The mystery of the solar corona. The Sun is a unique star The passage of starlight through the solar corona

Under the influence of gravity, S., like any star, tends to shrink. This compression is counteracted by the pressure drop resulting from the high internal temperature and density. layers S. In the center of S. temperature T ≈ 1.6. 10 7 K, density ≈ 160 g/cm -3. Such a high temperature in the central regions of the sun can be maintained for a long time only by the synthesis of helium from hydrogen. These reactions and phenomena. basic source of energy C.

At temperatures of ~10 4 K (chromosphere) and ~10 6 (corona), as well as in the transition layer with intermediate temperatures, ions of various elements appear. The emission lines corresponding to these ions are quite numerous in the short-wavelength region of the spectrum (λ< 1800 . Спектр в этой области состоит из отдельных эмиссионных линий, самые яркие из к-рых - линия водорода L a (1216 ) и линия нейтрального (584 ) и ионизованного (304 ) гелия. Излучение в этих линиях выходит из области эмиссии практически не поглощаясь. Излучение в радио- и рентг. областях сильно зависит от степени solar activity, increasing or decreasing several times over an 11-year period and noticeably increasing during solar flares.

Phys. The characteristics of the various layers are shown in Fig. 5 (the lower chromosphere with a thickness of ≈ 1500 km, where the gas is more homogeneous, is conventionally highlighted). The heating of the upper atmosphere of the North - the chromosphere and corona - may be due to mechanical factors. energy transferred by waves arising in the upper part of the convective zone, as well as dissipation (absorption) of electrical energy. currents generated by magnetic fields moving along with convective flows.

The existence of a surface convective zone in the north causes a number of other phenomena. Cells of the uppermost tier of the convective zone are observed on the surface of the sun in the form of granules (see). Deeper large-scale movements in the second tier of the zone appear in the form of supergranulation cells and a chromospheric network. There is reason to believe that convection in an even deeper layer is observed in the form of giant structures - cells with dimensions larger than supergranulation.

Large local mag. fields in the zone ± 30 o from the equator lead to the development of the so-called. active areas with spots included in them. The number of active regions, their position on the disk, and the polarity of sunspots in groups change with a period of ≈ 11.2 years. During the unusually high peak of 1957-58. activity affected almost the entire solar disk. In addition to strong local fields in the north, there is a weaker large-scale magnetic field. field. This field changes sign with a period of approx. 22 years and vanishes near the poles at maximum solar activity.

During a large flare, enormous energy is released, ~10 31 -10 32 erg (power ~10 29 erg/s). It is drawn from magnetic energy. active area fields. According to ideas, they have been successfully developing since the 1960s. In the USSR, when magnetic fluxes interact, current layers arise. Development in the current sheet can lead to acceleration of particles, and there are trigger (starting) mechanisms that lead to sudden development of the process.

Rice. 13. Types of impact of a solar flare on the Earth (according to D. X. Menzel).

X-ray radiation and solar cosmic rays coming from the flare (Fig. 13) cause additional ionization of the earth's ionosphere, which affects the conditions for the propagation of radio waves. The flow of particles ejected during the flare reaches the Earth's orbit in about a day and causes a magnetic storm and auroras on Earth (see,).

In addition to the corpuscular flows generated by flares, there is continuous corpuscular radiation C. It is associated with the outflow of rarefied plasma from the outside. regions of the solar corona into interplanetary space - by the solar wind. The loss of matter due to the solar wind is small, ≈ 3. 10 -14 per year, but it represents the basic. component of the interplanetary medium.

The solar wind carries large-scale magnetic field into interplanetary space. field C. Rotation C. twists the lines of the interplanetary magnetic field. fields (IMF) into the Archimedes spiral, which is clearly observed in the ecliptic plane. Since the main feature of large-scale magnetic fields S. yavl. two circumpolar regions of opposite polarity and the fields adjacent to them; with a calm north, the northern hemisphere of interplanetary space turns out to be filled with a field of one sign, the southern hemisphere of another (Fig. 14). Near the activity maximum, due to a change in the sign of the large-scale solar field, a reversal of the polarity of this regular magnetic field occurs. fields of interplanetary space. Magn. the flows of both hemispheres are separated by a current sheet. When rotating C. the Earth is located several times. days, then above and then below the curved “corrugated” surface of the current layer, i.e., it falls into the permafrost, directed either towards the north or away from it. This phenomenon is called. interplanetary magnetic field.

