On September 28, a strong magnetic storm (level 3 on a scale of five) occurs on Earth due to our planet being caught in a stream of fast solar wind. This is evidenced by data from the ACE spacecraft, located on the Sun-Earth line at the gravitational equilibrium point L1, reports the Laboratory of Solar X-ray Astronomy, Lebedev Physical Institute.
Sun, September 28, 2017. Image from SDO website
The solar wind, the increased impact of which our planet is now experiencing, is a stream of plasma that continuously flows from the atmosphere of the Sun in all directions and fills the entire solar system. The speed of the solar wind increases with distance from the Sun and at the level of the Earth's orbit averages about 400 km/sec. If the Sun were a perfectly symmetrical object without any features, the speed of the solar wind would be constant. However, since there are centers of activity on the Sun, as well as areas of higher and lower temperatures, this is reflected in the speed of outflowing plasma flows - it can either increase or decrease relative to the average value. As paradoxical as it sounds, the fastest flows of solar wind flow from the coldest parts of the solar corona, which, due to the lower temperature, appear darker and for this reason are called coronal holes.
Since coronal holes often “live” for several revolutions of the Sun (that is, for several months), the fast wind streams they produce are also stable formations. Some of them hit the Earth several times during this time - with each turn of the Sun towards the Earth with the corresponding side. The magnetic storms that arise during such impacts are repeating - they are separated by a step of 27 days, coinciding with the rotation period of the Sun. This makes it possible to predict such storms 27 days in advance, that is, it is fundamental for long-term forecasting.
The Earth entered a stream of fast wind yesterday at about 9:00 Moscow time, when the speed of the surrounding plasma increased from 300-350 km/sec (the level at which it had remained in recent days) to approximately 500 km/sec. The first contact with the flow turned the Earth's magnetic field into a disturbed state, in which it remained until the end of the day. Around midnight, the speed of the solar wind blowing the Earth increased to 650-700 km/sec, and is now at this level, almost 2 times the average value. Apparently, at the moment our planet is passing through the fastest part of the flow and is experiencing the greatest impact. The level of oscillations of the Earth's magnetic field, which receives the main impact, now corresponds to the level of Kp=7, which is classified as a strong magnetic storm.
Based on the angular size of the flow, the Earth will remain inside it for about another day. During this entire time, the likelihood of disturbances in the magnetic field of our planet will be significantly increased. However, the storm, apparently, is passing its peak right now and will no longer be able to reach a higher level. The Earth's magnetic field should completely calm down by the middle of tomorrow, September 29.
from the collection of PGI "Physics of Near-Earth Space", vol. 2, Apatity, 2000"
1. Introduction
2. QUASI-STATIONARY FLOWS
2.1High-speed flow from coronal holes
2.2 VSP edge
2.3 GTS and streamer
24 Interstream plasma
3. UNSTATIONARY FLOWS
3.1 Solar storms
3.2 Disappearance of fibers
3.3 Flare and filament streams
Introduction
Types of solar wind can be divided into two main groups: quasi-stationary and non-stationary.
Quasi-stationary solar wind flows are associated with structural formations of the solar magnetic field with a characteristic lifetime from several days to several weeks or months. Non-stationary flows include flows whose sources are non-stationary phenomena on the Sun with a lifetime of less than a day. There is no complete classification of solar wind types in the literature. .
If for quasi-stationary types of solar wind there are no special differences in the definition (these are high-speed streams from coronal holes (HSPs from CHs), a heliospheric current layer (HCS) with coronal streamers around it), then the definitions of non-stationary types and their solar sources are somewhat different. So Huddleston et al.,(1995) Unsteady flows include transient flows from coronal mass ejections (CME) and the region between interplanetary shock waves and the leading edges of coronal mass ejections following the shock wave.
Unsteady flows include flows from coronal mass ejections (CMEs) and shock wave plasma.
On the other side Ivanov(1996) non-stationary fluxes are determined by their solar sources, namely: sporadic phenomena such as flares, sudden disappearances of filaments in active regions of the Sun and sudden disappearances of filaments outside active regions. 
Rice. 1 Magnetic field topologies and associated types of solar wind
The topologies of the magnetic field and the associated types of solar wind are shown in Fig. 1.
Below will be a description of the different types of solar wind and their solar sources, as well as the identification of these types of flows in Earth's orbit.
