Corona

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Solar corona natural phenomena , means Solar atmosphere The outermost layer of the Chromosphere Layers and photosphere Layer), with a thickness of more than several million kilometers. Beyond the chromosphere is the corona layer, which is extremely hot. The temperature of the corona is 1 million degrees Celsius, Particle number density Is 10 fifteen /m three 。

There are coronal holes in the corona, and coronal holes are the wind source of the solar wind. The corona Total solar eclipse Sometimes it can be seen through the coronagraph^[1] Its shape varies with the size of solar activity. In the year of maximum solar activity, the shape of the corona is nearly circular, while in the year of minimum solar activity, it is elliptical.

Chinese name: Corona
Foreign name: solar corona

Structure: 3 layers of inner, middle and outer crowns
Observation: It can only be seen with a coronagraph during a total solar eclipse

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six Magnetic disturbance
seven Scientific research achievements

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The corona is Solar atmosphere Outermost layer of, from Chromosphere The edge extends outward to several solar radii, or even farther. branch Inner corona 、 Middle corona and Outer corona , the inner corona only extends to 1.3 times the solar radius from the solar surface; The outer corona can reach several solar radii or even farther. The corona is completely ionized by very thin Plasma Composition, mainly including proton , highly ionized ions and high-speed free electron 。 The temperature of the corona is hundreds of times that of the sun's surface.^[2]

The corona can be divided into three layers: inner corona, middle corona and outer corona. The inner corona extends from the top of the chromosphere to 1.3 times the solar radius; The middle corona ranges from 1.3 times the solar radius to 2.3 times the solar radius. Some people also call it the inner corona within 2.3 times the solar radius. The place greater than 2.3 times the solar radius is called the outer corona (the above distances are calculated from the heliocentric). The broad sense of the corona can include the range that the solar wind can reach.

The temperature of the corona is 1 million degrees Celsius, Particle number density Is 10 fifteen /m three 。 At high temperatures, hydrogen 、 helium Such atoms have been ionized into positively charged protons, helium nuclei and negatively charged free electrons. these ones here charged particle The speed of movement is so fast that charged particles break away from the gravitational binding of the sun and shoot towards the periphery of the sun. Form solar wind. The light from the corona is weaker than that from the chromosphere.

The corona is mainly composed of high-speed free electrons, protons and highly ionized ions (plasmas). Its material density is less than 2 × 10 -12 Kg/m three , temperature up to 1.5 × 10 six ～2.5×10 six K。 Due to the high temperature and low density of the corona, its radiation is very weak and Nonlocal thermodynamic balance Status, except Visible radiation Besides, there are Radio radiation ， X-ray ， Ultraviolet , far Ultraviolet radiation And highly ionized ions (i.e Coronal forbidden line )。

White corona There are three components: ① K crown. Within 2.3 solar radius free electron Continuous spectrum of scattered photosphere. ② F Crown. Beyond 2.3 solar radius, originated from Ecliptic surface Expert Interstellar dust The particle scatters the light of the photosphere, and its spectrum contains Fraunhofer Line, F-corona is also called "internal ecliptic light". ③ E Crown. Also called L-corona, it is the light of coronal gas ion emission lines. Coronal magnetic field intensity About 1/10000~1/100 Tesla And decreases with the increase of distance from the solar surface.

Corona

Shape structure

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shape

The shape of the corona is related to solar activity. In years of extreme solar activity, the corona is nearly circular, while in years of quiet sun, it is relatively flat, equator The area is relatively extended. The diameter of the corona is approximately 1.5~3 times of the diameter of the solar apparent circle. (See coronal periodic variation).

Corona (Fig. 1)

structure

The fine structure of the corona includes: Coronal flow and Polar feather 、 Coronal hole 、 Coronal condensation region Etc. The structure of the corona generally changes slowly with time. It is believed that the different structures observed may be the appearance of the same structure in different periods.

NASA Photographs the Giant "Corona Hole" at the Sun's South Pole

NASA Photographs the Giant "Corona Hole" at the Sun's South Pole

observation

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Coronal radiation has a wide range of wavelengths, ranging from X-ray 、 visible light To the radio wave with long wavelength, different instruments must be used for observation. By X-ray or Far ultraviolet In the photo, we can see that there is a large irregular dark area in the corona, which is called Coronal hole 。

Before the coronagraph was invented in 1931, people could only observe the corona during a total solar eclipse, because its brightness was only about one millionth of [[photosphere]], which was about the brightness of the full moon. At ordinary times, the scattered light of the atmosphere on the ground and the scattered light of the observation instrument will greatly exceed the brightness of the corona itself and submerge it. During a total solar eclipse, the solar photosphere is blocked by the moon, and the scattered light of the atmosphere and instruments is weakened accordingly, so that the corona can be observed easily. Although there are few opportunities for a total solar eclipse, astronomers still make great efforts to transport instruments and equipment to the place where a total solar eclipse occurs for observation, because there are some observations (such as verification Einstein The theory of relativity and the study of outer corona can only be carried out during the total solar eclipse.

