Sunny wind. Facts and theory
V.B. Baranov, Moscow State University them. M.V. Lomonosov
The article deals with the problem of supersonic expansion of the solar corona (solar wind). Four main problems are analyzed: 1) the reasons for the outflow of plasma from the solar corona; 2) whether such an outflow is homogeneous; 3) change in solar wind parameters with distance from the Sun and 4) how the solar wind flows out into the interstellar medium.
Introduction
Almost 40 years have passed since the American physicist E. Parker theoretically predicted a phenomenon called the "solar wind" and which, a couple of years later, was experimentally confirmed by the group of the Soviet scientist K. Gringauz using instruments mounted on the Luna- 2" and "Luna-3". sunny wind is a stream of fully ionized hydrogen plasma, that is, a gas consisting of electrons and protons of approximately the same density (quasi-neutrality condition), which moves from the Sun at a high supersonic speed. In the Earth's orbit (one astronomical unit (AU) from the Sun), the velocity VE of this stream is approximately 400-500 km/s, the concentration of protons (or electrons) ne = 10-20 particles per cubic centimeter, and their temperature Te is approximately 100,000 K (the electron temperature is somewhat higher).
In addition to electrons and protons, alpha particles (on the order of a few percent), a small amount of heavier particles, and a magnetic field have been detected in interplanetary space. average value whose induction turned out to be in the Earth's orbit of the order of several gammas (1
= 10-5 Gs).A bit of history related to the theoretical prediction of the solar wind
During not so much long history In theoretical astrophysics, it was believed that all the atmospheres of stars are in hydrostatic equilibrium, that is, in a state when the force of the gravitational attraction of a star is balanced by the force associated with the pressure gradient in its atmosphere (with a change in pressure per unit distance r from the center of the star). Mathematically, this equilibrium is expressed in the form of an ordinary differential equation
| (1) |
where G is the gravitational constant, M* is the mass of the star, p is the atmospheric gas pressure,
is its mass density. If the temperature distribution T in the atmosphere is given, then from the equilibrium equation (1) and the equation of state for an ideal gas| (2) |
where R is the gas constant, the so-called barometric formula is easily obtained, which in the particular case of a constant temperature T will have the form
| (3) |
In formula (3), p0 is the pressure at the base of the stellar atmosphere (at r = r0). It can be seen from this formula that for r
, that is, at very large distances from the star, the pressure p tends to a finite limit, which depends on the value of the pressure p0.Since it was believed that the solar atmosphere, as well as the atmospheres of other stars, is in a state of hydrostatic equilibrium, its state was determined by formulas similar to formulas (1), (2), (3) . Taking into account the unusual and not yet fully understood phenomenon of a sharp increase in temperature from about 10,000 degrees on the surface of the Sun to 1,000,000 degrees in the solar corona, Chapman (see, for example) developed the theory of a static solar corona, which should have smoothly passed into the interstellar medium surrounding the solar system.
However, in his pioneering work, Parker noticed that the pressure at infinity, obtained from a formula like (3) for the static solar corona, turns out to be almost an order of magnitude greater than the pressure value that was estimated for interstellar gas from observations. To eliminate this discrepancy, Parker suggested that the solar corona is not in a state of static equilibrium, but is continuously expanding into the interplanetary medium surrounding the Sun. At the same time, instead of the equilibrium equation (1), he proposed to use a hydrodynamic equation of motion of the form
| (4) |
where in the coordinate system associated with the Sun, the value V is the radial velocity of the plasma. Under
refers to the mass of the sun.For a given temperature distribution Т, the system of equations (2) and (4) has solutions of the type shown in Figs. 1. In this figure, a denotes the speed of sound, and r* is the distance from the origin at which the gas speed is equal to the speed of sound (V = a). Obviously, only curves 1 and 2 in Figs. 1 have a physical meaning for the problem of gas outflow from the Sun, since curves 3 and 4 have non-unique velocities at each point, and curves 5 and 6 correspond to very high velocities in solar atmosphere which is not observed in telescopes. Parker analyzed the conditions under which a solution corresponding to curve 1 is implemented in nature. He showed that in order to match the pressure obtained from such a solution with the pressure in the interstellar medium, the most realistic case is the transition of gas from a subsonic flow (at r< r*) к сверхзвуковому (при r >r*), and called this current the solar wind. However, this statement was disputed in the work by Chamberlain, who considered the most realistic solution corresponding to curve 2, which describes the subsonic "solar breeze" everywhere. At the same time, the first experiments on spacecraft (see, for example,), which discovered supersonic gas flows from the Sun, did not seem, judging by the literature, to Chamberlain sufficiently reliable.
| Rice. 1. Possible solutions of one-dimensional equations of gas dynamics for the velocity V of gas flow from the surface of the Sun in the presence of gravitational force. Curve 1 corresponds to the solution for the solar wind. Here a is the speed of sound, r is the distance from the Sun, r* is the distance at which the gas speed is equal to the speed of sound, is the radius of the Sun. |
The history of experiments in outer space brilliantly proved the correctness of Parker's ideas about the solar wind. Detailed material on the theory of the solar wind can be found, for example, in the monograph.
