SPATIAL PHYSICAL

SPATIAL, PHYSICAL .– Under this denomination there is a vast field of physical research that has taken on a vast and more precise physiognomy with the advent of artificial satellites, for the possibility of direct exploration of the space surrounding the Earth and more generally of the interplanetary space. As such, the fs has a distinctly interdisciplinary character, as in it coexist, among others, the physics of plasmas, solar physics, the physics of the planets, the geophysics. The space surrounding the Earth and the planets, and in general the interplanetary space constitute an immense laboratory in which electrically charged particles, magnetic and electric fields, electromagnetic radiation, thermodynamic and electromagnetic collective phenomena manifest and influence each other.

The denomination itself of fs must be interpreted in the sense of physics (“in” space (rather than in that of physics ([of “space), that is, of study and interpretation of the phenomena that occur in space. We limit ourselves in this context to consider some fundamental topics of fs and precisely the solar wind and its interaction with the outer shell of the planets up to the limits of the interplanetary space, as well as the formation of the magnetosphere and the radiation belts.

The solar wind . Historical background . – The solar origin of the electromagnetic radiation that reaches the Earth is well known. About half of the energy conveyed by this radiation is in the visible region of the spectrum. This, however, extends from the lowest frequencies, lower than those of radio waves, to much higher ones (for example, X and γ rays). That the Sun was also a source of corpuscular radiation has long been postulated on the basis of indirect arguments, but only recently verified experimentally using instruments carried by spacecraft in the interplanetary space.

GD Cassini (1672) has the idea that the zodiacal light, a weak luminosity observable near the equatorial plane, was light diffused by corpuscles. The idea of ​​particle beams, to which to attribute magnetic storms, was put forward for the first time at the end of the last century by G. Fitgerald (1892) and by Sir O. Lodge (1900). The experiments of K. Birkeland (1903) and the calculations of C. Störmer of orbits of electrically charged corpuscles, coming from the Sun and interacting with the magnetic field of the Earth, were the first concrete steps. In 1919 FA Lindemann specified the idea that the clouds of particles emitted by the Sun had to be made up of electric charges of the two signs in order to constitute an overall neutral gas. In the years 1931-40 the theory of S. Chapman and VCA finally took shape

The last step towards the interplanetary space scheme, now universally adopted in its general lines, is based on the study of the deviation of the tail of certain comets from the radial direction. While in the idea of ​​Chapman and Ferraro it was believed that bundles of charged particles originated on the Sun sporadically, the idea suggested by the observations of C. Hoffmeister (1943) and then more quantitatively elaborated and extended by L. Biermann in the years 1951-57 is that the Sun emits particles continuously, that is, it is a permanent source of corpuscular radiation. From a theoretical point of view the general properties of this radiation were predicted by EN Parker in the years 1957 and immediately following: the name “solar wind” was coined by Parker in 1958.Lunik III and Venus I ; shortly afterwards, the American satellite Explorer X (1961) and above all the Mariner 2 probe definitively confirmed the existence of a continuous, albeit variable over time, flow of solar particles. The fact that the particles are electrically charged, mainly protons and electrons, together with the observation that the Sun has intense magnetic fields, implies (see below) that the solar wind is also permeated by a magnetic field, that is, a magnetized plasma .

L and observation techniques . – Particle detection . – The detection of solar wind particles, whose energy is of the order of hundreds of eV, requires the use of instruments without a wall. These are essentially suitable charge collectors, mostly preceded by devices designed to deflect the trajectory of the incident particles, both to select them in energy and to remedy the disturbing effect produced by sunlight. The detectors can be Faraday wells, electrostatic deflectors, etc.

In the well (or cup) of Faraday, which is a real metal well at the entrance of which one or more metal grids are located, the flow of particles is detected by measuring the electric current that flows to the collector c (fig. 1 A). The geometric characteristics of the cup and appropriate electric fields created between the various grids and the collector allow to separate the contributions determined by particles in the various energy ranges, thus allowing to trace the direction of origin, the density of particles in the beam, their distribution in speed, at the “temperature” of the beam, etc. In electrostatic deflectors the selection in energy consists in passing the particles between two parallel spherical or cylindrical electrodes, between which an electric field is established; only the particles endowed with certain energies manage not to fall on one or the other of the electrodes, so as to finally reach the collector c (fig. 1 B ).

When the particle flows are modest, more sensitive detection devices are used, in particular certain multiplication detectors, in which each particle, affecting a special surface, on average gives rise to more than one secondary particle and these, under the action of an appropriate electric field, they again hit the same surface, so as to enhance the phenomenon of multiplication. In this way the flow that finally hits the collector can be increased by several orders of magnitude. And it is, at the limit, also possible to individually count the particles incident by the current pulses produced by each of them. The separation between electrons and positive ions, mainly protons, can be done with similar devices but using electric fields of opposite direction.

Detection of magnetic fields . – The determination of magnetic fields in the interplanetary space is done by saturation magnetometers ( flux – gate ), by nuclear effects or by induction. While the first two types were mainly used for constant or slowly variable field measurements, induction magnetometers are suitable for measuring rapidly variable fields.

In saturation magnetometers (fig. 2) we exploit the fact that by magnetizing a rod (or two rods) of ferromagnetic material up to saturation with a variable current of frequency f , a distortion of the product field detectable is determined on a secondary circuit at which a second harmonic induced voltage of the excitation voltage can be observed, i.e. at frequency 2f , the amplitude of which is proportional to the component of the ambient magnetic field in the direction of the rod. With three sensor elements, mutually perpendicular, it is possible to completely determine the magnetic field vector.

In the induction magnetometer the measurement of the field is traced back to that of the potential difference induced at the ends of a coil due to the effect of the electromagnetic induction in it caused by a variable magnetic field. The variation of the field can be both intrinsic to the magnetic field and determined by the fact that the coil is mounted on a vehicle that moves with respect to the environment that is permeated by the field. Since the coil is sensitive only to the component of the magnetic field perpendicular to the plane of its turns, also in this case the magnetic vector can be determined by means of three mutually perpendicular coils.

