Charged particles of the solar wind. What is the solar wind? Solar wind: origin, characteristics

V.B. Baranov, Moscow State University them. M.V. Lomonosov

The article examines 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) is such an outflow homogeneous; 3) changes in solar wind parameters with distance from the Sun and 4) how the solar wind flows into the interstellar medium.

Introduction

Almost 40 years have passed since the American physicist E. Parker theoretically predicted the phenomenon, which was called the “solar wind” and which a couple of years later was confirmed experimentally by the group of the Soviet scientist K. Gringaus using instruments installed on the Luna spacecraft. 2" and "Luna-3". sunny wind is a flow of fully ionized hydrogen plasma, that is, a gas consisting of electrons and protons of approximately the same density (quasineutrality condition), which moves from the Sun at high supersonic speed. In Earth's orbit (one astronomical unit (AU) from the Sun), the speed VE of this flow is approximately 400-500 km/s, the concentration of protons (or electrons) ne = 10-20 particles per cubic centimeter, and their temperature Te equal to approximately 100,000 K (electron temperature is slightly higher).

In addition to electrons and protons, alpha particles (of the order of several percent), a small amount of heavier particles, as well as a magnetic field were discovered in interplanetary space. average value the induction of which turned out to be on the order of several gammas in the Earth’s orbit (1

= 10-5 G).

A little history related to the theoretical prediction of solar wind

During the not so long history of theoretical astrophysics, it was believed that all stellar atmospheres are in hydrostatic equilibrium, that is, in a state where the gravitational pull of the star is balanced by the force associated with the pressure gradient in its atmosphere (with the change in pressure per unit distance r from the center stars). Mathematically, this equilibrium is expressed as the ordinary differential equation

(1)

where G is the gravitational constant, M* is the mass of the star, p is the atmospheric gas pressure,

- 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), the value p0 represents the pressure at the base of the star’s atmosphere (at r = r0). From this formula it is clear 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, like the atmospheres of other stars, is in a state of hydrostatic equilibrium, its state was determined by formulas similar to formulas (1), (2), (3). Considering the unusual and still not fully understood phenomenon of a sharp increase in temperature from approximately 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 was supposed to smoothly transition into the interstellar medium surrounding the solar system.

However, in his pioneering work, Parker drew attention to the fact that the pressure at infinity, obtained from a formula like (3) for a static solar corona, turns out to be almost an order of magnitude greater than the pressure value that was estimated for interstellar gas based on observations. To resolve this discrepancy, Parker proposed that the solar corona is not in a state of static equilibrium, but is continuously expanding into the interplanetary medium surrounding the Sun. Moreover, instead of the equilibrium equation (1), he proposed using the hydrodynamic equation of motion of the form

(4)

where in the coordinate system associated with the Sun, the value V represents the radial velocity of the plasma. Under

refers to the mass of the Sun.

For a given temperature distribution T, the system of equations (2) and (4) has solutions of the type presented in Fig. 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 Fig. 1 have a physical meaning for the problem of gas outflow from the Sun, since curves 3 and 4 have non-unique velocity values at each point, and curves 5 and 6 correspond to very high velocities at solar atmosphere, which is not observed in telescopes. Parker analyzed the conditions under which the solution corresponding to curve 1 is realized 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 such a flow the solar wind. However, this statement was disputed in the work of Chamberlain, who believed that the most real 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 be sufficiently reliable to Chamberlain.

Rice. 1. Possible solutions of one-dimensional gas dynamics equations for the speed V of gas flow from the surface of the Sun in the presence of gravity. 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, and is the radius of the Sun.

The history of experiments in outer space has brilliantly proven the correctness of Parker's ideas about the solar wind. Detailed material on the theory of solar wind can be found, for example, in the monograph.

Concepts of a uniform outflow of plasma from the solar corona

From the one-dimensional gas dynamics equations one can obtain known result: in the absence of mass forces, the spherically symmetric flow of gas from a point source can be everywhere either subsonic or supersonic. The presence of gravitational force in equation (4) (right side) leads to the appearance of solutions like curve 1 in Fig. 1, that is, with a transition through the speed of sound. Let's draw an analogy with the classical flow in a Laval nozzle, which is the basis of all supersonic jet engines. This flow is shown schematically in Fig. 2.