Near the maximum activity, flows of particles accelerated during flares most effectively affect the Earth’s atmosphere and magnetosphere. During the phase of activity decline, towards the end of the 11-year activity cycle, with a decrease in the number of flares and the development of the interplanetary current sheet, stationary flows of enhanced solar wind become more significant. Rotating together with the north, they cause geomagnetic waves that repeat every 27 days. indignation. This recurrent (repeating) activity is especially high at the ends of even-numbered cycles, when the direction of the magnetic field. The fields of the solar "dipole" are antiparallel to the earth's.

Lit.:
Martynov D. Ya., Course of general astrophysics, 3rd ed., M., 1978;
Menzel D. G., Our Sun, trans. from English, M., 1963; Solar and solar-terrestrial physics. Illustrated dictionary of terms, trans. from English, M., 1980;
Shklovsky I.S., Physics of the Solar Corona, 2nd ed., M., 1962;
Severny A.B., Magnetic fields of the Sun and stars, UFN, 1966, vol. 88, v. 1, p. 3-50; - Solar corona - granulation

Has a high temperature. At the surface it is about 5500 degrees Celsius. The Sun has an atmosphere called the corona. This area consists of superheated gas - plasma. Its temperature reaches more than 3 million degrees. And scientists are trying to understand why the outer layer of the Sun is so much hotter than everything that lies underneath.

The problem that confuses scientists is quite simple. Since the source of energy is at the center of the Sun, its body should become progressively cooler as one moves away from the center. But observations suggest the opposite. And so far scientists cannot explain why the Sun's corona is hotter than its other layers.

Old secret

Despite its temperature, the solar corona is usually not visible to an observer on Earth. This is due to the intense brightness of the rest of the Sun. Even sophisticated instruments cannot study it without taking into account the light emanating from the surface of the Sun. But this does not mean that the existence of the solar corona is a recent discovery. It can be observed in rare but predictable events that have fascinated people for thousands of years. These are complete.

In 1869, astronomers took advantage of such an eclipse to study the outer layer of the Sun that suddenly became visible to observation. They pointed spectrometers at the Sun to study the elusive corona material. Researchers discovered an unfamiliar green line in the spectrum of the corona. The unknown substance was named coronium. However, seventy years later, scientists realized that it was a familiar element - iron. But heated to unprecedented millions of degrees.

An early theory said that acoustic waves (think of the Sun's material compressing and expanding like an accordion) could be responsible for the temperature of the corona. In many ways, this is similar to how a wave throws drops of water at high speed onto the shore. But solar probes have been unable to find waves with the power to explain the observed coronal temperature.

For almost 150 years, this mystery has been one of the small but interesting mysteries of science. At the same time, scientists are confident that their knowledge of the temperature both on the surface and in the corona is quite correct.

The Sun's magnetic field: how does it work?

Part of the problem is that we don't understand many of the small events that happen on the Sun. We know how it does its job of warming our planet. But models of the materials and forces involved in this process simply do not exist yet. We cannot yet get close enough to the Sun to study it in detail.

The answer to most questions about the Sun these days is that the Sun is a very complex magnet. The earth also has a magnetic field. But, despite the oceans and underground magma, it is still much denser than the Sun. Which is simply a large clump of gas and plasma. The earth is a harder object.

The sun also rotates. But since it is not solid, its poles and equator rotate at different speeds. Matter moves up and down the layers of the Sun, like in a pan of boiling water. This effect causes disorder in the magnetic field lines. The charged particles that make up the outer layers of the Sun travel along lines such as trains on high-speed rail. These lines break and reconnect, releasing enormous amounts of energy (solar flares). Or they produce vortices full of charged particles, which can be freely thrown from these rails into space at colossal speed (coronal mass ejection).

We have many satellites that are already tracking the Sun. Solarer Pro, launched this year, is just beginning its observations. It will continue its work until 2025. Scientists hope that the mission will provide answers to many mysterious questions about the Sun.

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Earthly life owes its origin to the heavenly body. It warms and illuminates everything on the surface of our planet. It is not without reason that the worship of the Sun and its representation as a great heavenly god was reflected in the cults of the primitive peoples who inhabited the Earth.