The principle of operation of nuclear effect magnetometers is somewhat more complicated and is based on particular effects within certain atomic nuclei. In general it can be said that a very precise measurement of the value of the magnetic field can be traced back to that of the frequency of the electromagnetic radiation emitted in the quantum leap from a higher energy level to a lower energy level. In the case of the nuclear precession magnetometer, the transitions between energy levels of the hydrogen nuclei are exploited. In the case of “optical pumping” magnetometers, the transitions between electronic levels whose separation in energy is proportional to the ambient magnetic field are exploited. A third type of magnetometer, which takes advantage of the transitions between metastable helium levels,

Experimental observations in the interplanetary space between the Sun and the Earth . – The experimental data obtained with the detectors described above is on the one hand the flow of particles, their distribution in speed and direction of origin, on the other the magnetic vector.

The direction of origin of the solar wind is generally obtained by exploiting the rotation of the spacecraft around an axis, which is almost always perpendicular to the plane of the ecliptic. Under these conditions, the detector “looks” in all azimuth directions as time changes. The energy spectrum is instead obtained from consecutive measurements on different energy intervals, obtained by varying the electric fields involved in the detection devices.

In fig. 3 shows a typical example of an energy spectrum. The number of particles counted with the detector pointed towards the Sun has a very evident maximum at energy of about 0.88 keV, while the flow of particles observed at this energy is restricted to an angle of a few degrees centered around the direction that goes to the Sun. From trends of the type shown in the figure it is possible to determine the average velocity of the particles, as well as their “temperature” which measures the dispersion of the velocity around the average.

It is also possible to obtain information on the type of particles that make up the solar wind: for example, the small secondary maximum that appears at about 1.7 keV is attributable to particles (helium nuclei), present in the ratio of a few percent of the main maximum due to protons. Other heavy ions such as carbon, oxygen, silicon and iron have also been identified in small percentages.

The particles of the solar wind move in space moving away from the Sun in an almost radial direction with speeds that can vary considerably, remaining however within the approximate limits of 200 and 900 km / sec. The overall speed of the electrons and protons is substantially the same; their temperatures, on the other hand, are considerably different, of the order of 10 4 ° K for protons and 10 5 ° K for electrons. Particle densities vary within wide limits between 0.5 and 100 per cm 3 . These data and others of interest are collected in the table.

A very important fact is that the dispersion of velocities, in simpler terms the “width” of their distribution curve, is different according to the direction in which the detector is pointed. The maximum tends to manifest itself in a direction which near the Earth is inclined by about 45 ° with respect to the radial direction; the minimum is found in a direction approximately perpendicular to the first. This shows that the solar wind is thermally anisotropic.

As for the chemical composition, the existence, alongside the electrons, protons and α particles, of very small percentages of other nuclei, in particular nuclei of carbon, and of variously ionized oxygen, of silicon and iron, has been found.

As a result of the solar rotation, particles emitted from a given point on the solar surface at different times move in different directions. The succession of these particles is what constitutes a beam which appears curved, in the same way as a jet of water emitted horizontally from a rotating platform appears with respect to an observer standing on the platform itself.

As regards the magnetic field, the results are summarized in figs. 4a and b , where respectively the percentage distribution in intensity and the orientation of the field are reported, identified by the angles Θ and ϕ which are the angle that the field forms with the plane of the ecliptic and the angle that the projection of the field itself on this plane it forms with the direction that goes from the Earth to the Sun. It appears from the figure that the average value of the interplanetary field is about 5.5 γ ( i γ = 10 -5oersted), that is about 10,000 times less than that of the Earth on the ground; the direction of the field on the ecliptic plane is preferentially oriented at 135 ° from the Earth-Sun line, or the opposite one at 315 °, while the elevation angle on the ecliptic is fairly symmetrically distributed around the zero value, corresponding to a field medium lying on the plane of the ecliptic itself.

The study of magnetic data has highlighted the existence of a large structure, identified by the so-called “magnetic sectors”. These are regions of space in which the average direction of the field is leaving the Sun or entering the Sun. Fig. 5 clearly shows this behavior, observable on the angle of the field on the ecliptic plane. During the 27 days, corresponding to an entire solar rotation, up to 4 sectors were observed, alternatively positive, that is, with an outgoing field, and negative with an entering field.

The behavior of the solar wind and magnetic field within a sector is quite typical: the density of particles is maximum between 1 and 2 days from the entry into a sector; the field is maximum shortly after the maximum density; also the so-called geomagnetic activity (see magnetism: Earth magnetism, App. III, 11, p. 9) it is maximum a couple of days after the start of a sector. The existence of the sectors and above all their clear tendency to recur in one or more successive solar rotations shows that the magnetic field has its roots on the Sun, in fairly well-defined regions, which also tend to remain on the Sun for more than 27 days. This is precisely the reason that determines the reproduction of similar characteristics in the structure of the magnetic field in successive solar rotations. From statistical studies on the correlation between the interplanetary magnetic field and the magnetic field in the solar photosphere, there is a maximum correlation when the field observed near the Earth is compared with that observed about four days earlier on the Sun.

From the first observations of solar wind to today, 15 years have passed covering more than one cycle of solar activity. It is an important fact that appreciable variations in the characteristics of the solar wind and the interplanetary magnetic field have not been observed over long periods. In other words, there is no indication of significant variations with the solar cycle. It is not inappropriate to point out that up to now the experimental observations, of which we have referred above, have been carried out on the equatorial plane of the Sun or in its immediate vicinity. As regards the physical conditions of the interplanetary medium outside of this plan, there are only a few indirect observations, which shows how our knowledge of the interplanetary space is still incomplete and in need of further study.

