Prominences
The surface of the Sun that we see is known as the photosphere. This is the area where light from the core finally reaches the surface. The photosphere has a temperature of about 6000 K and glows white.
Just above the photosphere, the atmosphere extends for several hundred thousand kilometers. Let's take a closer look at the structure of the Sun's atmosphere.
The first layer in the atmosphere has a minimum temperature, and is located at a distance of about 500 km above the surface of the photosphere, with a temperature of about 4000 K. For a star, this is quite cool.
Chromosphere
The next layer is known as the chromosphere. It is located at a distance of only about 10,000 km from the surface. In the upper part of the chromosphere, temperatures can reach 20,000 K. The chromosphere is invisible without special equipment that uses narrow-band optical filters. Giant solar prominences can rise in the chromosphere to a height of 150,000 km.
Above the chromosphere there is a transition layer. Below this layer, gravity is the dominant force. Above the transition region, the temperature rises quickly because helium becomes fully ionized.
Solar corona
The next layer is the corona, and it extends from the Sun millions of kilometers into space. You can see the corona during a total eclipse, when the disk of the luminary is covered by the Moon. The temperature of the corona is about 200 times hotter than the surface.
While the temperature of the photosphere is only 6000 K, near the corona it can reach 1-3 million degrees Kelvin. Scientists still don’t fully know why it is so high.
Heliosphere
The upper part of the atmosphere is called the heliosphere. It is a bubble of space filled with solar wind and extends out to about 20 astronomical units (1 AU is the distance from the Earth to the Sun). Ultimately, the heliosphere gradually transitions into the interstellar medium.
Stars are made entirely of gas. But their outer layers are also called the atmosphere.
The atmosphere of the Sun begins at 200-300 km. deeper than the visible edge of the solar disk. These deepest layers of the atmosphere are called the photosphere. Since their thickness is no more than one three-thousandth of the solar radius, the photosphere is sometimes conventionally called the surface of the Sun. The density of gas in the photosphere is approximately the same as in the Earth's stratosphere, and hundreds of times less than at the Earth's surface. The temperature of the photosphere decreases to 8000 K at a depth of 300 km. up to 4000 K in the uppermost layers. In a telescope with high magnification, you can observe subtle details of the photosphere: it all seems strewn with small bright grains - granules, separated by a network of narrow dark paths. Granulation is the result of the mixing of warmer gas flows rising and colder ones descending. The temperature difference between them in the outer layers is relatively small, but deeper, in the convective zone, it is greater, and mixing occurs much more intensely. Convection in the outer layers of the Sun plays a huge role in determining the overall structure of the atmosphere. Ultimately, it is convection, as a result of a complex interaction with solar magnetic fields, that is the cause of all the diverse manifestations of solar activity. The photosphere gradually passes into the more rarefied outer layers of the solar atmosphere - the chromosphere and corona.
The chromosphere (Greek for “sphere of light”) is named for its reddish-violet color. It is visible during total solar eclipses as a ragged bright ring around the black disk of the Moon, which has just eclipsed the Sun. The chromosphere is very heterogeneous and consists mainly of elongated elongated tongues (spicules), giving it the appearance of burning grass. The temperature of these chromospheric jets is 2-3 times higher than in the photosphere, and the density is hundreds of thousands of times lower. The total length of the chromosphere is 10-15 thousand km. The increase in temperature in the chromosphere is explained by the propagation of waves and magnetic fields penetrating into it from the convective zone. The substance is heated in much the same way as if it were in a giant microwave oven. The speed of thermal motion of particles increases, collisions between them become more frequent, and atoms lose their outer electrons: the substance becomes a hot ionized plasma. These same physical processes also maintain the unusually high temperature of the outermost layers of the solar atmosphere, which are located above the chromosphere. Often during eclipses, bizarrely shaped “fountains”, “clouds”, “funnels”, “bushes”, “arches” and other brightly luminous formations of chromospheric matter can be observed above the surface of the sun. These are the most ambitious formations of the solar atmosphere - prominences. They have approximately the same density and temperature as the chromosphere. But they are above it and surrounded by higher, highly rarefied upper layers of the solar atmosphere. Prominences do not fall into the chromosphere because their matter is supported by the magnetic fields of active regions of the Sun. Some prominences, having remained for a long time without noticeable changes, suddenly seem to explode, and their matter is thrown into interplanetary space at a speed of hundreds of kilometers per second.