2. Quasi-stationary flows
2.1 High-speed flow from coronal holes
A description of the formation of CD and its properties is given in the work [Kovalenko, 1983]. Photospheric magnetic fields on the Sun are large regions within which one polarity dominates with an open magnetic field configuration. They are separated by neutral lines. Coronal holes can form inside large unipolar magnetic regions if the sizes of these regions are not less than 300. The boundaries of the CH follow the shape of the neutral line at some distance from it. There is a certain boundary zone between the edge of the CD and the neutral line forming the edge of the magnetic cell. Within the CD there are no neutral lines and there are no closed structures. Low-latitude CHs can form between active regions with a closed magnetic field configuration.
The evolution of a CD occurs with a change in the structure of the magnetic field at its boundary. The birth and destruction of CHs is clearly associated with changes in photospheric magnetic fields and a corresponding restructuring of the configuration of the coronal fields. CHs are long-lived formations with an average lifetime for the decline phase of the solar cycle from 3 to 20 solar revolutions, and for the phase around the solar maximum. activity is about 1-2 solar revolutions. The lifetime of unipolar structures exceeds the lifetime of the CD.
The size and position of the CH on the solar surface depend on the configuration of the solar magnetic fields, which is what changes the CH in the solar activity cycle. Polar CHs decrease in size during the phase of increasing activity and completely disappear at the maximum, and the CHs have their maximum sizes at. phase of decline in activity. Equatorial CHs, located between two active regions, change in the solar cycle depending on changes in the active regions: the number of CHs decreases sharply at the minimum and greatly increases at the decline of the cycle, when there are many bipolar magnetic regions, and the latitude at which they are observed has noticeably decreased. Small CDs can always form.
Patrol observations of CHs on Earth are carried out in the He1 1083 nm line, and CH localization is obtained on spectroheliograms. The main difference between CDs and a normal quiet corona is that their electromagnetic radiation is less over the entire wavelength range. CDs are especially visible on the disk in soft X-rays and extreme ultraviolet radiation. CDs are regions of the corona with an anomalously low concentration, and the plasma concentration decreases, and the value of the plasma velocity increases significantly with increasing degree of non-radial configuration of the magnetic field.
Coronal holes are the solar source of high-speed flow (HSF) of solar wind. The mechanism of formation of high-speed flows from the pressurizer is considered in the work [Kovalenko, 1983] and boils down to the fact that due to the divergence of the magnetic field, the plasma concentration decreases, and part of the wave energy of the Sun goes to increase the speed of the solar wind.
The main parameters of VSP have been studied, studied and known. [Ermolaev, 1990; Kovalenko, 1983]. The dimensions of a VSP in Earth's orbit are on average approximately twice as large as the corresponding CD. The maximum speed of the SSW depends on the degree of divergence of the magnetic field in the coronal hole. The duration of the Earth's intersection of the SSW body is from 1 to 10 days. The average parameter values for the VSP body are:
vp=450-650 km/s; np=6 cm-3; B=(4+9) nT, Tr=10.104 K. (increases with increasing speed); parameter β<1; высокое содержание гелия (4 –:6)% . [Ermolaev, 1990; Yermolaev, Stupin, 1997].
Figure 2. Typical example of parameter distribution. in the body of the VSP..
The parameters of the VSP from the CD vary greatly both from flow to flow and within the flow, but the main properties, namely, the magnitude of the magnetic field modulus that does not change in the body of the flow IN, low, often lower than for a calm solar wind, concentration n, high speed, very slowly falling over several days, remain mandatory for the VSP body from CD.
A characteristic feature of SSW is the existence in the body of a stream of long trains of Alfven waves propagating from the Sun (High Intensity Long Duration Continuous AE Activity, HDLDCAA). The period of these Alfven nylons near the Earth's orbit can average T=3+8 hours. These waves are responsible for the appearance of Bz components near the Earth's orbit. A typical example of parameter distribution. in the body of the VSP is shown in Fig. 2 .