Theoretical models. – Until not many years ago it was believed that interplanetary space was essentially empty, with the Sun as the source of electromagnetic energy. As mentioned, the interplanetary space was believed to be crossed only sporadically by beams of charged particles responsible for magnetic storms and, more or less directly, for polar auroras. A noteworthy step was taken by S. Chapman in 1950 with a model of a solar atmosphere in hydrostatic equilibrium in which heat was transmitted by thermal conduction through the gas of the solar corona. The result was a situation in which the temperature of the crown varied very slowly away from the Sun; as a consequence, the hydrostatic pressure of the gas also decreased slowly enough to give rise to a expansion of the gas not contained by the force of solar gravity which, although very conspicuous near the Sun, was also not sufficient to retain the solar atmosphere. A more advanced model, proposed a few years later (1958) by E. Parker, considers the solar atmosphere in hydrodynamic equilibrium: in these conditions there is a continuous flow of particles escaping from the Sun, whose characteristics can be expected at the height of Earth’s orbit and, more generally, in the interplanetary space up to beyond the planets. The model was further elaborated to include the effect of a magnetic field that permeates the plasma as well as to take into account other important factors such as thermal conduction, the composition of the gas with two interacting components (protons and electrons, having different temperatures ).

Not going into details, the results can be summarized quickly as follows: the particles escaping from the Sun have a rapidly increasing speed in the first 10 ÷ 20 million km from the Sun until reaching a value almost independent of the heliocentric distance (fig. 6) . This value, in the order of hundreds of km / sec, depends in practice, near the Earth’s orbit, on the temperature of the gas found at the base of the solar corona. Such high velocity values, higher than the velocity of sound, make the flow of solar particles supersonic. As for the magnetic field present on the solar surface, it, as has been said, has its roots in it. But here comes an important feature of the interplanetary magnetic field: that of being, as they say, ” this reacts with extreme readiness to any changes in the magnetic field that permeates it. In other words, the fundamental laws of electromagnetism impose the constancy of the magnetic field flux through any closed line, each element of which moves with the speed of the solar wind particles at that same point. If the line widens, as a geometric consequence of the propagation of the solar wind towards the Earth, the intensity of the magnetic field is substantially attenuated, with the law of inverse proportionality to the square of the heliocentric distance. this reacts with extreme readiness to any changes in the magnetic field that permeates it. In other words, the fundamental laws of electromagnetism impose the constancy of the magnetic field flux through any closed line, each element of which moves with the speed of the solar wind particles at that same point. If the line widens, as a geometric consequence of the propagation of the solar wind towards the Earth, the intensity of the magnetic field is substantially attenuated, with the law of inverse proportionality to the square of the heliocentric distance. electromagnetism imposes the constancy of the magnetic field flux through any closed line every element of which moves with the speed of the solar wind particles in that same point. If the line widens, as a geometric consequence of the propagation of the solar wind towards the Earth, the intensity of the magnetic field is substantially attenuated, with the law of inverse proportionality to the square of the heliocentric distance. electromagnetism imposes the constancy of the magnetic field flux through any closed line each element of which moves with the speed of the solar wind particles at that same point. If the line widens, as a geometric consequence of the propagation of the solar wind towards the Earth, the intensity of the magnetic field is substantially attenuated, with the law of inverse proportionality to the square of the heliocentric distance.

The consequence of what has been said is that the rotation of the Sun with a period of about 27 days causes the roots of the field to be “dragged” like elastic threads from the region where they originated, while simultaneously the solar plasma, which propagates towards the external, tends to “pull” the lines of force of the field radially. The fairly intuitive result is that of having a magnetic field situation just like the one described in fig. 5.

The field lines have a spiral shape; the individual particles of the plasma move in a radial direction, just as happens with the tip of a turntable that moves in a radial direction on the plate, within a groove that constitutes a spiral that wraps around the center of the disk. The angle Φ that the field forms with the radial direction varies with the distance r from the Sun and with the speed V of the solar wind. Also variable with the distance are the two radial components, r , and azimuth, ϕ , of the magnetic field; we have respectively, indicating with Ω the angular velocity of the Sun, 0 the heliocentric distance at which the radial field is B0 :

At the height of the Earth’s orbit, with V = 400 km / sec, there is Φ ≈ 45 °; as for r and ϕ , the calculated values ​​are in excellent agreement with the measured ones.

The existence of a continuous flow of ionized particles from the Sun towards the interplanetary space has posed, among others, the problem of the interaction between the solar wind and the planets of the solar system and the question of whether, how and where the solar wind exhausts the his push towards outer space. The first problem will be examined in the next paragraph, while the second will now be briefly examined.

The particle density of the solar wind obviously decreases with the distance from the Sun, as the flow originally emitted by it tends to be distributed on an increasingly large advance front. It follows that the dynamic pressure that the solar wind possesses for its motion becomes progressively weaker. The “end” (or termination, in physical jargon) is determined by the balance between this pressure and the pressure exerted overall by the interstellar gas, including in it including cosmic rays, and by the interstellar magnetic field. The pressure exerted by the latter can be understood by observing that the trajectories of the particles are curved by the magnetic field so that the solar wind can no longer freely propagate. The wind is then contained and its further penetration into the interstellar space is prevented. It is generally believed that the distance at which this containment occurs is a few dozen astronomical units, that is, beyond the outer solar planets.