Centuries and millennia have passed, but its importance in human life has only increased. We are all children of the Sun.

What is the Sun?

Star from the Galaxy Milky Way, with its geometric shape, representing a huge, hot, gaseous ball, constantly emitting streams of energy. The only source of light and heat in our star-planetary system. Now the Sun is at the age of a yellow dwarf, according to the generally accepted classification of the types of stars in the universe.

Characteristics of the Sun

The sun has the following parameters:

• Age –4.57 billion years;
• Distance to Earth: 149,600,000 km
• Mass: 332,982 Earth masses (1.9891·10³⁰ kg);
• The average density is 1.41 g/cm³ (it increases 100 times from the periphery to the center);
• The orbital speed of the Sun is 217 km/s;
• Rotation speed: 1.997 km/s
• Radius: 695-696 thousand km;
• Temperature: from 5,778 K at the surface to 15,700,000 K at the core;
• Corona temperature: ~1,500,000 K;
• The Sun is stable in its brightness, it is in the 15% of the brightest stars in our Galaxy. It emits less ultraviolet rays, but has more mass compared to similar stars.

What is the Sun made of?

In my own way chemical composition our luminary is no different from other stars and contains: 74.5% hydrogen (by mass), 24.6% helium, less than 1% other substances (nitrogen, oxygen, carbon, nickel, iron, silicon, chromium, magnesium and other substances). Inside the core there are continuous nuclear reactions that convert hydrogen into helium. The absolute majority of the mass solar system– 99.87% belongs to the Sun.

Already this Saturday, August 11, 2018, a new mission to study the Sun - Parker Solar Probe (or the Parker solar probe) will go into space. In a few years, the device will approach the Sun as close as any man-made object has ever achieved. Editorial N+1 with the help of Sergei Bogachev, chief researcher of the laboratory X-ray astronomy Sun FIAN, decided to figure out why scientists are sending a device to such a hot place and what results are expected from it.

When we look at the night sky, we see a huge number of stars - the largest category of objects in the Universe that can be observed from Earth. It is these huge shining balls of gas that many people produce in their thermonuclear “furnaces.” chemical elements heavier than hydrogen and helium, without which our planet, all living things on it, and ourselves would not exist.

The stars are at enormous distances from Earth - the distance to the nearest of them, Proxima Centauri, is estimated at several light years. But there is one star whose light takes only eight minutes to reach us - this is our Sun, and observing it helps us learn more about other stars in the Universe.

The sun is much closer to us than it seems at first glance. In a certain sense, the Earth is inside the Sun - it is constantly washed by the flow of solar wind emanating from the corona - the outer part of the star's atmosphere. It is the flows of particles and radiation from the Sun that control “space weather” near the planets. The appearance of auroras and disturbances in the magnetospheres of planets depends on these flows, while solar flares and coronal mass ejections disable satellites, influence the evolution of life forms on Earth and determine the radiation load on manned space missions. Moreover, similar processes occur not only in the Solar System, but also in other planetary systems. Therefore, understanding the processes in the solar corona and the inner heliosphere allows us to better understand the behavior of the plasma “ocean” surrounding the Earth.

Structure of the Sun

Wikimedia Commons

“Due to the remoteness of the Sun, we receive almost all information about it through the radiation it generates. Even some simple parameters, such as temperature, which on Earth can be measured with an ordinary thermometer, are determined for the Sun and stars in a much more complex way - by the spectrum of their radiation. This also applies to more complex characteristics, for example to a magnetic field. A magnetic field can influence the radiation spectrum by splitting the lines in it - this is the so-called Zeeman effect. And it is precisely because the field changes the spectrum of the star’s radiation that we are able to register it. If such an influence did not exist in nature, then we would know nothing about the magnetic field of stars, since there is no way to fly directly to a star,” says Sergei Bogachev.

“But this method also has limitations - take, for example, the fact that the absence of radiation deprives us of information. If we talk about the Sun, the solar wind does not emit light, so there is no way to remotely determine its temperature, density and other properties. Does not emit light or magnetic field. Yes, in the lower layers solar atmosphere magnetic tubes are filled with luminous plasma and this makes it possible to measure the magnetic field near the surface of the Sun. However, even at a distance of one radius of the Sun from its surface, such measurements are impossible. And there are quite a lot of such examples. What to do in such a situation? The answer is very simple: we need to launch probes that can fly directly to the Sun, plunge into its atmosphere and into the solar wind, and take measurements directly on the spot. Such projects are common, although less well known than space telescope projects, which make remote observations and produce much more spectacular data (such as photographs) than probes that produce a boring stream of numbers and graphs. But if we talk about science, then, of course, few remote observations can compare in power and persuasiveness with the study of an object that is nearby,” continues Bogachev.