In order to observe the corona at ordinary times, a coronagraph that can eliminate the scattered light of the instrument to the maximum extent must be used. To overcome Atmospheric scattering Under the influence of light, the coronagraph must be placed on a high mountain. However, only the inner corona can be observed with the coronagraph, and only part of the information of the white corona can be obtained. because Space exploration With the development of the cause, people have put the coronagraph on rockets Orbital observatory Or extraatmospheric observation in the sky laboratory. In this way, not only the visible light band of the corona can be observed, but also the ultraviolet, far ultraviolet and X-ray radiation can be detected Interplanetary space Sample the solar wind. There are several radio band radiations that can pass through the earth atmosphere , so it is available on the ground radio telescope Make routine observations of the corona (see Solar radio ）。

In 1868, French astronomer Pierre J.C. Johnson Once in India solar eclipse During the observation, the coronal spectral lines were recorded and sent to British astronomers Joseph Norman Lockell , he is a recognized spectroscopy Experts. Through careful research, Bill Lockyer He thought that these spectral lines meant that there was an unknown new element in the solar atmosphere. He named it "helium", which means "the sun" in Greek, that is, "the element contained in the sun". However, this assertion was soon overturned.

Scottish chemist in 1895 William Ramsey It is found that helium also exists on the earth. And "helium" is the only known element that was first found on celestial bodies outside the Earth.

In 1931, French astronomer Bonnard Ferdinand Leot Invented Coronagraph This invention enables people to observe the light generated by the corona when the sun is shining. With the help of this instrument, we finally found that the corona is part of the sun. At that time, when people studied the corona, they found that the spectral lines generated by the corona did not belong to a certain range of the spectrum.

The corona also produces some other strange spectral lines, but this does not mean that there are any unknown elements in the corona. On the contrary, these spectral lines show that the atoms of the elements in the corona contain different numbers of electrons, and some electrons will break away under high temperature conditions atom The shackles of.

In 1942, Sweden physical scientist Benjamin Edwin It is believed that some special spectral lines in the corona are iron , carbon and nickel An atom is produced without electrons. The corona has no prominent edge, but continues to extend, gradually integrates with the entire solar system, and gradually weakens during the extension, until planet The movement of can not form any appreciable influence until. The heat contained in the sun will drive charged particles to shoot out of the sun in different directions, American physicist Eugene Newman Barker predicted this in 1959.

In 1962, "Sailor 2" detector Up to Space arrive Venus The results detected at the time verify this prediction. The ejection of charged particles is called "solar wind", and its speed is 400-700 km/s. The role of "solar wind" makes comet The tails of both point away from the sun. At the same time, the charged particles that make up the "solar wind" will continue to hit each planet, and if the planet has the North and South Poles (as on the Earth), the charged particles will move from its North Pole to the South Pole.

Space probe observations in the 1970s found that there were large irregular dark areas in the corona, called coronal holes. Coronal holes are low-temperature and low-density regions of the corona, which can be roughly divided into three types: polar coronal holes, isolated coronal holes and extended coronal holes. Polar coronal holes often exist in the north and south polar regions. The isolated coronal holes in the middle and low latitudes are small in size. The coronal holes that extend from the polar region to the equator have a long life span and are an important source of high-speed solar wind. When there are strong X-ray flares on the sun and Coronal mass ejection When some powerful plasma streams fly near the earth, they often cause large magnetic storms and strong auroras, and also ionospheric disturbances, affecting short wave communication and satellite communication on the earth. The two poles of the earth will have beautiful auroras in various forms.^[3]

Coronal hole

On January 1, 2015, the atmospheric imaging module of NASA's Solar Dynamics Observation Satellite captured a mysterious phenomenon on the sun - a huge "black hole" appeared in the sun's Antarctic region, covering almost 1/4 of the sun. Scientists said that this black hole is a huge coronal hole, which is a dark, low-density region of the solar corona. Under the irradiation of far ultraviolet light, it looks bleak, as if it is a black abyss deep into the center of the sun.