Interaction of the solar wind with the planets . – The solar wind interacts in different ways with the obstacle represented by a planetary body, depending on its physical characteristics (existence of atmosphere, magnetic field, electrical conductivity of the solid part, etc.).

a ) Interaction with Earth. The first case supported by direct observation is that of the Earth. Already at the time when magnetic storms were attributed to bundles of charged particles sporadically incident on the Earth’s magnetic field, S. Chapman and VCA Ferraro had pointed out that the plane advance front of a very large beam of particles underwent considerable deformation when approaching the Earth, the Earth’s magnetic field began to be felt with its deflecting action on the individual particles of the beam, however the field itself was modified due to the fact that the motion of particles as a whole is equivalent to electric currents, which generate a field magnetic. Of course the regions of the beam furthest from the line joining the Earth to the Sun, little or no perturbed, continuing their movement beyond the Earth and overcoming the particles deflected by the Earth’s magnetic field, they give rise to a “cavity” dug in the beam which incorporates the Earth’s magnetic field and which, in the opposite direction to the Sun, ends up closing at a distance more or less large and in different ways according to the various models proposed. The solar wind affecting the external region, very little dense, of the ionized atmosphere that surrounds the Earth and in which the Earth’s magnetic field is still very weak, determines a violent compression surface, called the impact front. It is a supersonic shock wave, that is, a discontinuity surface in correspondence of which there is on one side the undisturbed solar plasma which advances with supersonic speed and from the another a region of turbulence in which the compressed gas heats up strongly and the magnetic field is strongly intensified; the deviated solar wind tends to propagate in a different direction from that of incidence, lapping within a region in which the geomagnetic field is fairly regular, even if with modified characteristics compared to the simple one that would exist in the absence of the solar wind ( fig. 7). There is a point even if with modified characteristics compared to the simple one that would exist in the absence of the solar wind (fig. 7). There is a point even if with modified characteristics compared to the simple one that would exist in the absence of the solar wind (fig. 7). There is a pointS , called “stagnation” or “stop”, which represents the point of maximum approach of the solar wind to Earth. This point is the closer to Earth the higher the solar wind speed and its particle density. In the typical case of 4 particles / cm 3 density and a speed of 500 km / sec, the stagnation point is at a distance equal to about 10 times the radius of the Earth, that is, about 60,000 km from the Earth’s surface.

The terrestrial shock wave is a phenomenon that in stationary conditions of the solar wind flow is also stationary, except for the slow diurnal variation determined by the variable inclination of the geomagnetic dipole with respect to the direction of the solar wind. The structure of the shock wave can be identified as that of a collision-free shock; this means that in the region where it is formed the density of matter is so low that there is no collision between the individual particles (as is the case with gases under normal conditions). This implies mechanisms of interaction between solar wind, ambient gas and magnetic field which are completely different from those of ordinary gases and on the other hand, as observation data show, very effective. The formation and properties of the wave collisions are described in a remarkably satisfactory way on the basis of the magnetofluid dynamics equations. It is found that the shock wave is what with technical term is called “fast magnetosonic wave” (magnetosonic fast wave ), which the solar wind penetrates through its surface for a thickness that is of few terrestrial rays, in front of the Sun, and that grows progressively moving towards the anti-solar part. From the mathematical point of view the problem is very complex, also because the equations that govern the characteristic equations of the shock wave are not linear and therefore allow a great variety of possible situations.

The surface inside which the solar plasma cannot penetrate is called “magnetopause”; this surface obviously includes the stagnation point. The whole region, whose extension goes from a minimum of 304 times the terrestrial ray to many tens of terrestrial rays, delimited by the magnetopause internally and by the shock wave externally, is called the transition region. As you move backwards away from Earth, the solar wind-field interaction gradually becomes more labile, but not less interesting for this. Still referring to fig. 7, a geomagnetic tail is given to a region several hundred long terrestrial rays, in which the magnetic field is substantially parallel to the direction of motion of the solar wind, with an orientation towards the Earth or away from the Earth, depending on whether you are above or below a certain plane coinciding with the terrestrial equatorial plane, at the equinoxes. It is recognized from the observations made by various spacecraft that the intensity of the field varies very slowly with the distance from the Earth and is zero in correspondence to a more or less flat surface called the neutral surface or plane. This tail constitutes, in a certain sense, a kind of magnetic analogue of the comet’s tail. The region delimited externally by the magnetopause and internally by the earth’s surface, called the magnetosphere (see below), is of extraordinary interest for the understanding of the physical phenomena associated with the motion of charged particles in the earth’s magnetic field, the northern lights and the external layers of the ionosphere.

b ) Interaction with the Moon . The type of interaction with the Moon is very different from that with the Earth, in that the Moon has no appreciable magnetic field: the instruments brought to its surface during the Apollo missionsand, on the other hand, the measurements made by satellites around it made it possible to evaluate the lunar magnetic field as several thousand times less than that of the Earth. Among other things, it is not excluded that this very weak surface field is due to meteorites lying near the measurement site. As for the solar wind, the nonexistence of an atmosphere causes its particles to fall directly on the lunar surface. Consequently, there is nothing similar to terrestrial shock wave and magnetopause; there is only, in the opposite direction to the Sun, the formation of an area free of particles, practically coinciding with the shadow zone of sunlight.

The magnetic field conveyed by the solar wind does not undergo particular changes due to the presence of the Moon, as shown by the field observed in front of the Sun and in the shaded area. This particularity allows to attribute a very low value to the electrical conductivity of the material constituting the Moon. If the conductivity were high, induced electric currents would be appreciable and, contrary to what direct observations show, the configuration of the magnetic field permeating the solar wind near the Moon would have changed.