Rice. 2. Flow diagram in a Laval nozzle: 1 - a tank called a receiver, into which very hot air is supplied at low speed, 2 - an area of geometric compression of the channel in order to accelerate the subsonic gas flow, 3 - an area of geometric expansion of the channel in order to accelerate the supersonic flow.

Gas heated to a very high temperature is supplied to tank 1, called the receiver, at a very low speed (the internal energy of the gas is much greater than its kinetic energy directional movement). By geometrically compressing the channel, the gas is accelerated in region 2 (subsonic flow) until its speed reaches the speed of sound. To further accelerate it, it is necessary to expand the channel (region 3 of the supersonic flow). In the entire flow region, gas acceleration occurs due to its adiabatic (without heat supply) cooling (the internal energy of chaotic motion transforms into the energy of directed motion).

In the problem of solar wind formation under consideration, the role of the receiver is played by the solar corona, and the role of the walls of the Laval nozzle is 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 accelerate 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 that are always present in the solar wind (sometimes called plasma turbulence), superimposed on the average 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 sunspots, the magnetic field can be orders of magnitude greater. Since plasma is a good conductor of electricity, it is natural that solar magnetic fields interact with its flow from the Sun. In this case, a purely gas-dynamic theory provides an incomplete description of the phenomenon under consideration. Influence magnetic field the flow of the solar wind can only be considered within the framework of a science called magnetic hydrodynamics. What results do such considerations lead to? According to pioneering work in this direction (see also), the magnetic field leads to the appearance of electric currents j in the solar wind plasma, which, in turn, leads to the appearance of a ponderomotive force j x B, which is directed in the perpendicular to the radial direction. As a result, the solar wind acquires 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 of not only the Sun, but also 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 class, the hypothesis of the transfer of rotational momentum to the planets formed around them is often invoked. The considered mechanism for the loss of angular momentum of the Sun through the outflow of plasma from it opens up the possibility of revising this hypothesis.

SUNNY WIND- a continuous stream of plasma of solar origin, spreading approximately radially from the Sun and filling the Solar System to the heliocentric. distances R ~ 100 a. e. S. v. is formed during gas-dynamic. expansion of the solar corona (see 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 matter, and the corona expands.

The first evidence of the existence of post. plasma flows from the Sun were obtained by L. Biermann in the 1950s. on the analysis of forces acting on the plasma tails of comets. In 1957, Yu. Parker (E. Parker), analyzing the equilibrium conditions of the corona matter, showed that the corona cannot be in hydrostatic conditions. equilibrium, as was previously assumed, but should expand, and this expansion, under the existing boundary conditions, should lead to the acceleration of coronal matter to supersonic speeds (see below). For the first time, a plasma flow of solar origin was recorded in the Soviet spacecraft. spacecraft "Luna-2" in 1959. Existence post. the outflow of plasma from the Sun was proven as a result of many months of measurements in America. space the Mariner 2 apparatus in 1962.

Wed. characteristics of S. v. are given in table. 1. S. flows. 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 flows come from regions of the solar corona, where the structure of the magnetic field. fields are close to radial. Some of these areas are coronal holes. Slow flows of the North century. are apparently connected with the regions of the crown, in which there is, therefore, a tangential magnetic component. fields.

Table 1.- Average characteristics of the solar wind in Earth orbit

Speed
Proton concentration
Proton temperature
Electron temperature
Magnetic field strength
Python flux density....	2.410 8 cm -2 c -1
Kinetic energy flux density	0.3 ergcm -2 s -1

Table 2.- Relative chemical composition solar wind

	Relative content

	Relative content

In addition to the main components of solar water are protons and electrons; highly ionized particles are also found in its composition. ions of oxygen, silicon, sulfur, iron (Fig. 1). When analyzing gases trapped in foils exposed on the Moon, Ne and Ar atoms were found. Wed. relative chem. composition of S. v. is given in table. 2. Ionization. state of matter S. v. corresponds to the level in the corona where the recombination time is short compared to the expansion time Ionization measurements temperature of ions S. v. make it possible to determine the electron temperature of the solar corona.