Although there is no solar activity in the coronal hole from the satellite image, it actually releases a violent solar storm and spews solar particles at a speed of 500 miles per second, which is three times the speed of the solar wind elsewhere. Scientists are still studying the specific cause of the coronal hole, but it seems to be related to the region where the magnetic activity is enhanced. NASA said that the coronal hole is one of the most prominent features of the sun. NASA also said, "Since the coronal hole is located in the southernmost part of the sun, the solar wind is unlikely to have an impact on human beings on the earth."^[4]

In May 2015, NASA announced the spectacular coronal ring on the sun's surface, Solar Dynamics Observatory The onboard atmospheric imaging module is responsible for photographing the solar atmosphere. It takes pictures in different wavebands, and collects ten imaging data of different wavelengths every ten seconds to reveal the relationship between the changes of the sun's surface and the internal changes. The coronal ring in Figure 1 is very clear. The blue area and yellow area respectively represent the two poles of the magnetic field, and the heliosphere is also covered and superimposed below magnetic field Magnetic field data observed by the observer.

Figure 1

two thousand and twenty-one The meteorological satellite FY-3E, launched in the summer of, was equipped with a solar X-ray extreme ultraviolet imager, which realized China's space coronal exploration for the first time^[8] 。

In the early morning of April 9, 2024, Beijing time, the only total solar eclipse in the world will sweep across North America. Many cities in Mexico, the United States and Canada can see this total solar eclipse, which occurs from noon to afternoon on April 8, local time^[9]

In April 2024, the total solar eclipse was photographed, and the corona was clearly visible. Liu Boyang/Photo

radiation

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radiation

The radiation of the corona is generated under the state of non local thermodynamic equilibrium, and there are the following situations: ① free electrons in the coronal gas scatter photosphere radiation, that is, white corona. ② Electrons in thermal motion are identical with protons, alpha particles and various Heavy ion collision When, generate Bremsstrahlung 。③ The forbidden transition of metastable ions is the source of the coronal forbidden line. ④ When the electron magnetic field During the middle movement, cyclotron acceleration radiation or synchrotron acceleration radiation is generated. This process is very important for the generation of long wavelength radiation (such as radio waves) of the corona. ⑤ Electrostatic oscillations and Alvinbo Radiation is also generated during the process.

The continuous radiation of the visible light band of the corona is the result of the continuous radiation of the photosphere scattered by the coronal matter, so the energy distribution of the continuous spectrum of the corona is very similar to that of the photosphere. The light of the white corona can be divided into: K Corona , F corona, E corona (sometimes called L corona). Solar spectrum The far ultraviolet and X-ray are mainly produced in the corona. The photosphere temperature is low, and the radiation in these two bands is far less than that in the corona. In order not to be interfered by the photosphere radiation, the images of the corona are usually taken in two bands of far ultraviolet and X-ray. Figure 4 shows the coronal image taken by X-ray. By comparing the monochromatic images in the visible band with the monochromatic images such as far ultraviolet and X-ray, we can study the physical state of the solar atmosphere at different levels (see Monochromatic image of the sun ）。

Radio radiation

quiet Coronal radio Radiation is similar to coronal X-ray in some aspects. Although they only account for a small part of the total solar radiation energy, they can provide a considerable amount of information. Bremsstrahlung, which is significant for X-ray, is also important for radio spectrum; Like X-ray, radio waves can be used to directly observe the radio radiation of the corona without the interference of photosphere radiation. Through spectral analysis, it is found that e="3"<the sum of electron density and Movement temperature 。

temperature

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The temperature of the corona is very high, up to 2 million degrees. Incredibly, the nearest photosphere The temperature is several thousand degrees. A little farther away, the temperature ranges from tens of thousands of degrees to tens of thousands of degrees. The temperature of the corona, which is farthest from the sun center, is up to a million degrees^[5] 。 Scientists have not yet found a reasonable explanation for what this abnormal phenomenon means. The temperature of the corona is very high, and its value reaches the order of millions. This is not a hypothesis, but the high energy emitted from the corona X-ray Based on. However, this super high temperature is only concentrated in individual atoms of the corona. Moreover, these atoms are widely distributed in the entire corona, and their total heat is not high.