c ) Interaction with Venus and Mars. Also in this case the magnetic field of the two planets is very small or even zero, as in the case of the Moon. However, the interaction with the solar wind is still of a different type, in that in both cases there exists an atmosphere which, like the terrestrial one, is ionized. Permeated by the variable magnetic field conveyed by the solar wind, the ionosphere of Venus and Mars becomes the seat of induced currents which deform the frozen magnetic field in the solar wind in a way that in the end is not too dissimilar from what happens in the terrestrial case. Even in this case, in fact, despite the remarkable diversity of interaction, an external shock wave and an internal surface is formed which has a configuration very similar to magnetopause. Some have proposed the name of anemopause,

d ) Interaction with Jupiter. The interaction of the solar wind with Jupiter is in many ways similar to that with Earth, in the sense that the planet has a magnetic field intense enough to exert an effective deflection action on the solar wind particles and shield against their penetration close to its surface. The distribution of the magnetic field around Jupiter is more difficult to interpret than in the terrestrial case, as it is strongly asymmetrical around the geometric axis of the planet. The description by means of a single large magnet (or dipole), as for the case of the Earth, is remarkably inaccurate, therefore additional contributions (quadrupole or octupole terms) must be taken into account. In any case, quite differently from Earth, the dipole is strongly eccentric: properly it is displaced by about a fifth of the planetary ray with respect to the center of Jupiter. The value of the magnetic field on the surface of the planet is 10 to 20 times higher than that of the Earth, that is to say, of the order of several oersted and oriented in the opposite direction to that of the Earth. This strong intensity of the field and, together, the enormous geometric dimensions of Jupiter (volume about 1000 times that of the Earth) make the planet itself a major obstacle on the path of the solar wind and, in any case, make its influence enormous on many phenomena of space interplanetary. As for the solar wind, the direct observations due, like those of the field, to the American probes The value of the magnetic field on the surface of the planet is 10 to 20 times higher than that of the Earth, that is to say, of the order of several oersted and oriented in the opposite direction to that of the Earth. This strong intensity of the field and, together, the enormous geometric dimensions of Jupiter (volume about 1000 times that of the Earth) make the planet itself a major obstacle on the path of the solar wind and, in any case, make its influence enormous on many phenomena of space interplanetary. As for the solar wind, the direct observations due, like those of the field, to the American probes The value of the magnetic field on the surface of the planet is 10 to 20 times higher than that of the Earth, that is to say, of the order of several oersted and oriented in the opposite direction to that of the Earth. This strong intensity of the field and, together, the enormous geometric dimensions of Jupiter (volume about 1000 times that of the Earth) make the planet itself a major obstacle on the path of the solar wind and, in any case, make its influence enormous on many phenomena of space interplanetary. As for the solar wind, the direct observations due, like those of the field, to the American probes the enormous geometric dimensions of Jupiter (volume about 1000 times that of the Earth) make the planet itself a major obstacle on the path of the solar wind and, in any case, make its influence enormous on many phenomena of interplanetary space. As for the solar wind, the direct observations due, like those of the field, to the American probes the huge geometric dimensions of Jupiter (volume about 1000 times that of the Earth) make the planet itself a major obstacle on the path of the solar wind and, in any case, make its influence enormous on many phenomena of interplanetary space. As for the solar wind, the direct observations due, like those of the field, to the American probesPioneer 10 and 11 (fig. 8 A ) show the existence of a shock wave similar to the terrestrial one at a distance generally slightly greater than 100 rays of Jupiter (about 70 million km). The situation is summarized in fig. 8 Bwhere various regions are indicated in which different regimes for particles are found. Inside the “magnetopause” there are trapped particles similar to those present in the terrestrial radiation belts. The fact that the magnetopause is at such great distances shows the existence of a considerable pressure of particles from inside the atmosphere of Jupiter which adds to the already remarkable containment action of the solar wind exerted by the magnetic field of the planet. It has also been found that at closer distances certain asymmetries in the distribution of the particles occur with the same periodicity of about ten hours, which represents the period of rotation of Jupiter around its axis. In an intermediate region, about 30 ÷ 50 rays of Jupiter, the lines of force of the magnetic field assume an “elongated” configuration, which in all probability in the anti-solar direction gives rise to a very long tail similar to that of Earth. An important fact is the discovery that Jupiter acts as a source of relativistic particles, in particular electrons of energy between 3 and 30 MeV, which appear channeled towards interplanetary space along the lines of force of the magnetic field. Much more observation data expect from a mission ( which appear channeled towards interplanetary space along the lines of force of the magnetic field. Much more observation data expect from a mission ( which appear channeled towards interplanetary space along the lines of force of the magnetic field. Much more observation data expect from a mission (Jupiter – orbiter ) currently in the definition phase which envisages the putting into orbit around Jupiter of artificial satellites.

e ) Interaction with Mercury . The electromagnetic environment surrounding Mercury is particularly interesting for its small size, just the opposite of Jupiter. The exploration is due to the Mariner 10 probe, passed three times near the planet. There is a more intense magnetic field than expected, although about 100 times less than the Earth’s on the planet’s surface. The presence of a magnetic cavity with a stagnation point of only 0.5 planetary rays was detected. A magnetic tail and a neutral layer are also evident, in the opposite direction to that of the Sun. The electric currents flowing on the magnetopause and on the neutral plane are intense enough to disrupt the distribution of the magnetic field to the surface of the planet itself. This leads us to believe (and observation data confirm this) that regions of trapping particles similar to the radiation belts around the Earth should not be expected around Mercury.

The terrestrial magnetosphere. – This region is represented, not in scale for reasons of clarity, in fig. 9, which shows the different regions that compose it and the configuration of the lines of force of the magnetic field. With reference to the latter, three distinct regions are identified: an innermost region in which the lines of the field are all the closer to those of a dipole, the closer they are to Earth; a second region in which the lines are, as it were, stretched and “stretched” in the opposite direction to the Sun while maintaining their connection with the earth’s surface; finally, a third region in which the lines are, so to speak, open, moving the terrestrial radius up to many hundreds of times to constitute, in the two half-spaces north and south of the equatorial plane, the two “lobes” north and south of the geomagnetic tail. As for the particles, four regions are distinguished: the first is a thin penetration region of the solar wind particles, called the mantle, and a kind of funnel that lowers down to the Earth to constitute the so-called polar “cusps”. A second region closest to Earth, called the “plasma layer”, whose plane of symmetry is what we have called the neutral plane above, constitutes a kind of reserve of high energy particles captured in the solar wind. In a third region, called the plasmasphere, permeated by geomagnetic field lines and close to Earth, particles of low energy ionospheric origin accumulate. Finally,