In the N. century. differences are observed. types of waves: Langmuir, whistlers, ion-sonic, magnetosonic, Alfven, etc. (see. Waves in plasma Some of the Alfvén-type waves are generated on the Sun, and some are excited in the interplanetary medium. The generation of waves smoothes out deviations of the particle distribution function from the Maxwellian one and, in combination with the influence of magnetism. fields on the plasma leads to the fact that S. v. behaves like a continuous medium. Alfvén-type waves play a large role in the acceleration of small components of solar waves. and in the formation of the proton distribution function. In the N. century. contact and rotational discontinuities characteristic of magnetized plasma are also observed.

Rice. 1. Mass spectrum of the solar wind. Along the horizontal axis is the ratio of the mass of a particle to its charge, along the vertical axis is the number of particles registered in the energy window of the device in 10 s. Numbers with a “+” sign indicate the charge of the ion.

Stream N. in. is supersonic in relation to the speeds of those types of waves that provide eff. transfer of energy to the S. century. (Alfven, sound and magnetosonic waves). Alfven and sound Mach number C.V. in Earth's orbit 7. When flowing around the northeast. obstacles capable of effectively deflecting it (magnetic fields of Mercury, Earth, Jupiter, Saturn or the conducting ionospheres of Venus and, apparently, Mars), a departing bow shock wave is formed. S.v. slows down and heats up at the front of the shock wave, which allows it to flow around the obstacle. At the same time, in the North century. a cavity is formed - the magnetosphere (either its own or induced), the shape and dimensions of the shape are determined by the balance of magnetic pressure. fields of the planet and the pressure of the flowing plasma flow (see. Magnetosphere of the Earth, Magnetospheres of the planets). In case of interaction with S. v. with a non-conducting body (for example, 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 from the plasma.

The stationary process of corona plasma outflow is superimposed by non-stationary processes associated with solar flares. During strong flares, substances are released from below. corona regions into the interplanetary medium. In this case, a shock wave is also formed (Fig. 2), which gradually slows down, spreading in the plasma of the solar system. The arrival of a shock wave to the Earth causes compression of the magnetosphere, after which the development of magnetism usually begins. storms (see 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, the lines without a caption are the magnetic field lines.

Rice. 3. Types of solutions to the corona expansion equation. Speed and distance are normalized to the critical speed vk and the critical distance Rk. Solution 2 corresponds to the solar wind.

The expansion of the solar corona is described by a system of equations of conservation of mass, angular momentum and energy equations. Solutions that meet various the nature of the change in speed with distance are shown in Fig. 3. Solutions 1 and 2 correspond to low velocities at the base of the crown. The choice between these two solutions is determined by the conditions at infinity. Solution 1 corresponds to low rates of expansion of the corona and gives large values of pressure at infinity, i.e., it encounters the same difficulties as the static model. crowns Solution 2 corresponds to the transition of the expansion rate through the speed of sound values ( v to) on some critical. distance R to and subsequent expansion at supersonic speed. This solution gives a vanishingly small value of pressure at infinity, which makes it possible to reconcile it with the low pressure of the interstellar medium. This type of flow was called S. by Yu. Parker. Critical the point is above the surface of the Sun if the temperature of the corona is less than a certain critical value. values , where m is the proton mass, is the adiabatic exponent, and is the mass of the Sun. In Fig. Figure 4 shows the change in expansion rate from heliocentric. distance depending on isothermal temperature. isotropic corona. Subsequent models of S. century. take into account variations in coronal temperature with distance, two-liquid nature of the medium (electron and proton gases), thermal conductivity, viscosity, non-spherical. nature of expansion.

Rice. 4. Solar wind speed profiles for the isothermal corona model at different values of coronal temperature.

S.v. provides the basic outflow of thermal energy from the corona, since heat transfer to the chromosphere, el-magn. Corona radiation and electron thermal conductivity are insufficient to establish the thermal balance of the corona. Electronic thermal conductivity ensures a slow decrease in the ambient temperature. with distance. S.v. does not play any noticeable role in the energy of the Sun as a whole, since the energy flow carried away by it is ~10 -7 luminosity Sun.

