave the solar surface. As a result, the cooler low density regions of corona appear in x-ray images. These are called CORONA HOLES.


Granulation: The surface of the photosphere is not uniform. The surface of the photosphere is covered with a pattern dark-edged bright cells called GRANULATION.
Convection: When hot liquid or gas rises up at the expense of cool liquid or gas falling down, the process is called CONVECTION.
Filtergram: Photographs taken by passing light through filters that admit only one wavelength of a strong absorption line.
Spicule: These are flame like irregular structures present on the surface of the photosphere, 100 to 1000 km wide and extending upto 10,000 km above the surface.
Supergranule: Like granule, supergranule is a dark-edged cell pattern much larger in area, about twice the diameter of the earth and caused by heat from larger convection cells deeper below the photosphere.
Solar wind: In the outskirt of Corona region, the temperature is so high (2,000,000 K) that charged particles such as electrons and protons easily leave the sun in a continuous breeze.
Helioseismology: Astronomers study the interior of the sun, by detecting and analyzing the modes of vibrations on the surface of sun, by a method called Helioseismology.
Sunspot: The darker regions appearing in pairs or groups in the sun's surface about twice the diameter of the earth moving from higher lattitudes to lower lattitude and vanishing near the equator of the sun in about a week or so.
Zeeman effect: Splitting of single spectral line into two or more components when atoms are under the influence of magnetic fields.
Maunder butterfly diagram: When the lattitudes wher the sunspots begin are plotted as a function time over many years, one finds a diagram that looks like a butterfly.
Maunder minimum: The patterns in the diagram show periodic activities of the sunspot that goes through a minimum. That is the butterfly pattern is repeated.
Differential rotation: The variation in the rotaion of the sun with 25 days at the equator and longer near the pole.
Dynamo effect: The dense ionized gas of the sun's interior is a good conductor of electricity. When the sun rotates, the dense ionized gas rotates, the outward flow of heat energy by convection produces magnetic field by an effect similar to Dynamo effect well known in physics. Such effect also produces magnetic field of the earth.
The atmosphere of the sun consists of three layers. They are photosphere, chromosphere and corona. The innermost layer of the three, is called the photosphere, meaning the visible surface of the sun. It is a thin layer of low density gas belt that emits a large amount of visible photons or light. The surface of the photosphere is marked by granulation, a pattern produced by gas currents rising from below the photosphere layer.
The layer above the photosphere is called the chromosphere. This layer is observed during the total solar eclipse when it flashes into view for a brief period of time (a few seconds). It is a thin hot layer of gas just above the photosphere, and its brilliant pink color is caused by the emission lines in its spectrum. Filtergrams of chromosphere show large jets called spicules extending up into the corona.
Corona is the outermost layer of sun's atmosphere. It is composed of very low density and very hot gas extending at least 30 solar radii. It has a high temperature of 2 million degrees Kelvin that is maintained by the magnetic field and the rotation of the sun.The solar wind starts out from the outer part of the corona. Solar wind is a low density ionized gas (mostly protons and electrons) streaming away from the sun. From time to time, the earth together with other planets in the solar system, is bathed with the solar wind. Solar astronomers study the motion, density and the temperature of the gases below the solar surface. This is done by analyzing the oscillations of the solar surface. Helioseismology is such a study. The subject requires large amount of data and extensive computer analysis.
The sunspots are the most convincing evidence of solar activity. A sunspot being cooler than the rest of the photosphere is generallty darker. The average sunspot has a size equal to twice the size of the earth. Its magnetic field strength is more than 1000 times stronger than the sun's average magnetic field. The sunspots are caused by presence of highly intense magnetic field that prevents the normal convectional flow of hot gas from underneath the photosphere. This allows cooling of the sunspot area.

The average number of sunspots vary over a period of about 11 years. This phenomenon seems to be tied up with the 22 year period of the magnetic cycle.

Updated 1/14/2014