Unlike the chromosphere and photosphere, the outermost part of the Sun's atmosphere - the corona - has a huge extent: it extends over millions of kilometers, which corresponds to several solar radii. The density of matter in the solar corona decreases with height much more slowly than the density of air in the earth's atmosphere. The corona is best observed during the total phase of a solar eclipse. The main feature of the crown is its radiant structure. Coronal rays have a wide variety of shapes: sometimes they are short, sometimes long, some rays are straight, and sometimes they are strongly curved. The general appearance of the solar corona changes periodically. This is due to the eleven-year cycle of solar activity. Both the overall brightness and the shape of the solar corona change. During the era of maximum sunspots, it has a relatively round shape. When there are few spots, the shape of the corona becomes elongated, while the overall brightness of the corona decreases. So, the corona of the Sun is the outermost part of its atmosphere, the thinnest and the hottest. Let us add that it is also the closest to us: it turns out that it extends far from the Sun in the form of a plasma stream constantly moving from it - the solar wind. In fact, we live surrounded by the solar corona, although protected from its penetrating radiation by a reliable barrier in the form of the earth's magnetic field.
Life experience tells us that the closer you bring your hand to the flame, the hotter your hand will be. However, in space, many things do not work as everyday experience suggests: for example, the temperature of the visible surface of the Sun is “only” 5800 K (5526.85 °C), but at a distance, in the outer layers of the star’s atmosphere, it rises to millions of degrees.
Try solving this little special problem known as the Solar Corona Heating Problem, one of the unsolved problems of modern physics! When the phenomenon was discovered, it seemed to scientists that the solar corona violated the second law of thermodynamics - after all, energy from inside the star cannot be transferred to the corona region, bypassing the surface.
Until 2007, there were two main theories explaining the heating of the solar corona. One said that magnetic fields accelerate the plasma of the corona to incredible energies, due to which it acquires a temperature above the surface temperature. The authors of the second theory were inclined to believe that energy breaks through into the atmosphere from inside the star.
Research by Bart De Pontieu and his colleagues has proven that shock waves emanating from the interior of a star have sufficient energy to constantly feed the corona with energy.
In 2013, NASA launched the IRIS probe, which continuously films the boundary between the surface of the Sun and the corona in different ranges. His goal was to answer the same question: does the solar corona have one constant source of heat, or does energy enter the solar atmosphere as a result of many explosions? The difference between these two explanations is very large, but it is very difficult to understand which one is correct due to the enormous thermal conductivity of the corona. As soon as energy is released at a single point on the Sun, the temperature almost instantly rises over a vast area around this point - and it seems that the temperature of the corona is more or less constant.
But the IRIS apparatus recorded changes in the temperature of the corona with such a small interval that scientists were able to see many “nanoflares” (nanoflares) where magnetic lines intersected or overlapped. Whether there is a source of thermal radiation that uniformly and continuously heats the corona remains an open question, but it is now clear that at least some of the energy enters the solar atmosphere from the interior of the star as a result of such explosions.
Later, the IRIS observations were confirmed by the EUNIS apparatus. Scientists are now almost certain that the solar corona is heating up precisely because of many small explosions that release hot plasma into the star’s atmosphere, the temperature of which is much higher than the temperature of the surface of the Sun.
Program questions:
Chemical composition of the solar atmosphere;
Rotation of the Sun;
Darkening of the solar disk towards the edge;
Outer layers of the solar atmosphere: chromosphere and corona;
Radio and X-ray radiation from the Sun.
Summary:
Chemical composition of the solar atmosphere;
In the visible region, solar radiation has a continuous spectrum, against which several tens of thousands of dark absorption lines, called Fraunhofer. The continuous spectrum reaches its greatest intensity in the blue-green part, at wavelengths of 4300 - 5000 A. On both sides of the maximum, the intensity of the spectrum decreases.
Extra-atmospheric observations have shown that the Sun emits radiation into the invisible short-wave and long-wave regions of the spectrum. In the shorter wavelength region, the spectrum changes sharply. The intensity of the continuous spectrum quickly decreases, and the dark Fraunhofer lines are replaced by emission lines.
The strongest line of the solar spectrum is in the ultraviolet region. This is the resonance line of hydrogen L with a wavelength of 1216 A. In the visible region, the resonance lines H and K of ionized calcium are most intense. After them in intensity come the first lines of the Balmer series of hydrogen H , H , H , then the resonance lines of sodium, lines of magnesium, iron, titanium, and other elements. The remaining numerous lines are identified with the spectra of about 70 known chemical elements from the table of D.I. Mendeleev. The presence of these lines in the spectrum of the Sun indicates the presence of corresponding elements in the solar atmosphere. The presence of hydrogen, helium, nitrogen, carbon, oxygen, magnesium, sodium, iron, calcium, and other elements in the Sun has been established.
The predominant element in the Sun is hydrogen. It accounts for 70% of the Sun's mass. Next is helium - 29% of the mass. The remaining elements combined account for a little more than 1%.