S.v. carries the coronal magnetic field with it into the interplanetary medium. field. The field lines of this field frozen into the plasma form an interplanetary magnetic field. field (MMP). Although the IMF intensity is low and its energy density is approx. 1% of kinetic density energy of solar energy, it plays a large role in the thermodynamics of solar energy. and in the dynamics of interactions of S. v. with the bodies of the solar system, as well as the streams of the north. between themselves. Combination of expansion of the S. century. with the rotation of the Sun leads to the fact that the mag. the lines of force frozen into the north century have a shape close to the Archimedes spiral (Fig. 5). Radial B R and azimuthal magnetic components. fields change differently with distance near the ecliptic plane:

where is ang. speed of rotation of the Sun, And- radial component of the velocity of the central air, index 0 corresponds to the initial level. At the distance of the Earth's orbit, the angle between the direction of the magnetic. fields and R about 45°. At large L magnetic. the field is almost perpendicular to R.

Rice. 5. Shape of the interplanetary magnetic field line. - angular velocity of rotation of the Sun, and - radial component of plasma velocity, R - heliocentric distance.

S. v., arising over regions of the Sun with different. magnetic orientation fields, forms flows with differently oriented permafrost. Separation of the observed large-scale structure of the solar system. for an even number of sectors with different the direction of the radial component of the IMF is called. interplanetary sector structure. Characteristics of S. v. (speed, temp-pa, particle concentration, etc.) also on Wed. change naturally in the cross section of each sector, which is associated with the existence of a fast flow of solar water inside the sector. The boundaries of the sectors are usually located within the slow flow of the north. Most often, 2 or 4 sectors are observed, rotating with the Sun. This structure, formed when the S. is pulled out. large-scale mag. corona fields, can be observed for several. revolutions of the Sun. The sector structure of the IMF is a consequence of the existence of a current layer (CS) in the interplanetary medium, which rotates together with the Sun. TS creates a magnetic surge. fields - the radial components of the IMF have different signs on different sides of the vehicle. This TS, predicted by H. Alfven, passes through those parts of the solar corona that are associated with active regions on the Sun, and separates these regions from the various regions. signs of the radial component of the solar magnet. fields. The TS 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 folds of the TC into a spiral (Fig. 6). Being near the ecliptic plane, the observer finds himself either above or below the TS, due to which he ends up in sectors with different signs of the IMF radial component.

Near the Sun in the north. There are longitudinal and latitudinal velocity gradients caused by the difference in the velocities of fast and slow flows. As you move away from the Sun and the boundary between the streams in the north becomes steeper. radial velocity gradients arise, which lead to the formation collisionless shock waves(Fig. 7). First, a shock wave is formed, propagating forward from the boundary of the sectors (a forward shock wave), and then a reverse shock wave is formed, propagating towards the Sun.

Rice. 6. Shape of the heliospheric current layer. Its intersection with the ecliptic plane (inclined to the solar equator at an angle of ~ 7°) gives the observed sector structure of the interplanetary magnetic field.

Rice. 7. Structure of the interplanetary magnetic field sector. Short arrows show the direction of solar wind plasma flow, lines with arrows - magnetic field lines, dash-dotted lines - sector boundaries (intersection of the drawing plane with the current layer).

Since the speed of the shock wave is less than the speed of the solar energy, the plasma entrains the 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. A. e. These shock waves, as well as interplanetary shock waves from solar flares and circumplanetary shock waves, accelerate particles and are, therefore, a source of energetic particles.

S.v. extends to distances of ~100 AU. e., where the pressure of the interstellar medium balances the dynamic. blood pressure The cavity swept by the S. v. in the interstellar medium, forms the heliosphere (see. Interplanetary environment). Expanding S. v. along with the magnet frozen into it. field prevents the penetration of galactic particles into the Solar System. space rays of low energies and leads to variations in cosmic. high energy rays. A phenomenon similar to the S.V. has also been discovered in certain other stars (see Stellar wind).

Lit.: Parker E. N., Dynamic processes in the interplanetary medium, trans. from English, M., 1965; Brandt J., Solar Wind, trans. from English, M., 1973; Hundhausen A., Corona Expansion and the Solar Wind, trans. from English, M., 1976. O. L. Weisberg.

sunny wind

- a continuous stream of plasma of solar origin, spreading approximately radially from the Sun and filling the Solar System to the heliocentric. distances ~100 AU S.v. is formed during gas-dynamic. expansion into interplanetary space. At high temperatures, which exist in the solar corona (K), the pressure of the overlying layers cannot balance the gas pressure of the corona matter, and the corona expands.