The dynamics of the complex system constituted by the magnetosphere can be roughly outlined as follows: a considerable flow of particles from the solar wind tends to penetrate through the geomagnetic tail and through the polar cusps towards the polar ionosphere, as shown in fig. 9. In the mantle the velocity of the particles is reduced compared to that of the solar wind becoming smaller and smaller penetrating the plasma layer. Associated with motion in the sun direction there is a slow drift effect (with a speed of a few km / sec) of the plasma towards the central part of the geomagnetic tail where the particles that make up the plasma layer accumulate. Of course, in undisturbed conditions, the accumulation of particles does not last indefinitely as it is establishes a state of dynamic equilibrium between those conveyed towards the plasma layer and those lost: in particular, a considerable flow of leakage can be identified in particles which, guided by the magnetic field lines in the part of the Earth opposite the Sun, go to precipitate towards the low atmosphere at high magnetic latitudes thus giving rise to the well-known auroral manifestations. The observation data give, in reality, good evidence of acceleration processes of particles up to energy of several keV, which are just in the order of those attributed to the particles that give rise to the polar auroras. guided by the magnetic field lines in the part of the Earth opposite the Sun, they precipitate towards the low atmosphere at high magnetic latitudes thus giving rise to the well-known auroral manifestations. The observation data give, in reality, good evidence of acceleration processes of particles up to energy of several keV, which are just in the order of those attributed to the particles that give rise to the polar auroras. guided by the magnetic field lines in the part of the Earth opposite the Sun, they precipitate towards the low atmosphere at high magnetic latitudes thus giving rise to the well-known auroral manifestations. The observation data give, in reality, good evidence of acceleration processes of particles up to energy of several keV, which are just in the order of those attributed to the particles that give rise to the polar auroras.

As for the plasmasphere, the observation data show that the typical energy of the particles is of various orders of magnitude lower than in the plasma layer and it is precisely this characteristic that makes one speak of “cold” plasma. The external limit of the plasmasphere is well identified by a sharp variation in the density of particles: upon entering the plasmasphere, an increase of 10 ÷ 100 times is observed with respect to the density present outside. The particles of the plasmasphere are sufficiently linked to the lines of force of the geomagnetic field to follow, as a whole, their rotation around the Earth’s axis: in other words they “corrode”.

It is important to note that the modification of the Earth’s magnetic field, the more relevant the further away from the Earth’s surface, implies the existence of electric currents (obviously associated with movements of charged particles). Of primary importance are the current system distributed over the magnetopause and that on the neutral plane: the first determines the state of separation between the magnetospheric and interplanetary fields conveyed by the solar wind; the second determines the typical configuration of the field in the two northern and southern lobes of the geomagnetic tail. Furthermore, the high electrical conductivity along the lines of the magnetic field (in reality electrically charged particles, moving along these lines, do not encounter obstacles to motion, as is the case of those moving transversally to them) implies the onset of electric currents that connect the various magnetospheric regions to the ionosphere. The same ionospheric current system, which has been postulated for many years to interpret the temporal variations of the geomagnetic field, is connected to magnetospheric current systems in a rather complex though intuitively plausible way.

F radiation axes or of V an A llen . – This designation indicates the internal part of the magnetosphere in which the lines of force of the geomagnetic field are closed and charged particles of high energy are “trapped”, up to several tens of MeV.

The discovery of the radiation belts occurred during the first American scientific mission on satellite, mounted on the Explorer I satellite . In fact, the experiment carried out by JA Van Allen (hence the name of Van Allen belts) aimed to study the trend with the share of cosmic radiation using a Geiger counter. The experiment was able to measure up to 200 ÷ 300 particles per cm 2and second, more than enough for the expected flows, which were significantly smaller. On the contrary, the result of the experiment was that the count went up gradually to a maximum and then, further increasing the quota, it went down to cancel itself. After some initial uncertainties, the interpretation, which later turned out to be correct, was that the particles incident on the meter became so numerous as to substantially modify their functioning: the counter was no longer capable, due to crowding, of “counting” the particles! Subsequent experiments designed to reveal much more intense flows, definitively confirmed the large increase in counting and above all allowed to establish the configuration of a vast region surrounding the Earth up to distances of a few tens of thousands of km: in the part closest to Earth there are large protons of energy up to a few tens of Mev, while in the most distant part there are electrons of energy up to a few MeV with great intensity. The physical characteristics of the measured flows are organized in the simplest and most natural way by referring to the Earth’s magnetic field: e.g. the distribution of the intensities measured by a given detector in the various points of a geographical meridian plane, that is, containing the Earth’s axis of geographical rotation, is not exactly the same on different meridian planes. In other words, there is no circular symmetry around the Earth’s axis. If, on the other hand, the distributions refer to the magnetic meridian planes, defined intormo to the axis of symmetry of the Earth’s magnetic field,

The first systematic study of the confinement region of energy particles took place with the mission of the Pioneer 3 probe , which, launched to reach the Moon, entered a very eccentric orbit around the Earth. Fig. 10 shows a summary of the measures, with indication of the two areas of maximum count that Van Allen indicated with the name of internal band and external band.

In reality it is now completely clear that the two maxima must be attributed to penetrating protons, of energy of several tens of MeV and beyond, the innermost one, and to penetrating electrons, of energy of the order of MeV, the most external one.

In other words, the different energy distribution of the particles in different points and the response characteristics of the detector used to the various types of particles can give results not immediately related to the real physical structure.

The discovery of radiation belts has posed numerous problems, in particular: the trajectories of trapped particles; the energy spectrum; the processes of origin and disappearance; the connection to other phenomena.