VSP edge
The SSW edge is the region of interaction between the SSW and the low-speed solar wind, separating plasma of distinctly different properties and origins (interface). The leading leading edge of the VSP from the CH is formed as a result of the rotation of the VSP together with the Sun, and here the fast wind catches up with the slow one, forming a compression region. Strictly speaking, the leading edge of a VSP is not a quasi-stationary flow; it should rather be classified as a non-stationary phenomenon, although it rarely becomes sharp enough to form shock jules within 1AU. . The following changes in parameters are characteristic of the edge: the speed increases from the level of the calm solar wind to the speed in the VSP body (on average from v = 350 to 550 km/s); the n concentration increases sharply from the calm solar wind (=5 cm-3) to 20 cm-3 and then sharply drops to 5 cm-3 or less; T increases from approximately (2K to (10-15).104 K in the body of the VSP; the distribution of B is bell-shaped with a maximum of about 12+15 nT.
That. for the VSP edge: vp=550 km/s; np=20 cm-3; Tr=(10-15).104 K.
In addition to the leading edge, the VSP also has a second, trailing edge, but it is very blurred and is identified only by small increases in n and V. The speed in this case is almost reduced to the speed of the calm solar wind, and this edge is not very geoeffective. The Earth's crossing of the VSP edge lasts about 12-15 hours.
Based on the characteristics of CHs and SSWs emanating from them described above, it is possible to identify high-speed flows in Earth’s orbit. In this work, we take only those streams for which there were coronal holes on the Sun of the corresponding magnetic polarity with a shift of about 2.5+3 days relative to the date of CH passage through the central meridian to take into account the time of transportation of solar plasma from the Sun.
GTS and streamer
Quasi-stationary types of solar wind also include the heliospheric current sheet (HCS) and the coronal streamer. The GTS is formed as a dividing surface between flows carrying large-scale magnetic fields of opposite polarity. The heliospheric current sheet encircles the Sun and it is the central part of the heliospheric plasma layer, which is a belt of coronal rays (streamers). These coronal rays start from the tops of helmet-shaped structures, which have a closed configuration of magnetic field lines at their base, but the magnetic fields of the rays themselves have an open, non-converging configuration (Fig. 2).
Due to the specific configuration of the magnetic field in the HCS and in the streamer, the flux density decreases with distance more slowly than in a conventional radial flow, thus providing a high plasma density to the flux [Kovalenko, 1983]. The heliospheric current layer is visible on the solar disk as a neutral line, where the radial component is equal to zero: Br=0.
The HCS is a very stable formation throughout the heliosphere and exists without significant changes for years, although the shape of the HCS, determined by the distribution of large-scale magnetic fields on the Sun, can change from one solar revolution to another. The shape of the HCS and its location change especially clearly during the solar activity cycle: during the years of minimum the HCS is located approximately in the equatorial plane of the Sun; at other times, especially at the maximum of the cycle, its shape and location can be arbitrary [Kovalenko, 1983]. In Earth's orbit, the GCS is identified as the boundary of the sector structure of the interplanetary magnetic field (IMF).
In the literature, when determining the types of solar wind flows, some authors consider the plasma layer and the GCS together, while others consider apart. However, the HTS has slightly different parameters in Earth’s orbit: it is in the HTS that the sign of the radial component of the IMF changes; here the solar wind has the lowest speed and the highest density. It is by these properties that the identification of hydraulic structures occurs. The streamer is characterized by a density that is lower than in the HTS, but still increased compared to the undisturbed wind, a speed greater than in the HTS, and an increase compared to the HTS of module B. In general, the most important difference from other types of salt wind for the heliospheric plasma layer and the HTS is a change in the sign of the permafrost, and as an inherent property of them, high density. On average, a quiet streamer is characterized by the following parameter values
vp=360 km/s; np=(10-15) cm-3; Tr=5.104 K; B=(7-10) nT,
and for a quiet GTS:
vp=350 km/s; np=(20-30) cm-3; Tr=5.104 K.
A quiet plasma layer is characterized by symmetry of the parameter values on both sides of the GCS.
A disturbed streamer in Earth's orbit appears as a result of its interaction with disturbed solar wind flows, which can be slowed down by the dense plasma of the streamer, forming a complex disturbance by the time it reaches the Earth. As a result of this, a violation of the symmetry of the streamer may occur, an increase in all parameters of the streamer and the GTS, which can differ greatly from one event to another: here, some of the highest values for the solar wind density are possible (n>50 cm-3), speeds can increase to (450-500) km/s, increasing module B, increasing mass flux and energy flux density. For HTS with increased concentration up to n=(30-40)cm-3, β
>1
.