The first evidence of the existence of a constant flow of plasma from the Sun was obtained by L. Biermann (Germany) in the 1950s. on the analysis of forces acting on the plasma tails of comets. In 1957, Yu. Parker (USA), analyzing the equilibrium conditions of the corona matter, showed that the corona cannot be in hydrostatic conditions. equilibrium, as previously assumed, should expand, and this expansion, under the existing boundary conditions, should lead to the acceleration of coronal matter to supersonic speeds.

Average characteristics of S.v. are given in table. 1. For the first time, a plasma flow of solar origin was recorded on the second Soviet spacecraft. rocket "Luna-2" in 1959. The existence of a constant outflow of plasma from the Sun was proven as a result of many months of measurements in America. AMS Mariner 2 in 1962

Table 1. Average characteristics of the solar wind in Earth orbit

Speed	400 km/s
Proton Density	6 cm -3
Proton temperature	TO
Electron temperature	TO
Magnetic field strength	E
Proton flux density	cm -2 s -1
Kinetic energy flux density	0.3 ergsm -2 s -1

Streams N.v. can be divided into two classes: slow - with a speed of km/s and fast - with a speed of 600-700 km/s. Fast flows come from those regions of the corona where the magnetic field is close to radial. Some of these areas are . Slow currents N.W. are apparently associated with the areas of the crown where there is meaning. tangential component mag. fields.

In addition to the main components of S.v. - protons and electrons; - particles, highly ionized ions of oxygen, silicon, sulfur, and iron were also found in its composition (Fig. 1). When analyzing gases trapped in foils exposed on the Moon, Ne and Ar atoms were found. Average chem. composition of S.v. is given in table. 2.

Table 2. Relative chemical composition of the solar wind

Element	Relative content
H	0,96
3 He
4 He	0,04
O
Ne
Si
Ar
Fe

Ionization state of matter S.v. corresponds to the level in the corona where the recombination time becomes small compared to the expansion time, i.e. on distance . Ionization measurements ion temperatures S.v. make it possible to determine the electron temperature of the solar corona.

S.v. carries the coronal magnetic field with it into the interplanetary medium. field. The field lines of this field frozen into the plasma form an interplanetary magnetic field. field (MMP). Although the IMF intensity is low and its energy density is approx. 1% of kinetic energy of solar energy, it plays a large role in the thermodynamics of solar energy. and in the dynamics of interactions between S.v. with the bodies of the Solar System and the streams of the North. between themselves. Combination of expansion S.v. with the rotation of the Sun leads to the fact that the mag. power lyoniums frozen in the S.V. have a shape close to Archimedes’ spirals (Fig. 2). Radial and azimuthal component of mag. fields near the ecliptic plane change with distance:
,
Where R- heliocentric distance, - angular speed of rotation of the Sun, u R- radial velocity component S.v., index “0” corresponds to the initial level. At the distance of the Earth's orbit, the angle between the magnetic directions. fields and direction to the Sun, on large heliocentric. IMF distances are almost perpendicular to the direction to the Sun.

S.v., arising over regions of the Sun with different magnetic orientations. fields, forms flows in differently oriented permafrost - the so-called. interplanetary magnetic field.

In N.v. Various types of waves are observed: Langmuir, whistlers, ion-sonic, magnetosonic, etc. (see). Some waves are generated on the Sun, some are excited in the interplanetary medium. The generation of waves smoothes out deviations of the particle distribution function from the Maxwellian one and leads to the fact that the S.V. behaves like a continuous medium. Alfvén-type waves play a large role in the acceleration of small components of the S.V. and in the formation of the proton distribution function. In N.v. Contact and rotational discontinuities, characteristic of magnetized plasma, are also observed.