Ideas about the uniform outflow of plasma from the solar corona
From the one-dimensional equations of gas dynamics, one can obtain the well-known result: in the absence of body forces, a spherically symmetric gas flow from a point source can be either subsonic or supersonic everywhere. The presence of the gravitational force (right side) in equation (4) leads to the appearance of solutions like curve 1 in Fig. 1, that is, with the transition through the speed of sound. Let us draw an analogy with the classical flow in the Laval nozzle, which is the basis of all supersonic jet engines. Schematically, this flow is shown in Fig. 2.
| Rice. Fig. 2. Scheme of flow in the Laval nozzle: 1 - a tank, called a receiver, into which very hot air is supplied at a low speed, 2 - the area of the geometric compression of the channel in order to accelerate the subsonic gas flow, 3 - the area of the geometric expansion of the channel in order to accelerate the supersonic flow. |
Tank 1, called the receiver, is supplied with gas heated to a very high temperature at a very low speed (the internal energy of the gas is much greater than its kinetic energy of directed motion). By means of a geometric compression of the channel, the gas is accelerated in region 2 (subsonic flow) until its speed reaches the speed of sound. For its further acceleration, it is necessary to expand the channel (region 3 of the supersonic flow). In the entire flow region, gas is accelerated due to its adiabatic (without heat supply) cooling (the internal energy of chaotic motion is converted into the energy of directed motion).
In the considered problem of the formation of the solar wind, the role of the receiver is played by the solar corona, and the role of the walls of the Laval nozzle is played by the gravitational force of solar attraction. According to Parker's theory, the transition through the speed of sound should occur somewhere at a distance of several solar radii. However, an analysis of the solutions obtained in the theory showed that the temperature of the solar corona is not enough for its gas to be accelerated to supersonic speeds, as is the case in the Laval nozzle theory. There must be some additional source of energy. Such a source is currently considered to be the dissipation of wave motions always present in the solar wind (sometimes called plasma turbulence), superimposed on the mean flow, and the flow itself is no longer adiabatic. Quantitative analysis of such processes still requires further research.
Interestingly, ground-based telescopes detect magnetic fields on the surface of the Sun. The average value of their magnetic induction B is estimated at 1 G, although in individual photospheric formations, for example, in spots, the magnetic field can be orders of magnitude larger. Since plasma is a good conductor of electricity, it is natural that the solar magnetic fields interact with its flows from the Sun. In this case, a purely gas-dynamic theory gives an incomplete description of the phenomenon under consideration. The influence of the magnetic field on the flow of the solar wind can only be considered within the framework of a science called magnetohydrodynamics. What are the results of such considerations? According to pioneering work in this direction (see also ), the magnetic field leads to the appearance of electric currents j in the plasma of the solar wind, which, in turn, leads to the appearance of a ponderomotive force j x B, which is directed in a direction perpendicular to the radial direction. As a result, the solar wind has a tangential velocity component. This component is almost two orders of magnitude smaller than the radial one, but it plays a significant role in the removal of angular momentum from the Sun. It is assumed that the latter circumstance may play a significant role in the evolution not only of the Sun, but also of other stars in which a "stellar wind" has been discovered. In particular, to explain the sharp decrease in the angular velocity of stars of the late spectral type, the hypothesis of the transfer of rotational momentum to the planets formed around them is often invoked. The considered mechanism of the loss of the angular momentum of the Sun by the outflow of plasma from it opens up the possibility of revising this hypothesis.
Imagine that you heard the words of the announcer in the weather forecast: “Tomorrow the wind will pick up sharply. In this regard, interruptions in the operation of radio, mobile communications and the Internet are possible. US space mission delayed. Intense auroras are expected in the north of Russia…”.
You will be surprised: what nonsense, what does the wind have to do with it? But the fact is that you missed the beginning of the forecast: “Last night there was a solar flare. A powerful stream of solar wind is moving towards the Earth…”.
Ordinary wind is the movement of air particles (molecules of oxygen, nitrogen and other gases). A stream of particles also rushes from the Sun. It is called the solar wind. If you do not delve into hundreds of cumbersome formulas, calculations and heated scientific disputes, then, in general, the picture appears as follows.
Thermonuclear reactions are going on inside our luminary, heating up this huge ball of gases. The temperature of the outer layer - the solar corona reaches a million degrees. This causes the atoms to move at such speed that when they collide, they smash each other to smithereens. It is known that a heated gas tends to expand and occupy a larger volume. Something similar is happening here. Particles of hydrogen, helium, silicon, sulfur, iron and other substances scatter in all directions.
They are gaining more and more speed and in about six days they reach the near-Earth borders. Even if the sun was calm, the speed of the solar wind reaches here up to 450 kilometers per second. Well, when the solar flare erupts a huge fiery bubble of particles, their speed can reach 1200 kilometers per second! And you can’t call it a refreshing “breeze” - about 200 thousand degrees.
Can a person feel the solar wind?
Indeed, since the flow of hot particles is constantly rushing, why don't we feel how it "blows" us? Suppose the particles are so small that the skin does not feel their touch. But they are not noticed by terrestrial devices either. Why?
Because the Earth is protected from solar vortices by its magnetic field. The flow of particles flows around it, as it were, and rushes further. It is only on days when solar emissions are particularly strong that our magnetic shield has a hard time. A solar hurricane breaks through it and bursts into the upper atmosphere. Alien particles cause . The magnetic field is sharply deformed, forecasters talk about "magnetic storms." 
Because of them, space satellites go out of control. Planes disappear from the radar screens. Radio waves are interfered with and communications are disrupted. On such days, satellite dishes are turned off, flights are canceled, and “communication” with spacecraft is interrupted. In electrical networks, railway rails, pipelines, an electric current is suddenly born. From this, traffic lights switch by themselves, gas pipelines rust, and disconnected electrical appliances burn out. Plus, thousands of people feel discomfort and discomfort.
The cosmic effects of the solar wind can be detected not only during flares on the Sun: it is, albeit weaker, but blows constantly.
It has long been observed that the tail of a comet grows as it approaches the Sun. It causes the frozen gases that form the comet's nucleus to evaporate. And the solar wind carries these gases in the form of a plume, always directed in the opposite direction from the Sun. So the terrestrial wind turns the smoke from the chimney and gives it one form or another.
During years of increased activity, the Earth's exposure to galactic cosmic rays drops sharply. The solar wind is gaining such strength that it simply sweeps them to the outskirts of the planetary system.