Interstream plasma
Among the quasi-stationary flows in work A type of low-speed cold dense plasma has also been identified, which arises in the solar wind between the streamer and high-speed flows from the CH. This type in Earth orbit is identified as a Type III non-compressional density enhancement Noncompressive Dencity Enhancement (NCDE) [Kovalenko, Filippov, 1982] and is characterized by a small value of the modulus B=3 nT; low T=2.104 K; low speed v = 350 km/s and slightly increased density n = (10-2 cm-3). This type of solar wind flow is especially common during the decline of the solar cycle, when up to 75% of all large-scale coronal holes were accompanied by NCDEs in the solar wind. The duration of the intersection of these flows with the Earth is approximately 14 hours.
3. Unsteady flows
Solar storms
Unsteady solar wind flows are caused by unsteady sporadic phenomena on the Sun. The most effective of them is the so-called solar storm, when a significant amount of energy (1erg) is released in a relatively short time (=2.103 s).
In the optical range, a solar storm is visible as a solar flare, manifested mainly in a sudden increase in the brightness of the Hα line radiation. At the same time, intense X-ray, ultraviolet and radio emissions, shock waves, and emissions of plasma clouds are observed. Historically, a solar storm is usually called simply a cromospheric flare, and all other events are called accompanying events, although all this is a single, very complex phenomenon that covers almost all layers from the photosphere to the corona and interplanetary space.
The parameters of the optical flash are a score determined by the size of the area on a five-point scale, duration and brightness. Flares are visible from several minutes to several hours, the most probable duration of a flare is about 1 hour for points 3 and 4. Based on the bursts of soft X-ray radiation accompanying the flare and their maximum intensity in the range of 1-8 A, flares are divided into 3 classes: ( S, M, X). There is no unambiguous correspondence between the characteristics of flares based on optical and X-ray characteristics. Most solar flares occur in complex multipolar active regions during the period of their rapid evolution.
The sequence of development of solar storms (the “scenario”) is not generally accepted. Below we present some of them. In progress [Mogilevsky, 1987] It is assumed that the fundamental basis of these events are nonlinear wave processes in the form of solitary disturbances (MHD solitons, MHD wave trains) emerging from the subphotospheric layers of active regions. The latter can provide: an appropriate output of energy and matter (=1016 g), sufficient not only for the appearance of optical flares, but also ensuring the generation of coronal transients. Coronal transients associated in some way with optical flares are called F transients. The energy of coronal transients is an order of magnitude greater than the energy of the largest optical flares, and they begin at the level of the photosphere and chromosphere 15-25 minutes earlier. Apparently, the entire complex of flare phenomena can be considered secondary, determined by the passage of the F transient through the active region. Coronal transients are better known as coronal mass ejections. (CME - Coronal Mass Injection).
In the works It is proposed that the main cause of solar activity is the evolution of the solar magnetic field. In this case, as a result of instabilities, reconnection, and the ascent of new photospheric material with a different polarity, a significant mass of matter (CME) is ejected, which, propagating in the corona and solar wind, can generate a shock wave and lead to the acceleration of some of the particles in the corona and solar wind to significant energies. Upon reaching the Earth's orbit, this interplanetary disturbance can cause a geomagnetic storm, when the Earth collides first with a shock wave and then with the CME itself, identified in Earth's orbit as a magnetic cloud, although it remains unclear whether the material inside the CME was born in the outburst , that is, in the chromosphere, or in the corona itself.
In the works Bravo a slightly different scenario is described. The emergence of new photospheric material of opposite polarity, which in itself is a common phenomenon on the Sun, leads to a restructuring of magnetic fields in the solar photosphere. If this occurs near the coronal helmet or coronal hole, then the restructuring of the magnetic field can lead to CME, which will propagate along open magnetic field lines up to the Earth's orbit.
Disappearance of fibers
Another possible source of sporadic unsteady solar wind flux is the EP-type coronal transient [Chertok, 1987] its manifestation on the surface of the sun is the sudden disappearance of large dark filaments observed on the disk in the absorption of the H>α line. The characteristic time of this event ranges from tens of minutes to hours. The filament visible on the limb is called a prominence, and its disappearance is visible as the eruption of this prominence, sometimes for a long duration and at distances of several solar radii.