Stream N.w. yavl. supersonic in relation to the speed of those types of waves that provide effective transfer of energy into the S.V. (Alfvén, sound and magnetosonic waves), Alfvén and sound Mach numbers S.v. in Earth orbit. When trimming the S.V. obstacles that can effectively deflect S.v. (magnetic fields of Mercury, Earth, Jupiter, Staurn or the conducting ionospheres of Venus and, apparently, Mars), a bow shock wave is formed. S.v. slows down and heats up at the front of the shock wave, which allows it to flow around the obstacle. At the same time, in N.v. a cavity is formed - the magnetosphere (either its own or induced), the shape and size of the structure is determined by the balance of magnetic pressure. fields of the planet and the pressure of the flowing plasma flow (see). The layer of heated plasma between the shock wave and the streamlined obstacle is called. transition region. The temperatures of ions at the front of the shock wave can increase by 10-20 times, electrons - by 1.5-2 times. Shock wave phenomenon. , the thermalization of the flow is ensured by collective plasma processes. The thickness of the shock wave front is ~100 km and is determined by the growth rate (magnetosonic and/or lower hybrid) during the interaction of the oncoming flow and part of the ion flow reflected from the front. In case of interaction between S.v. with a non-conducting body (the Moon), a shock wave does not arise: the plasma flow is absorbed by the surface, and behind the body a SW is formed which is gradually filled with plasma. cavity.

The stationary process of corona plasma outflow is superimposed by non-stationary processes associated with. During strong solar flares, matter is ejected from the lower regions of the corona into the interplanetary medium. In this case, a shock wave is also formed (Fig. 3), the edges gradually slow down when moving through the plasma of the SW. The arrival of a shock wave to the Earth leads to compression of the magnetosphere, after which the development of magnetism usually begins. storms

The equation describing the expansion of the solar corona can be obtained from the system of conservation equations for mass and angular momentum. The solutions to this equation, which describe the different nature of the change in speed with distance, are shown in Fig. 4. Solutions 1 and 2 correspond to low velocities at the base of the crown. The choice between these two solutions is determined by the conditions at infinity. Solution 1 corresponds to low rates of expansion of the corona (“solar breeze”, according to J. Chamberlain, USA) and gives large pressure values at infinity, i.e. encounters the same difficulties as the static model. crowns Solution 2 corresponds to the transition of the expansion rate through the speed of sound ( v K) on a certain rum critical. distance R K and subsequent expansion at supersonic speed. This solution gives a vanishingly small value of pressure at infinity, which makes it possible to reconcile it with the low pressure of the interstellar medium. Parker called this type of current the solar wind. Critical the point is above the surface of the Sun if the temperature of the corona is less than a certain critical value. values , where m- proton mass, - adiabatic index. In Fig. Figure 5 shows the change in expansion rate from heliocentric. distance depending on isothermal temperature. isotropic corona. Subsequent models of S.v. take into account variations in the coronal temperature with distance, the two-liquid nature of the medium (electron and proton gases), thermal conductivity, viscosity, and the nonspherical nature of the expansion. Approach to substance S.v. how to a continuous medium is justified by the presence of the IMF and the collective nature of the interaction of the SW plasma, caused by various types of instabilities. S.v. provides the basic outflow of thermal energy from the corona, because heat transfer to the chromosphere, electromagnet. radiation from highly ionized corona matter and electronic thermal conductivity of solar energy. insufficient to establish thermal balance of the crown. Electronic thermal conductivity ensures a slow decrease in the ambient temperature. with distance. S.v. does not play any noticeable role in the energy of the Sun as a whole, because the energy flux carried away by it is ~ 10 -8

In 1957, University of Chicago professor E. Parker theoretically predicted the phenomenon, which was called the “solar wind.” It took two years for this prediction to be confirmed experimentally using instruments installed on the Soviet Luna-2 and Luna-3 spacecraft by K.I. Gringauz’s group. What is this phenomenon?

The solar wind is a stream of fully ionized hydrogen gas, usually called fully ionized hydrogen plasma due to the approximately equal density of electrons and protons (quasineutrality condition), which accelerates away from the Sun. In the region of the Earth's orbit (at one astronomical unit or 1 AU from the Sun), its speed reaches an average value of V E » 400–500 km/sec at a proton temperature T E » 100,000 K and a slightly higher electron temperature (index “E” here and in hereinafter refers to the Earth's orbit). At such temperatures, the speed is significantly higher than the speed of sound by 1 AU, i.e. The flow of solar wind in the region of the Earth's orbit is supersonic (or hypersonic). The measured concentration of protons (or electrons) is quite small and amounts to n E » 10–20 particles per cubic centimeter. In addition to protons and electrons, alpha particles (of the order of several percent of the proton concentration), a small amount of heavier particles, as well as an interplanetary magnetic field were discovered in interplanetary space, the average induction value of which turned out to be on the order of several gammas in Earth’s orbit (1g = 10 –5 gauss).