There are planets in which the magnetic field is very weak, if not completely absent (for example, on Mars). Here nothing prevents the solar wind from roaming. Scientists believe that it was he who, over hundreds of millions of years, almost “blew out” its atmosphere from Mars. Because of this, the orange planet lost sweat and water and, possibly, living organisms.
Where does the solar wind subside?
Nobody knows the exact answer yet. Particles fly to the vicinity of the Earth, picking up speed. Then it gradually falls, but it seems that the wind reaches the farthest corners of the solar system. Somewhere there it weakens and is decelerated by rarefied interstellar matter.
So far, astronomers cannot say exactly how far this happens. To answer, you need to catch particles, flying farther and farther from the Sun, until they stop coming across. By the way, the limit where this will happen can be considered the boundary of the solar system. 
Traps for the solar wind are equipped with spacecraft that are periodically launched from our planet. In 2016, solar wind streams were captured on video. Who knows if he will not become the same familiar "character" of weather reports as our old friend - the earth's wind?
Solar wind and Earth's magnetosphere.
Sunny wind ( solar wind) is a stream of mega-ionized particles (mainly helium-hydrogen plasma) flowing from the solar corona at a speed of 300-1200 km/s into the surrounding space. It is one of the main components of the interplanetary medium.
Lots of natural phenomena associated with the solar wind, including space weather phenomena such as magnetic storms and auroras.
The concepts of "solar wind" (a stream of ionized particles flying from the Sun to 2-3 days) and "sunshine" (a stream of photons flying from the Sun to the Earth in an average of 8 minutes 17 seconds) should not be confused. In particular, it is the effect of sunlight pressure (and not wind) that is used in the projects of the so-called solar sails. A form of engine for using an impulse of solar wind ions as a thrust source - an electric sail.
History
The existence of a constant stream of particles flying from the Sun was first proposed by the British astronomer Richard Carrington. In 1859, Carrington and Richard Hodgson independently observed what was later called a solar flare. The following day, a geomagnetic storm occurred, and Carrington suggested a connection between these phenomena. Later, George Fitzgerald suggested that matter is periodically accelerated by the Sun and reaches the Earth in a few days.
In 1916, the Norwegian explorer Christian Birkeland wrote: "From a physical point of view, it is most probable that the rays of the sun are neither positive nor negative, but both." In other words, the solar wind is made up of negative electrons and positive ions.
Three years later, in 1919, Friederik Lindemann also suggested that particles of both charges, protons and electrons, come from the Sun.
In the 1930s, scientists determined that the temperature of the solar corona must reach a million degrees, since the corona remains bright enough at a great distance from the Sun, which is clearly visible during solar eclipses. Later spectroscopic observations confirmed this conclusion. In the mid-1950s, British mathematician and astronomer Sidney Chapman determined the properties of gases at such temperatures. It turned out that the gas becomes an excellent conductor of heat and should dissipate it into space beyond the Earth's orbit. At the same time, German scientist Ludwig Biermann became interested in the fact that comet tails always point away from the Sun. Biermann postulated that the Sun emits a constant stream of particles that pressurize the gas surrounding the comet, forming a long tail.
In 1955, Soviet astrophysicists S. K. Vsekhsvyatsky, G. M. Nikolsky, E. A. Ponomarev and V. I. Cherednichenko showed that an extended corona loses energy through radiation and can be in a state of hydrodynamic equilibrium only with a special distribution of powerful internal energy sources. In all other cases, there must be a flow of matter and energy. This process serves as a physical basis for an important phenomenon - the "dynamic corona". The magnitude of the matter flux was estimated from the following considerations: if the corona were in hydrostatic equilibrium, then the heights of a homogeneous atmosphere for hydrogen and iron would be related as 56/1, that is, iron ions should not be observed in the far corona. But it's not. Iron glows throughout the corona, with FeXIV observed in higher layers than FeX, although the kinetic temperature is lower there. The force that maintains the ions in a "suspended" state can be the momentum transmitted during collisions by the ascending proton flux to the iron ions. From the condition of the balance of these forces, it is easy to find the flux of protons. It turned out to be the same as that followed from the hydrodynamic theory, subsequently confirmed by direct measurements. For 1955, this was a significant achievement, but no one then believed in the "dynamic crown".
Three years later, Eugene Parker concluded that the hot current from the Sun in Chapman's model and the stream of particles blowing away cometary tails in Biermann's hypothesis are two manifestations of the same phenomenon, which he called "solar wind". Parker showed that even though the solar corona is strongly attracted by the Sun, it conducts heat so well that it remains hot at great distances. Since its attraction weakens with distance from the Sun, a supersonic outflow of matter into interplanetary space begins from the upper corona. Moreover, Parker was the first to point out that the effect of weakening gravity has the same effect on the hydrodynamic flow as the Laval nozzle: it produces a transition of the flow from the subsonic to the supersonic phase.
Parker's theory has been heavily criticized. An article submitted in 1958 to the Astrophysical Journal was rejected by two reviewers and only thanks to the editor, Subramanyan Chandrasekhar, made it to the pages of the journal.
However, in January 1959, the first direct measurements of the characteristics of the solar wind (Konstantin Gringauz, IKI RAS) were carried out by the Soviet Luna-1, using a scintillation counter and a gas ionization detector installed on it. Three years later, the same measurements were carried out by the American Marcia Neugebauer using data from the Mariner-2 station.
Yet the acceleration of the wind to high speeds was not yet understood and could not be explained from Parker's theory. The first numerical models of the solar wind in the corona using the equations of magnetohydrodynamics were created by Pneumann and Knopp in 1971.