The problem of trajectories has its distant origins in the calculations elaborated by the Swedish physicist C. Störmer who studied systematically at the beginning of our century the motion of charged particles in the presence of a dipole magnetic field, such as the earth’s magnetic one. A very important fact mathematically found by him was that of the existence of certain particular areas around the Earth in which particles of a given energy could not penetrate from the outside or, vice versa, from which they could not escape if they were already there. indoor. In other words, particles of that particular energy within the areas that Störmer called “forbidden” were “trapped”.

As for the actual form of the trajectories, the study is extremely complicated and approximate methods have been very useful. Trapped particles are substantially subject to three types of movements (fig. 11) that overlap each other: a ) a rotation motion around the lines of force of the magnetic field; b ) a back and forth motion along the lines of force; c) a drift motion transversal to the lines themselves, around the axis of symmetry of the Earth’s magnetic field. The first type of motion is the well-known Larmor precession. The motion along the lines of the field occurs with a longitudinal speed that gradually decreases as the particle approaches the Earth, in points where the magnetic field grows in intensity, until it disappears at a well-defined point. At the same time, the trajectory becomes increasingly narrow around the lines themselves. The third type of motion causes a progressive displacement of the trajectory around the dipole axis. The fastest movement is that of type a ); then follows, less quickly, the type b ); even slower the drift motion c). As an example, the period corresponding to an entire oscillation or a revolution around the Earth is respectively in the three cases: several μsec, 0.1 sec and 1 hour, for electrons of 1 MeV; 4 • 10 -3 sec, 2 sec and 1/2 hour, for energy protons always of a MeV, in the center of the internal band.

The two extreme points 1 , 2 on a line of force, between which the trapped particles oscillate, have the name of mirror points ; it is near these points that the particles can escape entrapment, since being closer to the Earth where the atmosphere is denser (although always very tenuous) the higher the probability of interaction with the atmospheric particles.

As for the energy characteristics of the trapped particles, they are expressed by the energy spectrum. If E is indicated as the kinetic energy of the particles, the “distribution function” of the energy f ( E ) is defined as the ratio ΔΝ / ΔΕ between the number ΔΝ of particles, having kinetic energy between the values E and E + Δ E , and the energy interval Δ E (assuming that ΔΕ is sufficiently small). In many cases the function f ( E ) takes the form kE  , where a is a positive number having values ​​of the order of 1 andk is a proportionality constant. The value of a can itself be energy dependent. The constant k depends on the geometric characteristics of the detector, as well as, obviously, on the place and conditions of observation. In certain cases, or places, the function f ( E ) is much more complicated than the simple function  , since it can also have relative maxima and minima.

In general, it can be said that the density of the particles becomes gradually smaller with the increase of their energy, that the density of energetic protons decreases with the increase of the distance from the Earth, while instead that of the electrons grows up to a maximum achieved around two terrestrial rays away on the equator.

Sources and disappearance processes . – Many and different mechanisms of origin of the trapped particles have been proposed.

We mention the so-called albedo neutrons, which are protons produced by the interaction of cosmic rays with atmospheric neutral particles at low altitude; part of these neutrons is emitted upwards and within a few tens of minutes it disintegrates giving rise to energetic protons and electrons which are then trapped. Another process mentioned above is the diffusion of charged particles, which, although constrained in their movements by the Earth’s magnetic field, can slowly move from one region of the magnetosphere to another, precisely the entrapment region. The diffusion is accompanied by a variation of kinetic energy, sometimes positive and sometimes negative. Whether it’s increase, i.e. acceleration of particles or decrease, i.e. deceleration,

As for the loss of trapped particles, an important cause is the absorption by impact with neutral particles of the atmosphere, as mentioned above. But there are other mechanisms connected to the interaction with the same charged particles, with variable electric or magnetic fields, instability phenomena, etc.

From what has been said it is clear that the radiation belts constitute the result of various competitive mechanisms, so their physical conditions are determined by a sort of dynamic equilibrium, which can also be substantially modified due to changes in one or more of the factors that regulate it.

Very expressly, Van Allen introduced the analogy of the “laundry” bucket: the bands behave like a bucket that collects the water that is conveyed into it, losing it through holes.

Reasons for sudden changes in equilibrium factors can be sudden changes in the ambient magnetic field, such as, for example, those associated with magnetic storms; or sudden changes in properties, e.g. of speed, of the solar wind that affects the magnetosphere by altering the structure of the magnetic field; or electromagnetic phenomena such as those associated with auroral phenomena typical of high latitudes. A very special reason for long-term disturbance of the radiation belts was the nuclear explosions at high altitude, both American ( Starfish) and Russian in the summer-autumn 1962. The result of the explosions was to produce large numbers of charged particles at high altitudes (400 km in the Starfish case) which overlapped the pre-existing ones of natural origin. In particular positions the additional flows are far greater than the pre-existing ones. Given the long average life of some of the products of nuclear explosions, the so-called artificial strips have produced a “pollution” that has lasted years and is not yet completely extinct.

The solar wind in disturbed conditions. – What has been said above generally describes the main characteristics of the solar wind in “quiet” conditions. However, the observation data clearly show that superimposed on an average state which is the one described above, a continuous, incessant succession of variations can be observed which in part are due to actual temporal variations and in part, instead, to the spatial irregularities that the instruments observe at different times as they are dragged “rigidly” by the solar wind that instant by instant affects the instruments themselves at high speed (note that the absolute speed with which spacecraft move in space is a few km / sec, i.e. about two orders of magnitude lower than that of the solar wind). Discontinuities of various kinds are thus observable that it has been possible to classify into various theoretically foreseeable categories (tangential, rotational, shock waves), waves of various kinds (in particular Alfvén waves). Here we briefly dwell on the most striking phenomenon, that of shock waves or interplanetary shocks, very often clearly associated with solar flares.