The collapse of the idea 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 gravitational attraction of a given star is balanced by the force associated with the pressure gradient (the change in pressure in the star’s atmosphere at a distance r from the center of the star. Mathematically, this equilibrium is expressed as an ordinary differential equation,

Where G– gravitational constant, M* – mass of the star, p and r – pressure and mass density at some distance r from the star. Expressing 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– gas constant), which in the particular case of constant temperature T looks like

Where p 0 – represents 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, was 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 approximately 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 was supposed to smoothly transition into the local interstellar medium surrounding the Solar system. 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 point of view has been shared by astrophysicists for a long time.

Parker dealt a blow to these already established ideas. 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 in magnitude 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. Moreover, instead of the equation of hydrostatic equilibrium, he proposed using 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 precisely 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, therefore, it is this solution, called the solar wind, that is carried out in nature.

The first direct measurements of the parameters of interplanetary plasma, which were carried out on the first spacecraft entering interplanetary space, confirmed the correctness of Parker’s idea about the presence of supersonic solar wind, and it turned out that already in the region of the Earth’s orbit the speed of the solar wind far exceeds the speed of sound. Since then, there has been 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 have “stellar winds” similar to the solar wind.

Despite the fact that the solar wind was predicted theoretically based on a spherically symmetric hydrodynamic model, the phenomenon itself turned out to be much more complex.

What is the real pattern of solar wind movement? For a long time, the solar wind was considered spherically symmetric, i.e. independent of solar latitude and longitude. Because the spacecraft Until 1990, when the Ulysses spacecraft was launched, most of the flights were in the ecliptic plane, and measurements on such spacecraft gave distributions of solar wind parameters only in this plane. Calculations based on observations of the deflection of cometary tails indicated an 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 solar wind parameters was measured by instruments installed on spacecraft, it was nevertheless either insignificant and associated with the interplanetary magnetic field of solar origin, or with short-term non-stationary processes on the Sun (mainly with solar flares).

Measurements of plasma and magnetic field parameters in the ecliptic plane have shown that so-called sector structures with different parameters of the solar wind and different directions of the magnetic field can exist in interplanetary space. Such structures rotate with the Sun and clearly indicate that they are a consequence of a similar structure in the solar atmosphere, the parameters of which thus depend on solar longitude. The qualitative four-sector structure is shown in Fig. 1.

At the same time, ground-based telescopes detect the 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 greater. Since plasma is a good conductor of electricity, solar magnetic fields somehow interact with the solar wind due to the appearance of ponderomotive force j ґ B. This force is small in the radial direction, i.e. it has virtually no effect on the distribution of the radial component of the solar wind, but its projection onto a direction perpendicular to the radial direction 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 detected. In particular, to explain the sharp decrease in the angular velocity of stars of the late spectral class, the hypothesis that they transfer rotational momentum to the planets formed around them is often invoked. The considered mechanism for the loss of 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, almost coinciding 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 Parkerian spiral of Archimedes, the quantities B r, B j – radial and azimuthal components of the magnetic induction vector, respectively, W – angular velocity of the Sun’s rotation, V– 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 European Space Agency's launch of the Ulysses spacecraft in October 1990, whose trajectory was calculated so that it now orbits the Sun in a plane perpendicular to the ecliptic plane, completely changed the idea that the solar wind is spherically symmetric. In Fig. Figure 2 shows the distributions of 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 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 ecliptic plane the radial velocity is on average ~ 450 km/sec, and the proton density is ~15 cm–3, then, for example, at 75° solar latitude these values are ~700 km/sec 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 (stationarity of the phenomenon under consideration). However, the processes occurring on the Sun, generally speaking, are not stationary, and therefore the solar wind is not stationary. The characteristic times of changes in parameters have very different scales. In particular, there are changes in solar wind parameters associated with the 11-year cycle of solar activity. In Fig. Figure 3 shows the average (over 300 days) dynamic pressure of the solar wind measured using the IMP-8 and Voyager-2 spacecraft (r V 2) in the area of the Earth’s orbit (at 1 AU) for one 11-year solar cycle solar activity (upper part of the figure). On the bottom of Fig. Figure 3 shows the change in the number of sunspots over the period 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 ecliptic plane, 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. For example, flares on the Sun and different rates 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. The mechanism of their formation is shown qualitatively in Fig. 4. When a fast flow of any gas (for example, solar plasma) catches up with a slower one, an arbitrary gap in the parameters of the gas appears at the point of their contact, in 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 (on them the laws of conservation of mass, momentum and energy lead to the so-called Hugoniot relations) and a tangential discontinuity (the same conservation laws lead to the fact that on it the pressure and the normal velocity component must be continuous). In Fig. 4 this process is shown in the simplified form of a spherically symmetrical flare. It should be noted here that such structures, consisting of a forward shock wave, a tangential discontinuity and a second shock wave (reverse shock), move from the Sun in such a way that the forward shock moves at a speed greater than the speed of the solar wind, the reverse shock moves from the Sun at a speed slightly lower than the speed of the solar wind, and the speed of the tangential discontinuity is equal to the speed of the solar wind. Such structures are regularly recorded by instruments installed on spacecraft.