In the late 1990s, using the Ultraviolet Coronal Spectrometer ( Ultraviolet Coronal Spectrometer (UVCS) ) observations were made on board of the regions where the fast solar wind originated at the solar poles. It turned out that the wind acceleration is much greater than expected from purely thermodynamic expansion. Parker's model predicted that the wind speed becomes supersonic at 4 solar radii from the photosphere, and observations have shown that this transition occurs much lower, at about 1 solar radii, confirming that there is an additional mechanism for accelerating the solar wind.
Characteristics
The heliospheric current sheet is the result of the influence of the Sun's rotating magnetic field on the plasma in the solar wind.
Due to the solar wind, the Sun loses about one million tons of matter every second. The solar wind consists mainly of electrons, protons, and helium nuclei (alpha particles); the nuclei of other elements and non-ionized particles (electrically neutral) are contained in a very small amount.
Although the solar wind comes from the outer layer of the Sun, it does not reflect the real composition of the elements in this layer, since as a result of differentiation processes, the abundance of some elements increases and some decreases (FIP effect).
The intensity of the solar wind depends on changes in solar activity and its sources. Long-term observations in the Earth's orbit (about 150 million km from the Sun) have shown that the solar wind is structured and is usually divided into calm and disturbed (sporadic and recurrent). Calm flows, depending on the speed, are divided into two classes: slow(approximately 300-500 km / s near the Earth's orbit) and fast(500-800 km/s near the Earth's orbit). Sometimes the region of the heliospheric current sheet, which separates regions of different polarity of the interplanetary magnetic field, is referred to as a stationary wind, and is close in its characteristics to a slow wind.
slow solar wind
The slow solar wind is generated by the "calm" part of the solar corona (the region of coronal streamers) during its gas-dynamic expansion: at a corona temperature of about 2 10 6 K, the corona cannot be in hydrostatic equilibrium, and this expansion, under the existing boundary conditions, should lead to acceleration of the matter to supersonic speeds. The heating of the solar corona to such temperatures occurs due to the convective nature of heat transfer in the solar photosphere: the development of convective turbulence in plasma is accompanied by the generation of intense magnetosonic waves; in turn, when propagating in the direction of decreasing the density of the solar atmosphere, sound waves are transformed into shock waves; shock waves are effectively absorbed by the material of the corona and heat it up to a temperature of (1-3) 10 6 K.
fast solar wind
Streams of the recurrent fast solar wind are emitted by the Sun for several months and have a return period of 27 days (the rotation period of the Sun) when observed from the Earth. These streams are associated with coronal holes - regions of the corona with a relatively low temperature (approximately 0.8·10 6 K), reduced plasma density (only a quarter of the density of quiet regions of the corona) and a magnetic field radial with respect to the Sun.
Disturbed flows
Disturbed flows include the interplanetary manifestation of coronal mass ejections (CMEs), as well as compression regions in front of fast CMEs (called Sheath in the English literature) and in front of fast flows from coronal holes (called the Corotating interaction region - CIR in the English literature). About half of the cases of Sheath and CIR observations may have an interplanetary shock ahead of them. It is in perturbed solar wind types that the interplanetary magnetic field can deviate from the ecliptic plane and contain a southern field component, which leads to many effects of space weather (geomagnetic activity, including magnetic storms). Disturbed sporadic outflows were previously thought to be caused by solar flares, but sporadic outflows in the solar wind are now believed to be due to CMEs. At the same time, it should be noted that both solar flares and coronal mass ejections are associated with the same energy sources on the Sun, and there is a statistical dependence between them.
According to the observation time of various large-scale solar wind types, fast and slow streams make up about 53%, the heliospheric current sheet 6%, CIR - 10%, CME - 22%, Sheath - 9%, and the ratio between the observation time of various types varies greatly in the solar cycle. activity.
Phenomena generated by the solar wind
Due to the high conductivity of the solar wind plasma, the solar magnetic field is frozen into the outflowing wind currents and is observed in the interplanetary medium in the form of an interplanetary magnetic field.
The solar wind forms the boundary of the heliosphere, due to which it prevents penetration into. The magnetic field of the solar wind significantly weakens the galactic cosmic rays coming from outside. A local increase in the interplanetary magnetic field leads to short-term decreases in cosmic rays, Forbush decreases, and large-scale field decreases lead to their long-term increases. Thus, in 2009, during the period of a protracted minimum of solar activity, the intensity of radiation near the Earth increased by 19% relative to all previously observed maxima.
The solar wind generates in the solar system, possessing a magnetic field, phenomena such as the magnetosphere, aurora and radiation belts of planets.
Constant radial flux of solar plasma. crowns in interplanetary production. The flow of energy coming from the bowels of the Sun heats the plasma of the corona up to 1.5-2 million K. Post. heating is not balanced by the loss of energy due to radiation, since the corona is small. Excess energy means. degree carry away h-tsy S. century. (=1027-1029 erg/s). The crown, therefore, is not in hydrostatic. equilibrium, it is constantly expanding. According to the composition of S. century. does not differ from the plasma of the corona (S. century contains chiefly arr. protons, electrons, a few helium nuclei, oxygen ions, silicon, sulfur, and iron). At the base of the corona (10,000 km from the solar photosphere) h-tsy have a radial order of hundreds of m / s, at a distance of several. solar radii, it reaches the speed of sound in plasma (100 -150 km / s), near the Earth's orbit, the speed of protons is 300-750 km / s, and their space. - from several h-ts up to several tens of hours in 1 cm3. With the help of interplanetary space. stations found that up to the orbit of Saturn, the density flow h-c S. v. decreases according to the law (r0/r)2, where r is the distance from the Sun, r0 is the initial level. S. v. carries with it the loops of the lines of force of the suns. magn. fields, to-rye form interplanetary magn. . Combination of radial movement of h-c S. century. with the rotation of the Sun gives these lines the shape of spirals. Large-scale structure of the magnet. The field in the vicinity of the Sun has the form of sectors, in which the field is directed away from the Sun or towards it. The size of the cavity occupied by the SV is not exactly known (its radius, apparently, is not less than 100 AU). At the boundaries of this cavity dynamic. S. v. must be balanced by the pressure of interstellar gas, galactic. magn. fields and galactic space rays. In the vicinity of the Earth, the collision of the flow of c-c S. v. with geomagnetic field generates a stationary shock wave in front of the Earth's magnetosphere (from the side of the Sun, Fig.).