The lifetime of the filaments ranges from minutes to weeks, the prominence is characterized by a high density and lower temperature than the surrounding coronal plasma. According to the nature of movement and variability, they are divided into three classes: calm, active and eruptive. Active fibers usually have a loop shape (one or more one after another). Eruptive filaments are characterized by violent and sudden changes. Some of them are closely related to solar flares, forming part of the flare process. However, the disappearance of a fiber can also be an independent process both in the active region and outside it.
The disappearance of a fiber may be accompanied in the radio range by a noise storm and/or a weak type IV burst. At a heliocentric distance r=1.5+10 Rc, coronal transients of the EP type have the form of an expanding loop, a bubble, or a whole system of loops. Although there may be other forms: fan-shaped, luminous halos, diffuse clouds. The characteristic expansion speed is from 100 to 400 km/s, sometimes up to 800 km/s.
The released energy averages 1 erg. Is there a close relationship between moving fiber and CME? Most likely, the fiber in the corona can be considered as a CME or part of it. Thus, at the exit from the corona there is ejected material (CME) associated with other forms of solar activity, such as solar flares and eruptive prominences. CMEs are born in regions with closed magnetic field lines in the lower corona. Typically, these closed magnetic field regions are located at the base of the coronal streamer, but CMEs can also appear at much higher heliolatitudes and without connection to active regions.
In those sporadic solar activity events when CME and flares are in close temporal relationship, CME begins 15-25 minutes earlier, and often the flare location is near one of the edges of the CME, since the CME is much wider (tens of degrees). CME often (1/3 of all cases) occurs in combination with events of long duration (many hours) in the soft X-ray range (LDE - Long Duration Events). LDE is likely related to the rearrangement of the solar corona after the CME ejection and involves the formation of new loops of hot material low in the corona.
The leading edges of fast CMEs have radial velocities from the Sun much greater than the solar wind speeds, so a shock wave should form in front of the CME. Indeed, virtually all shocks in the solar wind originate from the motion of CMEs, which at 1AU are characterized by the following features:
- 1. Counterstreaming (along the field) of a halo of electrons; 2. Counterstreaming of energetic protons (>20 keV); 3. Increased helium content (He++/H+ >-0.08); 4. Reduced temperature of ions and electrons; 5. Strong magnetic fields (> 8 nT); 6. Low plasma number β<1);
7. Small variations in magnetic field strength; 8. Rotation of the magnetic field.
However, the most reliable of them is the counterstreaming flow of superthermal halo electrons with energy >80 eV, meaning a closed magnetic field topology typical of CME, in contrast to the open topology of field lines inside the normal solar wind.
Only 1/3 of the CME is accompanied by a shock wave, and only 1/6 of the CME directed towards the Earth causes a large geomagnetic storm. Interplanetary current ropes are usually known as magnetic clouds if the magnetic field strength exceeds 1AU≈10 nT. The frequency of occurrence of CME varies significantly in the solar activity cycle, amounting to about 6 cases per month in years of maximum and 8 cases per year in minimum solar activity. Interplanetary disturbances associated with fast CMEs, which are characterized by high speed and high magnetic field strengths (often with a large southerly component), can be very geoeffective. The very strong magnetic fields in such disturbances are mainly the result of compression in the interplanetary medium. The orientation of the field ahead of the CME (this is the space between the shock front and the CME itself, called the shock layer) is an effect of the draping of field lines near the OME, while the field orientation within the CME itself is determined by conditions on the Sun.
Very large geomagnetic storms are caused by a CME with a shock wave or a shock wave only, large storms can also be caused by a CME only. It is obvious that a shock wave can be observed at a separate point without CME, since the shock wave occupies a much larger space () than the causing CME (50-700).
Thus, transient ejections of material from the Sun in the form of CMEs are the best link between solar activity and non-recurrent events in the Earth's magnetosphere.
The behavior of CME over time is modeled
.
Unsteady flows in interplanetary space in Earth's orbit have two large structural areas: shock waves and magnetic clouds. The arrival of a shock wave on Earth is identified according to two main criteria [ Zastenker, Borodkova, 1984; Borrini et al., 1982; Ivanov, 1996]:
- 1. Registration in the Earth's magnetic field of a sudden onset of SC or a sudden pulse of SI; 2. Large sharp and simultaneous change in solar wind parameters:dv>150 km/s; nAndTmay increase several times;dB>0,increased fluctuations of the electric field and plasma flow, a sharp increase in energy flow.