When one of these events occurs, there is generally a strong increase in temperature which causes an abrupt explosive pressure wave that propagates with high speed (500 ÷ 1500 km / sec) through the coronal gas near the Sun. This wave is supersonic, as the speed of sound in the crown is in the order of 100 km / sec. There is thus a shock wave that propagates in space. A theoretical description of the phenomenon, first formulated by E. Parker and then further elaborated and generalized, leads to predicting the evolution of the wave during its propagation towards, and beyond, the Earth. Under highly idealized hypotheses, in particular the spherical symmetry around the Sun, magnetic field configurations are obtained such as those shown in fig. 12: depending on the case, the deformation can exhibit one or two discontinuity surfaces of the field. In a different model due to T. Gold one thinks of configurations of magnetic field lines, emitted like languages ​​from the Sun, which end up investing the Earth. The two views expressed here are in all probability extreme cases of a very complex situation which precisely for this reason cannot be described in a simple mathematical way. In fact, the observational data clearly confirm the existence of shock waves. The two views expressed here are in all probability extreme cases of a very complex situation which precisely for this reason cannot be described in a simple mathematical way. In fact, the observational data clearly confirm the existence of shock waves. The two views expressed here are in all probability extreme cases of a very complex situation which precisely for this reason cannot be described in a simple mathematical way. In fact, the observational data clearly confirm the existence of shock waves.

The origin of magnetic storms, at least in large part, is attributable to the arrival of shock waves of this type, as a consequence of the interaction they undergo with the earth’s magnetic field at the external limits of the magnetosphere. It can thus happen that this is compressed more than it normally is, which is equivalent to saying that the intensity of the magnetospheric magnetic field, and more particularly on the earth’s surface, is significantly increased. The collateral effects induced in the radiation belts lead to further complicated field variations which can be observed over time following the initial compression phase.

The magnetosphere in perturbed conditions. – The physical state of the magnetosphere, described above, is what generally identifies its “quiet” conditions. But in reality the continuous variations of speed, density, magnetic field, etc., sometimes more intense and showy but always present, of the solar wind determine an incessant state of variability within the magnetosphere: and this causes the incident flux to vary. particles, energy and momentum. On the other hand, the magnetic field frozen in the solar wind also interacts with the earth’s one. There are very complex physical conditions, which only in general and recently begin to be identified. Sometimes it happens that there is a situation of strong global instability of the magnetosphere, which can generate the so-called magnetospheric sub-storm (magnetospheric substorm ).

A description of the general lines of the sub storms can be given with reference to fig. 13, which shows a section of the magnetosphere according to the midday and midnight meridians. When the interplanetary magnetic field in its continuous fluctuations of direction (and intensity) acquires an orientation in the north-south celestial direction, that is in the opposite direction to the earth’s magnetic field, there is a phenomenon of “reconnection” of the lines of force, that is, a real reconfiguration of the distribution of the magnetic field throughout the interaction region. It follows the appearance of a real connection between magnetic field externally and internally to the magnetosphere, in correspondence of the magnetopause: the magnetic field is discontinuous and its component perpendicular to the magnetopause forms a channel for the penetration of the particles of the solar wind into the mantle which thus tends to become thicker. Furthermore, there is a real convective transport of geomagnetic lines of force from the day side towards the geomagnetic tail. As a consequence, the density (and therefore the dynamic pressure) of particles in the tail as well as the electric current in the neutral layer and the magnetic field of the tail increase. This process remains active as long as the magnetic field of the solar wind is oriented towards the south. We then arrive, when the phenomenon lasts for times longer than a few tens of minutes, to a borderline situation beyond which the geomagnetic tail cannot bear, so to speak, further increases in density and field.

In different terms, there is a rapid contraction of the magnetic lines of force towards Earth and, in a similar way to a wrung dropper, part of the charged particles immersed in the field are accelerated (and expelled) along the lines of least resistance , that is, along the lines of the field, towards the polar caps, which gives rise to the auroral manifestation. The radiation belts are also affected by this state of perturbation, as results from the observation of sudden changes in their physical characteristics (energy spectrum and density of trapped particles, magnetic field, etc.).

The development of space research in future years . – Among the programs in progress we mention what goes by the name of “international magnetospheric collaboration” which is a coordinated research program on a global scale for the study of the magnetosphere, considered as a single immense system made up of mutually correlated regions. This program includes a series of launches of satellites, rockets, and high altitude balloons as well as an extensive series of observations on the earth’s surface for the years 1976-79. Italy participates in this large international enterprise with various experiments on spacecraft as well as with a network of ground observations.

As far as the study of the solar wind in the interplanetary space is concerned, missions outside the ecliptic plane are being advanced which have so far not been possible due to the power limitations of even the largest carriers so far. A project is in an advanced phase which envisages the launch of one or maybe two probes towards Jupiter, with such orbit modalities that the deviation introduced by Jupiter’s gravitational action allows a radical modification of the orbit which will become essentially polar around the Sun. A mission of this type, with a foreseeable launch in the early 1980s, will allow for the first time a “stereoscopic” study of the Sun, that is, outside the ecliptic.

Missions to distant planets (Saturn and beyond), as well as to some comets, are also being prepared or discussed. The transition to the construction phase and the first significant results can be expected around 1980-90. Among other things, it is necessary to take into account the long times necessary for the probes launched by the Earth to reach their planetary goal (already in the case of Jupiter it takes about two years!).

At the other extreme in the scale of distances from the Sun, two Helios 1 and Helios 2 probes have been in orbit since December 1974 and January 1976 respectively , which have a very close perihelion, of about 0.3 astronomical units, and on which a experiment in collaboration between Italy and the United States. But a further and more complex mission is foreseen for which the definition study is in progress: that of a solar probe that penetrates up to heliocentric distances of a few solar rays, that is, precisely in the region where the acceleration and solar wind emission.

 

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