S. v. as if it flows around the magnetosphere, limiting its extent in the pr-ve. Changes in the intensity of S. century associated with solar flares, yavl. main the cause of geomagnetic disturbances. fields and magnetospheres (magnetic storms).
Over the Sun loses with S. in. \u003d 2X10-14 part of its mass Msun. It is natural to assume that an outflow of water, similar to S. V., also exists in other stars (""). It should be especially intense for massive stars (with a mass = several tens of Msolns) and with a high surface temperature (= 30-50 thousand K) and for stars with an extended atmosphere (red giants), because in In the first case, parts of a highly developed stellar corona have a sufficiently high energy to overcome the attraction of the star, and in the second, they have a low parabolic. speed (escape speed; (see SPACE SPEEDS)). Means. mass losses with the stellar wind (= 10-6 Msol/yr and more) can significantly affect the evolution of stars. In turn, the stellar wind creates "bubbles" of hot gas in the interstellar medium - sources of X-rays. radiation.
Physical Encyclopedic Dictionary. - M.: Soviet Encyclopedia. . 1983 .
SOLAR WIND - a continuous flow of plasma of solar origin, the Sun) into interplanetary space. At high temperatures, which exist in the solar corona (1.5 * 10 9 K), the pressure of the overlying layers cannot balance the gas pressure of the corona substance, and the corona expands.
The first evidence of the existence of post. plasma flux from the Sun obtained by L. Birman (L. Biermann) in the 1950s. on the analysis of the forces acting on the plasma tails of comets. In 1957, J. Parker (E. Parker), analyzing the equilibrium conditions of the substance of the crown, showed that the crown cannot be in hydrostatic conditions. Wed S.'s characteristics are given in table. 1. Flows of S. in. can be divided into two classes: slow - with a speed of 300 km / s and fast - with a speed of 600-700 km / s. Fast streams come from regions of the solar corona, where the structure of the magnetic. field is close to radial. coronal holes. Slow streams. in. associated, apparently, with the areas of the crown, in which there is a means Tab. one. - Average characteristics of the solar wind in Earth's orbit
Speed | |
Proton concentration | |
Proton temperature | |
Electron temperature | |
Magnetic field strength | |
Python Flux Density.... | 2.4*10 8 cm -2 *c -1 |
Kinetic energy flux density | 0.3 erg*cm -2 *s -1 |
Tab. 2.- Relative chemical composition solar wind
Relative content |
|
Relative content |
|
In addition to the main the components of S. century - protons and electrons, - particles were also found in its composition. Measurements of ionization. temperature of ions S. century. make it possible to determine the electron temperature of the solar corona.
In S. century. differences are observed. types of waves: Langmuir, whistlers, ion-sound, Plasma waves). Some of the Alfvén type waves are generated on the Sun, and some are excited in the interplanetary medium. The generation of waves smooths out the deviations of the function of the distribution of particles from the Maxwellian and, in conjunction with the influence of the magnetic. field on the plasma leads to the fact that S. century. behaves like a continuum. Waves of the Alfvén type play a large role in the acceleration of the small components of C. 
Rice. 1. Massive solar wind. On the horizontal axis - the ratio of the mass of the particle to its charge, on the vertical - the number of particles registered in the energy window of the device for 10 s. The numbers with a "+" sign indicate the charge of the ion.
S.'s stream in. is supersonic in relation to the speeds of those types of waves, to-rye provide eff. energy transfer in S. century. (Alvenov, sound). Alvenovskoye and sound Mach number C. in. 7. When flowing around S. in. obstacles capable of effectively deflecting it (the magnetic fields of Mercury, Earth, Jupiter, Saturn or the conducting ionospheres of Venus and, apparently, Mars), an outgoing bow shock wave is formed. waves, which allows it to flow around an obstacle. At the same time in S. century. a cavity is formed - the magnetosphere (own or induced), the shape and size of the swarm are determined by the balance of magnetic pressure. field of the planet and the pressure of the flowing plasma flow (see Fig. Magnetosphere of the Earth, Magnetosphere of planets). In the case of interaction S. century. with a non-conducting body (eg, the Moon), a shock wave does not occur. The plasma flow is absorbed by the surface, and a cavity is formed behind the body, which is gradually filled with plasma C. in.
The stationary process of corona plasma outflow is superimposed by nonstationary processes associated with flares on the sun. With strong outbreaks, matter is ejected from the bottom. regions of the corona into the interplanetary medium. magnetic variations). 
Rice. 2. Propagation of an interplanetary shock wave and ejecta from a solar flare. The arrows show the direction of motion of the solar wind plasma,
Rice. 3. Types of solutions to the corona expansion equation. Speed and distance are normalized to critical speed v to and critical distance R to. Solution 2 corresponds to the solar wind.
The expansion of the solar corona is described by a system of ur-tions of conservation of mass, v k) on some critical. distance R to and subsequent expansion at supersonic speed. This solution gives a vanishingly small value of the pressure at infinity, which makes it possible to match it with the low pressure of the interstellar medium. Yu. Parker called the course of this type S. century. 
, where m is the mass of the proton, is the adiabatic index, is the mass of the Sun. On fig. 4 shows the change in expansion rate with heliocentric. thermal conductivity, viscosity, 
Rice. Fig. 4. Solar wind velocity profiles for the isothermal model of a miconic corona at different values coronal temperature.