Observations show that the temperature of the solar atmosphere has an abnormal distribution, that is, it slowly drops from 5770K of the photosphere to 4600K at the top of the photosphere (the junction of the photosphere and the chromosphere), then slowly rises to tens of thousands of degrees about 2000 kilometers above the photosphere, and then extends up about 1000 kilometers to form the chromosphere corona transition layer, and the temperature rises sharply to hundreds of thousands of degrees, The low corona region is already a high temperature region of more than one million degrees. What causes this abnormal temperature rise is still one of the most important unsolved problems in solar physics for many years. In the past decades, many studies have been carried out on the causes of the abnormal high temperature in the transition layer and the corona. Sound wave heating mechanism, shock wave heating mechanism, Alvin wave heating mechanism, and non resonant turbulent wave particle heating mechanism have all been proposed, but the theoretical research in this area is still in the exploratory stage.

The Cause of Coronal High Temperature and High-energy particle Momentum is not conserved.

Law of conservation of momentum : There are strict requirements for momentum conservation in basic physics. The prerequisite is that the system object must be a rigid body and the system is free from external forces. Loose systems, such as cotton balls, are not suitable for the conservation of momentum principle. Similarly, high-energy particles in some extreme physical environments will not strictly follow the momentum conservation principle. Just like the cotton ball absorbs kinetic energy, under the strong gravitational field and extreme high pressure environment, the internal system of high-energy particles will also absorb additional energy to ensure the stability of its system in the extreme environment.

This phenomenon may exist in the evolution of cosmic particles. A certain high-energy particle body A, which is free at a high speed in a certain space, is a high-energy particle body with full energy and may overflow electrons or photons at any time, and its energy carrying capacity is far beyond its stable state. However, in the end, this high-energy particle body A did not overflow any energy, but transformed into other kinds of particle body B, and this new particle B can stably exist in its current environment.

We can see that the overall energy has not changed during the whole transformation process. The most obvious change is the mass change when particle A becomes particle B. It changes from the high-energy overflow state of particle A at any time to the stable particle B. In the process of condensation, the speed of particle A becomes smaller after it is transformed into particle B. We can simply draw a conclusion from the following kinetic energy formula. (See Sketch 2)

Figure 2

(Note: particles A and B are only conceptual symbols, and their particles may not become other particles in the process, but have changed in mass or speed.)

This situation can no longer be solved by momentum conservation. This particle transformation may be realized in some special environments. However, it is this principle of particle transformation that can provide us with a research direction of kinematics. The space environment we usually study is relatively stable. All our assumptions are in an ideal environment. This strange phenomenon may not be observed in our already stable space environment or stable laboratory.

From the formula of non momentum conservation, I can estimate the possible cause of 1 million high temperatures in the solar corona. The high-energy particles ejected from the sun may experience a process of decreasing mass and increasing speed after leaving a certain gravitational and pressure effective area of the sun, resulting in the particles in this area becoming quite active. (See Sketch 3)

Figure 3

Figure 4

This non decay mass change of particles may be common in some high-density stars or early universe. This process can be described as bubbles on the sea floor. A bubble is emerging from the sea bottom thousands of meters deep. At the beginning, because of the high water pressure at the bottom of the sea, the bubbles were very small. However, as the bubble rises closer to the water surface, the pressure of the sea water decreases, and the bubble begins to expand or overflow several new bubbles to reach a stable state (see Figure 4 for bubble diagram). The same is true for high-energy particles spilling from the sun. (This conjecture comes from Han Tongyi's book, Stargate - Principles of Superluminal Velocity of Spacecraft)

Magnetic disturbance

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Corona

from Magnetohydrodynamics From the point of view, the magnetic field in the solar atmosphere should be a unified whole, that is, the coronal magnetic field is closely related to the photosphere magnetic field and chromosphere magnetic field. Large scale inhomogeneous structure of the corona seen in coronal photos: coronal flow Polar feather , condensation areas and helmets are mostly caused by the uneven distribution of the coronal magnetic field. For example, the plumes at the poles are like iron filings near the poles of magnets, which were used to calculate the dipole field of the corona. However, unlike the light field and color field, due to the difficulties in observation, it is difficult to directly calculate the magnetic field of the corona from the Zeeman splitting distance of the measured spectral line (see Zeeman effect), so it can only be calculated by indirect observation methods or theoretical calculations. Nowadays, the method of calculating the coronal magnetic field from the photosphere magnetic field is widely used, because the photosphere magnetic field can be measured more accurately and recorded every day. Assuming that the magnetic field in the low coronal region is a force free field and a current free field, the intensity and direction of the coronal magnetic field can be obtained by solving the current free field equation using the observed photosphere magnetic field data as the boundary condition.