Rotation of the Sun
Observations of individual features on the solar disk, as well as measurements of shifts of spectral lines at various points on it, indicate the movement of solar matter around one of the solar diameters, called axis of rotation Sun.
The plane passing through the center of the Sun and perpendicular to the axis of rotation is called the plane of the solar equator. It forms an angle of 7 0 15’ with the plane of the ecliptic and intersects the surface of the Sun along the equator. The angle between the equatorial plane and the radius drawn from the center of the Sun to a given point on its surface is called heliographic latitude.
The angular speed of rotation of the Sun decreases as it moves away from the equator and approaches the poles.
On average = 14º.4 - 2º.7 sin 2 B, where B is the heliographic latitude. Angular velocity is measured by the angle of rotation per day.
The sidereal period of the equatorial region is 25 days; near the poles it reaches 30 days. Due to the rotation of the Earth around the Sun, its rotation seems to be slower and equals 27 and 32 days, respectively (synodic period).
Darkening of the solar disk towards the edge
The photosphere is the main part of the solar atmosphere in which visible radiation is formed, which is continuous. Thus, it emits almost all of the solar energy that comes to us. The photosphere is a thin layer of gas several hundred kilometers long, quite opaque. The photosphere is visible when directly observing the Sun in white light in the form of its apparent “surface”.
When observing the solar disk, its darkening towards the edge is noticeable. As you move away from the center, the brightness decreases very quickly. This effect is explained by the fact that in the photosphere the temperature increases with depth.
Various points of the solar disk are characterized by the angle , which makes up the line of sight with the normal to the surface of the Sun at the location in question. At the center of the disk, this angle is 0, and the line of sight coincides with the radius of the Sun. At the edge = 90 and the line of sight slides along the tangent to the layers of the Sun. Most of the radiation from a certain layer of gas comes from a level located at optical depth 1. When the line of sight intersects the layers of the photosphere at a large angle, optical depth1 is achieved in the outer layers, where the temperature is lower. As a result, the intensity of radiation from the edges of the solar disk is less than the intensity of radiation from its middle.
The decrease in the brightness of the solar disk towards the edge can, to a first approximation, be represented by the formula:
I () = I 0 (1 - u + cos),
where I () is the brightness at the point at which the line of sight makes an angle with the normal, I 0 is the brightness of the radiation from the center of the disk, u is the proportionality coefficient, depending on the wavelength.
Visual and photographic observations of the photosphere reveal its fine structure, reminiscent of closely spaced cumulus clouds. Light round formations are called granules, and the entire structure is granulation. The angular dimensions of the granules are no more than 1″ arc, which corresponds to 700 km. Each individual granule exists for 5-10 minutes, after which it disintegrates and new granules form in its place. The granules are surrounded by dark spaces. The substance rises in the granules and falls around them. The speed of these movements is 1-2 km/s.
Granulation is a manifestation of the convective zone located under the photosphere. In the convective zone, mixing of matter occurs as a result of the rise and fall of individual masses of gas.
The reason for the occurrence of convection in the outer layers of the Sun is two important circumstances. On the one hand, the temperature directly below the photosphere increases very quickly in depth and radiation cannot ensure the release of radiation from deeper hot layers. Therefore, energy is transferred by the moving inhomogeneities themselves. On the other hand, these inhomogeneities turn out to be tenacious if the gas in them is not completely, but only partially ionized.
When passing into the lower layers of the photosphere, the gas is neutralized and is not able to form stable inhomogeneities. therefore, in the very upper parts of the convective zone, convective movements are slowed down and convection suddenly stops. Oscillations and disturbances in the photosphere generate acoustic waves. The outer layers of the convective zone represent a kind of resonator in which 5-minute oscillations are excited in the form of standing waves.
Outer layers of the solar atmosphere: chromosphere and corona
The density of matter in the photosphere quickly decreases with height and the outer layers turn out to be very rarefied. In the outer layers of the photosphere, the temperature reaches 4500 K, and then begins to rise again. There is a slow increase in temperature to several tens of thousands of degrees, accompanied by the ionization of hydrogen and helium. This part of the atmosphere is called chromosphere. In the upper layers of the chromosphere, the density of the substance reaches 10 -15 g/cm 3 .
1 cm 3 of these layers of the chromosphere contains about 10 9 atoms, but the temperature increases to a million degrees. This is where the outermost part of the Sun's atmosphere, called the solar corona, begins. The reason for the heating of the outermost layers of the solar atmosphere is the energy of acoustic waves arising in the photosphere. As they propagate upward into lower-density layers, these waves increase their amplitude to several kilometers and turn into shock waves. As a result of the occurrence of shock waves, wave dissipation occurs, which increases the chaotic velocities of particle movement and an increase in temperature occurs.