The delay time of the shock wave relative to the solar storm is dT = tsc - tstorm = 24-48 hours.
Flare and filament streams
Historically, sporadic showers initiated by large solar storms with solar flares are called flare(an example of the behavior of parameters in a flare flow is shown in Fig. 3), and those initiated by sudden disappearances of filaments - fiber. Since they have slightly different characteristics in Earth’s orbit, we will consider them separately and call them flares and filaments. In models of flare streams, either, as in the work, [Hundhausen, 1976], two boundaries: the front of a fast MHD shock wave and the boundary of the flare ejection and two structural regions: the shock layer and the flare ejection, or as in the works of Ivanov
five structural boundaries: fast shock front Sf, slow shock front Ss, magnetopause of the magnetic cloud Ri; internal boundary of the boundary layer Rl"; boundary of He++-enriched plasma (plasmopause) Rп/SUB> and, accordingly, five structural regions: Sf - Ss - head shock layer of the fast wave (dense, hot turbulent plasma with an increased magnetic field, dt - hours;) Ss - Ri - shock layer of a slow wave (dense, n=nmax for the entire flow, hot turbulent plasma with a reduced magnetic field, B=Bmin for the entire flow); Ri - Ri" boundary layer in a strong field with decreasing n, relatively high level turbulence; Ri" - Rп - the inner part of the magnetic cloud with a strong B=Bmax for the entire flux, a regular field, the direction of which, as a rule, differs from the direction in the environment, and with low density values; beyond Rп - the plasmasphere. 
Figure 3. Typical distribution of parameters in a flare stream.
For filament streams, which are likely caused by EP-type transients, the most striking is the large increase in density (2-7 times) of the relatively quiet solar wind. Often these density increases can be uncompressed (NCDE type 1 [Kovalenko, Filippov, 1982], they are characterized by: a sharp front, short duration (dt=10 hours), propagation time to the Earth is 3-4 days, high density (n>≈ 25 cm ~), speed v>400 km/s and increased IMF value ( B>10 nT). There is often no shock wave in front of them. However, in approximately half of these phenomena, the increase in density occurs simultaneously with an increase in the speed and temperature of the protons [Ivanov, Kharshiladze, 1994]. For such "compressed" density increases there were often sudden onsets (SC and SI) and a shock wave. Compared to flare flows, filament flows are dense, slow, and cold.
Let us dwell on one more aspect of solar-terrestrial interaction. Often solar activity develops in such a way that streams from several solar sources can simultaneously enter the Earth’s orbit; this depends both on the solar storm scenario and on the location of these sources, when both quasi-stationary and transient flows interact. As a result, a composite flow with very complex characteristics appears in the Earth's orbit, often with several maxima and with significantly higher parameters than those characteristic of a single source. It is these composite flows in the solar wind that can cause the largest geomagnetic and auroral events on Earth.
Thus, fluxes from different sources on the Sun have different, but well-defined limits of parameters in the Earth’s orbit. In addition, quasi-stationary flows in the solar wind do not change their characteristics during the time required for the Earth to cross these flows during its orbit around the Sun. The day of non-stationary processes is characterized by a rapid change in the parameters of the flow both during its formation and during its propagation, and the most typical example of a non-stationary flow is a shock wave.
The main parameters of various types of solar wind are summarized in the table.
Characteristics of various types of solar wind streams
options | streamer | VSP edge | Shock layer | |||
V, km/s | ||||||
At a speed of 300–1200 km/s into the surrounding outer space.
Characteristics
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 (); the nuclei of other elements and non-ionized particles (electrically neutral) are contained in very small quantities.
Although the solar wind comes from the outer layer of the Sun, it does not reflect the actual composition of the elements in this layer, since as a result of differentiation processes the content of some elements increases and some decreases (FIP effect).
The intensity of the solar wind depends on changes in activity and its sources. Depending on the speed, solar wind flows are divided into two classes: slow(approximately 300-400 km/s around orbit) and fast(600–700 km/s around the Earth’s orbit).
There are also sporadic high speed(up to 1200 km/s) short-term flows.
Slow solar wind
The slow solar wind is generated by the “quiet” part during its gas-dynamic expansion: at a coronal temperature of about 2 × 10 6 K, the corona cannot be in conditions of hydrostatic equilibrium, and this expansion, under the existing boundary conditions, should lead to the acceleration of coronal matter to supersonic speeds. Heating of the solar corona to such temperatures occurs due to the nature of heat transfer in: the development of convective turbulence in the 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; are effectively absorbed by the corona matter and heat it to a temperature of 1 - 3 × 10 6 K.