S. v. provides the main outflow of thermal energy of the corona, since heat transfer to the chromosphere, el.-mag. coronas and electronic thermal conductivitypp. in. insufficient to establish heat balance crowns. Electronic thermal conductivity provides a slow decrease in the temperature of S. in. with distance. luminosity of the sun.
S. v. carries the coronal magnetic field with it into the interplanetary medium. field. The lines of force of this field frozen into the plasma form the interplanetary magnetic field. field (MMP). Although the intensity of the IMF is small and its energy density is approx. 1% of the density of the kinetic. energy S. v., it plays an important role in the thermodynamics of S. in. and in the dynamics of S.'s interactions. with the bodies of the solar system, as well as the flows of S. in. between themselves. Combination of S.'s expansion. with the rotation of the Sun leads to the fact that the magn. the lines of force frozen in the S. century have the form, B R and the azimuth components of the magnetic. fields change differently with distance near the plane of the ecliptic: 
where - ang. sun rotation speed And - radial component of velocity c., index 0 corresponds to the initial level. At a distance of the Earth's orbit, the angle between the direction of the magnetic. fields and R about 45°. At large L magn. 
Rice. 5. The shape of the field line of the interplanetary magnetic field. - the angular velocity of the rotation of the Sun, and - the radial component of the plasma velocity, R - the heliocentric distance.
S. v., arising over the regions of the Sun with decomp. magnetic orientation. fields, speed, temp-pa, concentration of particles, etc.) also cf. regularly change in the cross section of each sector, which is associated with the existence of a fast S. flow within the sector. The boundaries of the sectors are usually located in the intraslow flow of S. at. Most often, 2 or 4 sectors are observed, rotating with the Sun. This structure which is formed at S.'s pulling out of century. large-scale magnetic field of the crown, can be observed for several. revolutions of the sun. The sectoral structure of the IMF is a consequence of the existence of a current sheet (TS) in the interplanetary medium, which rotates together with the Sun. TS creates a magnetic surge. fields - radial IMF have different signs according to different sides TS. This TS, predicted by H. Alfven, passes through those parts of the solar corona, which are associated with active regions on the Sun, and separates these regions from decomp. signs of the radial component of the solar magnet. fields. The TC is located approximately in the plane of the solar equator and has a folded structure. The rotation of the Sun leads to the twisting of the CS folds into a spiral (Fig. 6). Being near the plane of the ecliptic, the observer turns out to be either above or below the CS, due to which he falls into sectors with different signs of the IMF radial component.
Near the Sun in the N. century. there are longitudinal and latitudinal velocity gradients of collisionless shock waves (Fig. 7). First, a shock wave is formed that propagates forward from the boundary of the sectors (a direct shock wave), and then a reverse shock wave is formed that propagates towards the Sun. 
Rice. 6. Shape of the heliospheric current sheet. Its intersection with the plane of the ecliptic (tilted to the equator of the Sun at an angle of ~ 7°) gives the observed sectoral structure of the interplanetary magnetic field.
Rice. 7. Structure of the sector of the interplanetary magnetic field. The short arrows show the direction of the solar wind, the arrow lines show the magnetic field lines, the dash-dotted line shows the sector boundaries (the intersection of the figure plane with the current sheet).
Since the speed of the shock wave less speed S. v., entrains a reverse shock wave in the direction away from the Sun. Shock waves near the sector boundaries are formed at distances of ~1 AU. e. and can be traced to distances of several. but. e. These shock waves, like interplanetary shock waves from solar flares and circumplanetary shock waves, accelerate particles and are thus a source of energetic particles.
S. v. extends to distances of ~100 AU. That is, where the pressure of the interstellar medium balances the dynamic. S.'s pressure The cavity swept up by S. in. interplanetary environment). ExpandingS. in. together with the magnet frozen into it. field prevents penetration into the solar system galactic. space rays of low energies and leads to cosmic variations. beams of high energy. A phenomenon similar to S. V., found in some other stars (see. Stellar wind).
Lit.: Parker E. N., Dynamics in the interplanetary medium, O. L. Vaisberg.
Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1988 .
See what "SOLAR WIND" is in other dictionaries:
SOLAR WIND, the solar corona plasma flow that fills the solar system up to a distance of 100 astronomical units from the Sun, where the pressure of the interstellar medium balances the dynamic pressure of the flow. The main composition is protons, electrons, nuclei ... Modern Encyclopedia
SOLAR WIND, a steady flow of charged particles (mainly protons and electrons) accelerated high temperature solar corona to speeds large enough for the particles to overcome the gravity of the sun. The solar wind deflects... Scientific and technical encyclopedic dictionary
In 1957, E. Parker, a professor at the University of Chicago, theoretically predicted a phenomenon that was called the "solar wind". It took two years for this prediction to be confirmed experimentally with the help of instruments installed on the Soviet spacecraft "Luna-2" and "Luna-3" by the group of K.I. Gringhaus. What is this phenomenon?
The solar wind is a flow of fully ionized hydrogen gas, usually called a fully ionized hydrogen plasma due to approximately the same density of electrons and protons (quasi-neutrality condition), which moves with acceleration from the Sun. In the region of the Earth's orbit (at one astronomical unit or, 1 AU from the Sun), its velocity reaches an average value VE » 400–500 km/sec at a proton temperature TE » 100,000 K and a slightly higher electron temperature (subscript "E" here and in hereinafter refers to the orbit of the Earth). At such temperatures, the speed by 1 AU significantly exceeds the speed of sound, i.e. the flow of the solar wind in the region of the Earth's orbit is supersonic (or hypersonic). The measured concentration of protons (or electrons) is quite low and amounts to n E » 10–20 particles per cubic centimeter. In addition to protons and electrons, alpha particles (on the order of a few percent of the proton concentration), a small amount of heavier particles, and an interplanetary magnetic field were detected in interplanetary space, the average induction of which turned out to be on the Earth's orbit of the order of several gammas (1g = 10 –5 gauss).