The integral brightness of the chromosphere is hundreds of times less than the brightness of the photosphere. Therefore, to observe the chromosphere, it is necessary to use special methods that make it possible to isolate its weak radiation from the powerful flux of photospheric radiation. The most convenient methods are observations during eclipses. The length of the chromosphere is 12 - 15,000 km.
When studying photographs of the chromosphere, inhomogeneities are visible, the smallest ones are called spicules. The spicules are oblong in shape, elongated in the radial direction. Their length is several thousand km, thickness is about 1,000 km. At speeds of several tens of km/s, spicules rise from the chromosphere into the corona and dissolve in it. Through spicules, the substance of the chromosphere is exchanged with the overlying corona. Spicules form a larger structure, called a chromospheric network, generated by wave motions caused by much larger and deeper elements of the subphotospheric convective zone than granules.
Crown has very low brightness, so it can only be observed during the total phase of solar eclipses. Outside of eclipses, it is observed using coronagraphs. The crown does not have sharp outlines and has an irregular shape that changes greatly over time. The brightest part of the corona, removed from the limb no more than 0.2 - 0.3 radii of the Sun, is usually called the inner corona, and the remaining, very extended part is called the outer corona. An important feature of the crown is its radiant structure. The rays come in different lengths, up to a dozen or more solar radii. The inner crown is rich in structural formations resembling arcs, helmets, and individual clouds.
Corona radiation is scattered light from the photosphere. This light is highly polarized. Such polarization can only be caused by free electrons. 1 cm 3 of corona matter contains about 10 8 free electrons. The appearance of such a number of free electrons must be caused by ionization. This means that 1 cm 3 of the corona contains about 10 8 ions. The total concentration of the substance should be 2 . 10 8 . The solar corona is a rarefied plasma with a temperature of about a million Kelvin. A consequence of high temperature is the large extent of the corona. The length of the corona is hundreds of times greater than the thickness of the photosphere and amounts to hundreds of thousands of kilometers.
Radio and X-ray radiation from the Sun
WITH The solar corona is completely transparent to visible radiation, but poorly transmits radio waves, which experience strong absorption and refraction in it. At meter waves, the brightness temperature of the corona reaches a million degrees. At shorter wavelengths it decreases. This is due to an increase in the depth from which the radiation emerges, due to a decrease in the absorbing properties of the plasma.
Radio emission from the solar corona has been traced over distances of several tens of radii. This is possible due to the fact that the Sun annually passes by a powerful source of radio emission - the Crab Nebula and the solar corona eclipses it. The nebula's radiation is scattered in the inhomogeneities of the corona. Bursts of radio emission from the Sun are observed, caused by plasma oscillations associated with the passage of cosmic rays through it during chromospheric flares.
X-ray radiation studied using special telescopes installed on spacecraft. The X-ray image of the Sun has an irregular shape with many bright spots and a “clumpy” structure. Near the optical limb, there is a noticeable increase in brightness in the form of an inhomogeneous ring. Particularly bright spots are observed above centers of solar activity, in areas where there are powerful sources of radio emission at decimeter and meter waves. This means that X-rays originate primarily from the solar corona. X-ray observations of the Sun make it possible to conduct detailed studies of the structure of the solar corona directly in projection onto the solar disk. Next to the bright areas of the corona glow above the sunspots, extensive dark areas were found that were not associated with any noticeable formations in visible rays. They're called coronal holes and are associated with areas of the solar atmosphere in which magnetic fields do not form loops. Coronal holes are a source of increased solar wind. They can exist for several revolutions of the Sun and cause on Earth a 27-day periodicity of phenomena sensitive to corpuscular radiation from the Sun.
Control questions:
What chemical elements predominate in the solar atmosphere?
How can you find out about the chemical composition of the Sun?
With what period does the Sun rotate around its axis?
Do the rotation periods of the equatorial and polar regions of the Sun coincide?
What is the photosphere of the Sun?
What is the structure of the solar photosphere?
What causes the darkening of the solar disk towards the edge?
What is granulation?
What is the solar corona?
What is the density of matter in the corona?
What is the solar chromosphere?
What are spicules?
What is the temperature of the corona?
What explains the high temperature of the corona?
What are the features of radio emission from the Sun?
Which regions of the Sun are responsible for the appearance of X-rays?
Literature:
Kononovich E.V., Moroz V.I. General astronomy course. M., Editorial URSS, 2004.
Galuzo I.V., Golubev V.A., Shimbalev A.A. Planning and methods of conducting lessons. Astronomy in 11th grade. Minsk. Aversev. 2003.
Whipple F.L. Family of the Sun. M. Mir. 1984
Shklovsky I. S. Stars: their birth, life and death. M. Science. 1984
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