What is solar wind. sunny wind

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.

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, nevertheless, it was either insignificant and associated with the interplanetary magnetic field solar origin, or with short-term non-stationary processes on the Sun (mainly 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 the 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

The atmosphere of the Sun is 90% hydrogen. The part farthest from the surface is called the solar corona, and is clearly visible during total solar eclipses. The temperature of the corona reaches 1.5-2 million K, and the corona gas is completely ionized. At this plasma temperature, the thermal speed of protons is about 100 km/s, and that of electrons is several thousand kilometers per second. To overcome solar gravity, an initial speed of 618 km/s is sufficient, the second cosmic speed of the Sun. Therefore, plasma constantly leaks from the solar corona into space. This flow of protons and electrons is called the solar wind.

Having overcome the gravity of the Sun, solar wind particles fly along straight trajectories. The speed of each particle almost does not change with distance, but it can be different. This speed depends mainly on the condition solar surface, from the “weather” on the Sun. On average it is equal to v ≈ 470 km/s. The solar wind travels the distance to Earth in 3-4 days. In this case, the density of particles in it decreases in inverse proportion to the square of the distance to the Sun. At a distance equal to the radius of the earth's orbit, 1 cm 3 on average there are 4 protons and 4 electrons.

The solar wind reduces the mass of our star - the Sun - by 10 9 kg per second. Although this number seems large on an earthly scale, in reality it is small: the loss of solar mass can only be noticed over times that are thousands of times greater than the modern age of the Sun, which is approximately 5 billion years.

The interaction of the solar wind with the magnetic field is interesting and unusual. It is known that charged particles usually move in a magnetic field H in a circle or along helical lines. This is true, however, only when the magnetic field is strong enough. More precisely, for charged particles to move in a circle, it is necessary that the energy density of the magnetic field H 2 /8π be greater than the kinetic energy density of the moving plasma ρv 2 /2. In the solar wind the situation is the opposite: the magnetic field is weak. Therefore, charged particles move in straight lines, and the magnetic field is not constant, it moves along with the flow of particles, as if carried away by this flow to the periphery solar system. The direction of the magnetic field throughout interplanetary space remains the same as it was on the surface of the Sun at the moment the solar wind plasma emerged.

When traveling along the equator of the Sun, the magnetic field usually changes its direction 4 times. The sun rotates: points on the equator complete a revolution in T = 27 days. Therefore, the interplanetary magnetic field is directed in spirals (see figure), and the entire pattern of this figure rotates following the rotation of the solar surface. The angle of rotation of the Sun changes as φ = 2π/T. The distance from the Sun increases with the speed of the solar wind: r = vt. Hence the equation of the spirals in Fig. has the form: φ = 2πr/vT. At a distance of the earth's orbit (r = 1.5 10 11 m), the angle of inclination of the magnetic field to the radius vector is, as can be easily verified, 50°. On average, this angle is measured spaceships, but not quite close to Earth. Near the planets, the magnetic field is structured differently (see Magnetosphere).

There is a constant stream of particles ejected from the Sun's upper atmosphere. We see evidence of the solar wind all around us. Powerful geomagnetic storms can damage satellites and electrical systems on Earth, and cause beautiful auroras. Perhaps the best evidence of this is the long tails of comets when they pass close to the Sun.

Dust particles from a comet are deflected by the wind and carried away from the Sun, which is why the tails of comets are always directed away from our star.

Solar wind: origin, characteristics

It comes from the Sun's upper atmosphere, called the corona. In this region, the temperature is more than 1 million Kelvin, and the particles have an energy charge of more than 1 keV. There are actually two types of solar wind: slow and fast. This difference can be seen in comets. If you look at the image of a comet closely, you will see that they often have two tails. One of them is straight and the other is more curved.

Solar wind speed online near Earth, data for the last 3 days

Fast solar wind

It is moving at a speed of 750 km/s, and astronomers believe it originates from coronal holes - regions where magnetic field lines make their way to the surface of the Sun.

Slow solar wind

It has a speed of about 400 km/s, and comes from the equatorial belt of our star. The radiation reaches the Earth, depending on the speed, from several hours to 2-3 days.

The slow solar wind is wider and denser than the fast solar wind, which creates the comet's large, bright tail.

If not for the Earth's magnetic field, it would have destroyed life on our planet. However, the magnetic field around the planet protects us from radiation. The shape and size of the magnetic field is determined by the strength and speed of the wind.