The collapse of the concept of a static solar corona.
For quite a long time, it was believed that all stellar atmospheres are in a state of hydrostatic equilibrium, i.e. in a state where the force of the gravitational attraction of a given star is balanced by the force associated with the pressure gradient (change in pressure in the atmosphere of a star at a distance r from the center of the star. Mathematically, this equilibrium is expressed as an ordinary differential equation,
where G is the gravitational constant, M* is the mass of the star, p and r are pressure and mass density at some distance r from a star. Expressing the mass density from the equation of state for an ideal gas
R= r RT
through pressure and temperature and integrating the resulting equation, we obtain the so-called barometric formula ( R is the gas constant), which in the particular case of constant temperature T has the form
where p 0 is the pressure at the base of the star's atmosphere (at r = r 0). Since before Parker's work it was believed that the solar atmosphere, like the atmospheres of other stars, is in a state of hydrostatic equilibrium, its state was determined by similar formulas. Taking into account the unusual and not yet fully understood phenomenon of a sharp increase in temperature from about 10,000 K on the surface of the Sun to 1,000,000 K in the solar corona, S. Chapman developed the theory of a static solar corona, which should smoothly pass into the local interstellar medium surrounding the Solar system. From this it followed that, according to the ideas of S. Chapman, the Earth, making its revolutions around the Sun, is immersed in a static solar corona. This view was shared by astrophysicists for a long time.
The blow to these already established notions was dealt by Parker. He drew attention to the fact that the pressure at infinity (at r® Ґ), which is obtained from the barometric formula, is almost 10 times greater than the pressure that was accepted at that time for the local interstellar medium. To eliminate this discrepancy, E. Parker suggested that the solar corona cannot be in hydrostatic equilibrium, but must continuously expand into the interplanetary medium surrounding the Sun, i.e. radial speed V solar corona is not zero. At the same time, instead of the equation of hydrostatic equilibrium, he proposed to use a hydrodynamic equation of motion of the form, where M E is the mass of the Sun.
For a given temperature distribution T, as a function of distance from the Sun, solving this equation using the barometric formula for pressure, and the mass conservation equation in the form
can be interpreted as the solar wind, and it is with the help of this solution with the transition from subsonic flow (at r r *) to supersonic (at r > r*) pressure can be adjusted R with pressure in the local interstellar medium, and, consequently, it is this solution, called the solar wind, that occurs in nature.
The first direct measurements of the parameters of interplanetary plasma, which were carried out on the first spacecraft that went into interplanetary space, confirmed the correctness of Parker's idea about the presence of a supersonic solar wind, and it turned out that even in the region of the Earth's orbit, the solar wind speed far exceeds the speed of sound. Since then, there is no doubt that Chapman's idea of the hydrostatic equilibrium of the solar atmosphere is erroneous, and the solar corona is continuously expanding at supersonic speed into interplanetary space. Somewhat later, astronomical observations showed that many other stars also have "stellar winds" similar to the solar wind.
Despite the fact that the solar wind was predicted theoretically on the basis of a spherically symmetric hydrodynamic model, the phenomenon itself turned out to be much more complicated.
What is the real picture of the motion of the solar wind? For a long time, the solar wind was considered to be spherically symmetrical, i.e. independent of solar latitude and longitude. Since spacecraft before 1990, when the Ulysses spacecraft was launched, mainly flew in the plane of the ecliptic, measurements on such spacecraft gave distributions of solar wind parameters only in this plane. Calculations based on observations of comet tail deflection indicated the approximate independence of solar wind parameters from solar latitude, however, this conclusion based on cometary observations was not sufficiently reliable due to the difficulties in interpreting these observations. Although the longitudinal dependence of the solar wind parameters was measured by instruments installed on spacecraft, it was nevertheless either insignificant and was associated with the interplanetary magnetic field of solar origin, or with short-term non-stationary processes on the Sun (mainly solar flares).
Measurements of plasma and magnetic field parameters in the plane of the ecliptic showed that so-called sector structures with different solar wind parameters and different magnetic field directions can exist in interplanetary space. Such structures rotate with the Sun and clearly indicate that they are the result of a similar structure in the solar atmosphere, the parameters of which thus depend on solar longitude. Qualitatively, the four-sector structure is shown in fig. one.
At the same time, ground-based telescopes detect a general magnetic field on the surface of the Sun. Its average value is estimated at 1 G, although in individual photospheric formations, for example, in sunspots, the magnetic field can be orders of magnitude larger. Since plasma is a good conductor of electricity, the solar magnetic fields somehow interact with the solar wind due to the appearance of a ponderomotive force. j ґ B. This force is small in the radial direction, i.e. it practically does not affect the distribution of the radial component of the solar wind, but its projection onto a direction perpendicular to the radial leads to the appearance of a tangential velocity component in the solar wind. Although this component is almost two orders of magnitude smaller than the radial one, it plays a significant role in the removal of angular momentum from the Sun. Astrophysicists suggest that the latter circumstance may play a significant role in the evolution not only of the Sun, but also of other stars in which a stellar wind has been discovered. In particular, to explain the sharp decrease in the angular velocity of late-type stars, the hypothesis that they transfer rotational momentum to the planets formed around them is often invoked. The considered mechanism of the loss of the angular momentum of the Sun by the outflow of plasma from it in the presence of a magnetic field opens up the possibility of revising this hypothesis.