Mysteries of the Sun

Observations of the Sun were carried out back in Ancient Greece and in Ancient Egypt, and over the past 70 years, more than a dozen space satellites, interplanetary stations and telescopes, starting from Sputnik-2 and ending with space observatories operating today, such as SDO, SOHO or STEREO, have closely monitored (and are monitoring) the behavior of the closest to us the stars and its surroundings. However, astronomers still have many questions related to the structure of the Sun and its dynamics.

For example, for more than 30 years, scientists have been faced with the problem of solar neutrinos, which consists in the lack of detected electron neutrinos produced in the solar core as a result of nuclear reactions, compared to their theoretically predicted number. Another mystery involves the anomalous heating of the corona. This outermost layer of the star's atmosphere has a temperature of more than a million degrees Kelvin, while the visible surface of the Sun (photosphere), above which the chromosphere and corona are located, is heated to only six thousand degrees Kelvin. This seems strange, because logically, the outer layers of the star should be cooler. Direct heat transfer between the photosphere and the corona is not enough to ensure such temperatures, which means that other mechanisms for heating the corona are at work here.

The Sun's corona during a total solar eclipse in August 2017.

NASA's Goddard Space Flight Center/Gopalswamy

There are two main theories to explain this anomaly. According to the first, magnetoacoustic waves and Alfven waves, which, scattering in the corona, increase the plasma temperature, are responsible for the transfer of heat from the convective zone and photosphere of the Sun to the chromosphere and corona. However, this version has a number of disadvantages, for example, magnetoacoustic waves cannot ensure the transfer of a sufficiently large amount of energy into the corona due to scattering and reflection back into the photosphere, and Alfven waves relatively slowly convert their energy into thermal energy of the plasma. In addition, for a long time there was simply no direct evidence of wave propagation through the solar corona - only in 1997, the SOHO space observatory first recorded magnetoacoustic solar waves at a frequency of one millihertz, which provide only ten percent of the energy required to heat the corona to the observed temperatures

The second theory associates the anomalous heating of the corona with constantly occurring microflares that arise due to the continuous reconnection of magnetic lines in local regions of the magnetic field in the photosphere. This idea was proposed in the 1980s by American astronomer Eugene Parker, whose name the probe is named after and who also predicted the presence of the solar wind, a stream of high-energy charged particles continuously emitted by the Sun. However, the theory of microflares has also not yet been confirmed. It is possible that both mechanisms work on the Sun, but this needs to be proven, and for this you need to fly to the Sun at a fairly close distance.

Another mystery of the Sun is connected with the corona - the mechanism for the formation of the solar wind, which fills the entire Solar System. It is on this that space weather phenomena such as the northern lights or magnetic storms. Astronomers are interested in the mechanisms of the emergence and acceleration of the slow solar wind generated in the corona, as well as the role of magnetic fields in these processes. Here, too, there are several theories that have both evidence and shortcomings, and the Parker probe is expected to help dot the i’s.

“In general, there are now fairly well-developed models of the solar wind that predict how its characteristics should change as it moves away from the Sun. The accuracy of these models is quite high at distances on the order of the Earth's orbit, but how accurately they describe the solar wind at close distances from the Sun is not clear. Perhaps Parker can help with this. Another rather interesting question is the acceleration of particles on the Sun. After flares, streams come to the Earth large number accelerated electrons and protons. It is not entirely clear, however, whether their acceleration occurs directly on the Sun, and then they simply move towards the Earth by inertia, or whether these particles are additionally (and perhaps completely) accelerated on their way to the Earth by interplanetary magnetic field. Perhaps, when data collected by a probe near the Sun comes to Earth, this issue can also be sorted out. There are several more similar problems, the solution of which can be advanced in the same way - by comparing similar measurements near the Sun and at the level of the Earth's orbit. In general, the mission is aimed at resolving such issues. We can only hope that the device will be successful,” says Sergei Bogachev.