On changes in solar wind parameters with distance from the sun.

The change in solar wind speed with distance from the Sun is determined by two forces: the force of solar gravity and the force associated with changes in pressure (pressure gradient). Since the force of gravity decreases as the square of the distance from the Sun, its influence is insignificant at large heliocentric distances. Calculations show that already in Earth's orbit its influence, as well as the influence of the pressure gradient, can be neglected. Consequently, the speed of the solar wind can be considered almost constant. Moreover, it significantly exceeds the speed of sound (hypersonic flow). Then from the above hydrodynamic equation for the solar corona it follows 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 from the Sun of several tens of astronomical units, confirmed these ideas about the parameters of the solar wind. They also confirmed the theoretically predicted Parker Archimedes spiral 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 warm up. Such heating may 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 braking of the solar wind at large heliocentric distances, detected on the above-mentioned spacecraft.

Conclusion.

Thus, the solar wind is physical phenomenon, which is not only of purely academic interest associated with the study of processes in plasma located in the natural conditions of outer space, but also a factor that must be taken into account when studying processes occurring in the vicinity of the Earth, since these processes, to one degree or another, influence our life. In particular, high-speed solar wind flows 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 important practical activities person of solar-terrestrial connections. However, this is a different area scientific research, which we will not touch upon in this article.

Vladimir Baranov

It can be used not only as a propulsion device for space sailing ships, but also as a source of energy. The most famous use of the solar wind in this capacity was first proposed by Freeman Dyson, who suggested that a highly developed civilization could create a sphere around a star that would collect all the energy it emitted. Based on this, another method of searching for extraterrestrial civilizations was also proposed.

Meanwhile, a team of researchers at the University of Washington (Washington State University), led by Brooks Harrop, proposed a more practical concept for using solar wind energy - the Dyson-Harrop satellites. They are fairly simple power plants that harvest electrons from the solar wind. A long metal rod pointed at the sun is energized to generate a magnetic field that will attract electrons. At the other end is an electron trap receiver consisting of a sail and a receiver.

According to Harrop's calculations, a satellite with a 300-meter rod, 1 cm thick and a 10-meter trap in Earth orbit will be able to “collect” up to 1.7 MW. This is enough to power approximately 1,000 private homes. The same satellite, but with a kilometer-long rod and a sail of 8400 kilometers, will be able to “collect” 1 billion billion gigawatts of energy (10 27 W). All that remains is to transfer this energy to Earth in order to abandon all other types of it.

Harrop's team proposes to transmit energy using a laser beam. However, if the design of the satellite itself is quite simple and quite feasible at the current level of technology, then the creation of a laser “cable” is still technically impossible. The fact is that in order to effectively collect solar wind, the Dyson-Harrop satellite must lie outside the ecliptic plane, which means it is located millions of kilometers from the Earth. At this distance, the laser beam will produce a spot thousands of kilometers in diameter. An adequate focusing system will require a lens from 10 to 100 meters in diameter. In addition, many dangers from possible system failures cannot be excluded. On the other hand, energy is also required in space itself, and small Dyson-Harrop satellites may well become its main source, replacing solar panels and nuclear reactors.