Measurements of the average magnetic field not only in the region of the Earth's orbit, but also at large heliocentric distances (for example, on the Voyager 1 and 2 and Pioneer 10 and 11 spacecraft) showed that in the ecliptic plane, which almost coincides with the plane of the solar equator , its magnitude and direction are well described by the formulas
received by Parker. In these formulas, which describe the so-called Parker spiral of Archimedes, the quantities B r , B j are the radial and azimuthal components of the magnetic induction vector, respectively, W is the angular velocity of the Sun's rotation, V is the radial component of the solar wind, index "0" refers to the point of the solar corona at which the magnitude of the magnetic field is known.
The launch by the European Space Agency in October 1990 of the Ulysses spacecraft, whose trajectory was calculated so that it currently orbits the Sun in a plane perpendicular to the plane of the ecliptic, completely changed the idea that the solar wind is spherically symmetrical. On fig. Figure 2 shows the distributions of the radial velocity and density of solar wind protons measured on the Ulysses spacecraft as a function of solar latitude.
This figure shows a strong latitudinal dependence of the solar wind parameters. It turned out that the speed of the solar wind increases, and the density of protons decreases with heliographic latitude. And if in the plane of the ecliptic the radial velocity is on average ~ 450 km/s, and the proton density is ~15 cm–3, then, for example, at 75° solar latitude these values are ~700 km/s and ~5 cm–3, respectively. The dependence of solar wind parameters on latitude is less pronounced during periods of minimum solar activity.
Non-stationary processes in the solar wind.
The model proposed by Parker assumes the spherical symmetry of the solar wind and the independence of its parameters from time (the stationarity of the phenomenon under consideration). However, the processes occurring on the Sun, generally speaking, are not stationary, and, consequently, the solar wind is not stationary either. The characteristic times of parameter variation have very different scales. In particular, there are changes in solar wind parameters associated with the 11-year cycle of solar activity. On fig. Figure 3 shows the average (over 300 days) dynamic pressure of the solar wind (r V 2) in the region of the Earth's orbit (per 1 AU) during one 11-year solar cycle of solar activity ( top part drawing). On the bottom of Fig. Figure 3 shows the change in the number of sunspots from 1978 to 1991 (the maximum number corresponds to the maximum solar activity). It can be seen that the parameters of the solar wind change significantly over a characteristic time of about 11 years. At the same time, measurements on the Ulysses spacecraft showed that such changes occur not only in the plane of the ecliptic, but also at other heliographic latitudes (at the poles, the dynamic pressure of the solar wind is slightly higher than at the equator).
Changes in solar wind parameters can also occur on much smaller time scales. So, for example, flares on the Sun and different velocities of plasma outflow from different regions of the solar corona lead to the formation of interplanetary shock waves in interplanetary space, which are characterized by a sharp jump in speed, density, pressure, and temperature. Qualitatively, the mechanism of their formation is shown in fig. 4. When a fast flow of any gas (for example, solar plasma) catches up with a slower one, then at the place of their contact an arbitrary discontinuity of gas parameters occurs, on which the laws of conservation of mass, momentum and energy are not satisfied. Such a discontinuity cannot exist in nature and breaks up, in particular, into two shock waves (the laws of conservation of mass, momentum and energy on them lead to the so-called Hugoniot relations) and a tangential discontinuity (the same conservation laws lead to pressure and the normal velocity component must be continuous). On fig. 4 this process is shown in a simplified form of a spherically symmetric flash. It should be noted here that such structures, consisting of a forward shock wave (forward shock), a tangential discontinuity and a second shock wave (reverse shock) move away from the Sun in such a way that the forward shock moves at a speed greater than the solar wind speed, the reverse shock moves from the Sun at a speed slightly less than the solar wind speed, and the tangential discontinuity speed is equal to the solar wind speed. Such structures are regularly recorded by instruments installed on spacecraft.
On the change in solar wind parameters with distance from the sun.
The change in the speed of the solar wind with distance from the Sun is determined by two forces: the force of solar gravity and the force associated with a change in pressure (pressure gradient). Since the force of gravity decreases as the square of the distance from the Sun, then at large heliocentric distances its influence is insignificant. Calculations show that already in the Earth's orbit, its influence, as well as the influence of the pressure gradient, can be neglected. Therefore, the speed of the solar wind can be considered almost constant. At the same time, it significantly exceeds the speed of sound (the flow is hypersonic). Then it follows from the above hydrodynamic equation for the solar corona that the density r decreases as 1/ r 2. The American spacecraft Voyager 1 and 2, Pioneer 10 and 11, launched in the mid-1970s and now located at distances of several tens of astronomical units from the Sun, confirmed these ideas about the parameters of the solar wind. They also confirmed the theoretically predicted Parker spiral of Archimedes for the interplanetary magnetic field. However, the temperature does not follow the adiabatic cooling law as the solar corona expands. At very large distances from the Sun, the solar wind even tends to heat up. This heating can be due to two reasons: energy dissipation associated with plasma turbulence and the influence of neutral hydrogen atoms penetrating into the solar wind from the interstellar medium surrounding solar system. The second reason also leads to some deceleration of the solar wind at large heliocentric distances, which was discovered on the above-mentioned spacecraft.
Conclusion.
Thus, the solar wind is a physical phenomenon that is not only of purely academic interest associated with the study of processes in plasma in natural space conditions, but also a factor that must be taken into account when studying processes occurring in the vicinity of the Earth, since these processes in one way or another affect our lives. In particular, high-speed solar wind streams, flowing around the Earth's magnetosphere, affect its structure, and non-stationary processes on the Sun (for example, flares) can lead to magnetic storms that disrupt radio communications and affect the well-being of weather-sensitive people. Since the solar wind originates in the solar corona, its properties in the region of the Earth's orbit are a good indicator for studying solar-terrestrial relations important for practical human activity. However, this is another area of scientific research, which we will not touch on in this article.
Vladimir Baranov