Fast solar wind
Streams of recurrent fast solar wind are emitted over several months, and have a return period when observed from Earth of 27 days (the period of rotation of the Sun). These flows are associated with - regions of the corona with relatively low temperature (about 0.8 × 10 6 K), reduced density (only a quarter of the density of the quiescent regions of the corona), and radial to the Sun.
High speed streams
Sporadic flows, when moving in space filled with slow solar wind, condense the plasma in front of their front, forming a plasma moving with it. It was previously assumed that such streams were caused by solar flares, but it is now (2005) believed that sporadic high-speed streams in the solar wind are caused by coronal ejections. At the same time, it should be noted that both solar flares and coronal ejections are associated with the same active regions on the Sun and there is a relationship between them.
Solar radiation is the energy of electromagnetic radiation from the Sun.
Solar radiation, which arrived at the upper boundary of the atmosphere, on its way to the earth's surface undergoes a number of changes caused by its absorption and scattering in the atmosphere.
Radiation that comes to Earth directly from the solar disk is called direct solar radiationS. (The radiation that came from the Sun into the atmosphere and then onto the earth’s surface in the form of a parallel beam of rays is called direct solar radiation).
Scattered radiationD comes to the earth's surface from the entire vault of heaven and is assessed by the flux of solar radiation, i.e. the amount of energy that comes per unit time per unit horizontal surface. (Part of solar radiation is scattered by molecules of atmospheric gases and aerosols and enters the earth's surface in the form scattered radiation).
The part of solar radiation that is reflected from the earth's surface and atmosphere (mainly from clouds) is called reflected radiation.
The earth and atmosphere continuously emit invisible infrared radiation. The Earth's radiation is almost completely absorbed by the atmosphere. The portion of atmospheric radiation directed towards the Earth is called counter radiation from the atmosphere.
Part of the atmospheric radiation directed upward and passing through the entire thickness of the atmosphere is directed into outer space and is called outgoing atmospheric radiation.
All of the listed radiant energy flows differ from each other in spectral composition, that is, in wavelengths. In meteorology, radiation is considered:
Short wavelength (wavelength 0.1-4 microns);
Long wavelength (4 – 120 microns).
Solar radiation is mainly short-wave (ultraviolet, visible, infrared). Radiation from the earth's surface and atmosphere is long-wave.
Radiant energy is characterized by the flow of radiation.
Radiation flux- this is the amount of radiant energy that enters per unit time per unit surface. Measured in W/m2.
The amount of direct radiation S that comes per unit time per unit surface perpendicular to the sun's rays is called direct radiation flux density.
The branch of meteorology that studies solar, terrestrial and atmospheric radiation is called actinometry. The main task of actinometry is to measure radiant energy fluxes. Radiation is dissipated in the atmosphere mainly by molecules of atmospheric gases and aerosols (dust, fog droplets, clouds). The intensity of scattering depends on the number of scattering particles per unit volume, on their size and nature, as well as on the wavelengths of the radiation itself that is scattered.
According to Rayleigh's law, the intensity of molecular scattering is inversely proportional to the fourth power of the wavelength, that is:
K – scattering intensity coefficient;
λ – wavelength,
C is a coefficient that depends on the number of gas molecules per unit volume of gas and on the nature of the gas.
Table 1.1 - The value of the dissipation coefficient in clean and dry air at
normal pressure for different wavelengths
The table shows that the shorter the wavelength, the more strongly the rays are scattered. Violet rays are scattered 14 times more strongly than red ones. This explains the blue color of the sky. Although violet and blue rays are scattered even more than blue rays, their energy is much less. Therefore, in diffused light, blue color predominates.
In 1957, University of Chicago professor E. Parker theoretically predicted a phenomenon that became known as 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. Since spacecraft before 1990, when the Ulysses spacecraft was launched, mainly flew in the ecliptic plane, 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) during one 11-year solar cycle of 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 a physical phenomenon that is of not only 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 influence our lives to one degree or another. 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 solar-terrestrial connections that are important for practical human activity. However, this is another area of scientific research, which we will not touch upon in this article.
Vladimir Baranov