Straight to hell

The Parker probe will be launched on August 11, 2018 from the SLC-37 launch complex at the US Air Force Base at Cape Canaveral, it will be launched into space by a heavy launch vehicle Delta IV Heavy - this is the most powerful rocket in operation, it can be launched into low orbit almost 29 tons of cargo. It is surpassed only in terms of carrying capacity, but this carrier is still in the testing stage. To get to the center of the solar system, it is necessary to reduce the very high speed that the Earth (and all objects on it) has relative to the Sun - about 30 kilometers per second. In addition to a powerful rocket, this will require a series of gravity maneuvers near Venus.

According to the plan, the process of approaching the Sun will last seven years - with each new orbit (there are 24 in total), the device will come closer and closer to the star. The first perihelion will be passed on November 1, at a distance of 35 solar radii (about 24 million kilometers) from the star. Then, after a series of seven gravity maneuvers near Venus, the device will approach the Sun to a distance of about 9-10 solar radii (about six million kilometers) - this will happen in mid-December 2024. This is seven times closer than the perihelion of Mercury's orbit, never before man-made spacecraft did not get so close to the Sun (the current record belongs to the Helios-B apparatus, which approached the star at 43.5 million kilometers).

Scheme of the flight to the Sun and the main working orbits of the probe.

The main stages of work on each of the orbits.

The choice of such a position for observations is not accidental. According to scientists' calculations, at a distance of ten radii from the Sun there is the Alfven point - the region where the solar wind accelerates so much that it leaves the Sun, and waves propagating in the plasma no longer affect it. If the probe can get close to the Alfven point, then we can assume that it has entered the solar atmosphere and touched the Sun.

The Parker probe, assembled, during installation on the third stage of the launch vehicle.

“The probe’s task is to measure the main characteristics of the solar wind and solar atmosphere along its trajectory. The scientific instruments on board are not unique and do not have record-breaking characteristics (except for the ability to withstand solar radiation fluxes at the perihelion of the orbit). Parker Solar Probe is a vehicle with conventional instruments, but in a unique orbit. Most (and perhaps even all) scientific instruments are planned to be kept off in all parts of the orbit except at perihelion, where the vehicle is closest to the Sun. In a sense, this scientific program additionally emphasizes that the main objective of the mission is to study the solar wind and solar atmosphere. When the device moves away from perihelion, data from the same instruments will turn into ordinary data, and in order to preserve the resource of scientific instruments, they will simply be switched to the background until the next approach. In this sense, the ability to enter a given trajectory and the ability to live on it for a given time are the factors on which the success of the mission will primarily depend,” says Sergei Bogachev.

The Parker heat shield device.

Greg Stanley/Johns Hopkins University

View of the heat protection shield at the stage of installation on the probe.

NASA/Johns Hopkins APL/Ed Whitman

Parker probe with installed heat shield.

NASA/Johns Hopkins APL/Ed Whitman

To survive close to the star, the probe is equipped with a heat shield that acts as an “umbrella” under which all scientific instruments will hide. The front part of the shield will withstand temperatures of more than 1400 degrees Celsius, while the temperature of its rear part, where the scientific instruments are located, should not exceed thirty degrees Celsius. This temperature difference is ensured by the special design of this “solar umbrella”. With a total thickness of just 11.5 centimeters, it consists of two panels made of carbon-graphite composite, between which is a layer of carbon foam. The front of the shield has a protective coating and a white ceramic layer that increases its reflective properties.

In addition to the shield, the problem of overheating is designed to be solved by a cooling system that uses 3.7 liters of deionized water under pressure as a coolant. The device's electrical wiring is made using high-temperature materials such as sapphire tubes and niobium, and during approaches to the Sun, the solar panels will be retracted under a thermal shield. In addition to the intense heat, mission engineers will have to take into account the strong light pressure from the Sun, which will throw off the correct orientation of the probe. To facilitate this work, solar sensors are installed on the probe in different places to help monitor the protection of scientific equipment from the sun.

Tools

Almost all of the probe’s scientific instruments are “tailored” to study electromagnetic fields and the properties of the solar plasma surrounding it. The only exception is the WISPR (Wide-field Imager for Solar PRobe) optical telescope, whose task will be to obtain images of the solar corona and solar wind, the inner heliosphere, shock waves and any other structures observed by the device.