In 1968, Newcock and others first carried out this research. They compared the calculated coronal magnetic field structure with the shape of the corona, and the results were quite satisfactory. The results show that the magnetic field intensity of the corona is between 1 and 100 Gaussian Within the range, it decreases with the increase of the distance from the solar surface. In a Astronomical unit Measured directly from space Interplanetary magnetic field The average is about 5 × 10 -5 Gauss, with archimedes spiral The magnetic structure of. When the solar activity is strong, the intensity of the local magnetic field in the corona coexisting with the active object is much higher, and the intensity of the interplanetary magnetic field also increases greatly. There are two kinds of coronal magnetic field structures: one is closed field structure, and its corresponding optical structure is helmet like coronal current; The other is open structure, and its counterpart is Coronal hole 。 And with solar flare The symbiotic local disturbance region is often a partially open and partially closed field structure.

The corona or some part of it will be disturbed in a short period of time, which is manifested in the influence on the movement of matter, particle acceleration, coronal density and temperature changes within a few seconds to an hour. Coronal disturbances can be divided into three categories: ① long-term disturbances, lasting from a few days to a few months, which show that the changes in the coronal structure are controlled by the changes in the large-scale photosphere magnetic field. Long term disturbances control the solar wind and interplanetary magnetic field. ② Rapid disturbance, from minutes to hours. It shows the enhancement of visible light, radio continuous radiation and soft X-ray radiation. The fast disturbance causes strong interplanetary shock waves. ③ Pulse disturbance, time is less than a few seconds. As Radio burst And hard X-ray bursts. In case of such disturbance, particle acceleration process and non thermal radiation occur (see Solar radio burst and Solar pulsed hard X-ray bursts ）。

The study of coronal disturbances is related to other solar activities and interplanetary disturbances. The research work in this field is very active now.

Scientific research achievements

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According to foreign media reports, U.S.A NASA plans to launch the latest probe on July 31, 2018, which will approach at an unprecedented close range sunlight 。 This space program is called "Solar Probe Plus". It will carry out four experiments on the solar corona layer to study the solar wind and the energy particles released from the solar surface.

During the close approach to the sun, detector The nearest distance to the sun is 6.11 million kilometers, and its external temperature will reach 1399 degrees Celsius. It is reported that according to the original plan, this probe will be launched in Florida on July 31, 2018 Cape Canaveral Air Base It was launched by Delta 4 heavy rocket, and the launch window was opened for 20 days.

For a long time, scientists expect to launch detectors through the solar corona (the outermost layer of the solar atmosphere) to better understand the solar wind and the materials entering the solar system. Solar detector The main scientific task of the additional task is to track the solar energy flow, understand the heating of the sun's corona, and explore the physical principles that promote the activity of the solar wind and energy particles.^[6]

On December 4, 2019, the British journal Nature simultaneously published four important astrophysical research results - the original data of the "Parker" solar probe. The detector went beyond the heliosphere of the sun and reached about 24 million kilometers away from the sun. The instruments on the detector observed the activities in the corona, providing new insights for humans to understand the origin of the solar wind and high-energy particle physics.

The corona produces the solar wind, a high-energy particle continuously emitted by the sun. Remote observation has revealed some details of the mechanism behind the formation of the solar wind, but other processes have been difficult to explore. Most measurements are made at a distance of 1 astronomical unit from the sun（ Average distance between the sun and the earth ）At. It is known that the solar wind will change when it moves from the sun to the earth, but the extent and origin of these changes have been unclear.

Now, the Parker Solar Probe has provided the closest coronal observation result to the sun so far, realizing the unprecedented observation of the sun. For example, past missions have shown that the solar wind will accelerate from the corona, but the reason behind this is not clear. In a study published this time, the team of the University of Michigan, Ann Arbor, reported that the magnetic field changes increased the speed of the solar wind outflow. Their measured speed is higher than the speed predicted by the model study.

In another study, the University of California, Berkeley, focused on the slow solar wind (speed less than 500 km/s), whose origin is not as clear as the fast solar wind. They found that the slow solar wind originated from the coronal hole near the solar equator.

University College London Daniel Vischalen of Murad Space Science Laboratory commented that in the next five years, the "Parker" probe will continue to approach the sun and eventually reach just over 6 million kilometers from the sun's surface, and will continue to bring new discoveries. During this period, the sun will enter a more active phase of its 11 year activity cycle, so we can expect more exciting results in the coming years.^[7]

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