Designed as a text for upper-level undergraduate and first-year graduate students, this volume attempts to establish the curriculum for the modern student entering the fledgling field of space physics. The book provides a broad, yet selective, treatment of the subject, covering virtually all aspects of space plasmas in the solar system. There are sections on the sun and solar wind, the magnetized and unmagnetized planets, and the fundamental process of space plasmas including shocks, plasma waves, ULF waves, wave-particle interactions, and auroral processes. In addition to emphasizing analysis, the authors also place importance on underlying phenomenology with extensive attention to observations.
Karttunen H., Kroger P., et al.
1.2 EARLY MEASUREMENTS OF THE GEOMAGNETIC FIELD 3 time, studies of geomagnetism and the aurora became more firmly linked. 1.2 EARLY MEASUREMENTS OF THE GEOMAGNETIC FIELD The earliest indication of the existence of the geomagnetic field was the direction-finding capability of the compass. As compasses were improved, more and more was learned about the geomagnetic field. The earliest reliable evidence of Chinese knowledge that a compass points north or south dates from the eleventh century. The encyclopedist Shon- Kau (a.d. 1030-93) stated that "fortune-tellers rub the point of a needle with the stone of the magnet in order to make it properly indicate the south." In the European literature, the earliest mention of the compass and its application to navigation appeared in two works by Alexander Neekan, a monk of St. Albans (a.d. 1157-217), entitled De Untensilibus and De Rerum. In the former he described the use of the magnetic needle to indicate north and noted that mariners used that means to find their course when the sky was cloudy. In the second, he described the needle as being placed on a pivot, a second-generation form of the compass. In neither work did he describe the instrument as a novelty; it was in common use at that time. Official records indicate that by the fourteenth century, many sailing ships carried compasses. The direction of magnetic north and that of geographic or true north differ over most of the globe. The measure of this difference is called the declination. It is not clear when magnetic declination was actually discovered. However, a letter written by Georg Hartmann, vicar of St. Sebald's at Nurnberg, to Duke Albrecht of Prussia in 1544 showed that he had observed the declination of Rome in 1510 to be 6° east, whereas it was 10° at Nurnberg. Also, it is known that between the years 1538 and 1541, Joao de Castro made 43 determinations of declination during a voyage along the west coast of India and in the Red Sea. The geomagnetic field is also inclined to the horizontal. To measure this inclination, one must pivot a needle about a horizontal axis. Georg Hartmann's letter also discussed such an observation, but the angle of inclination was incorrect for his point of observation. William Gilbert ascribed the discovery of the magnetic dip or inclination to an En- glishman, Robert Norman, who in 1576 published a work with the title The newe Attractiue containyng a short discourse of the Magnes or Lodestone, and amongest other his verities, of a newe discouered secret and subtill propertie, concernyng the Declinyng of the Needle, touched there with onder the plaine of the Horizon. Now first found out by ROBERT NORMAN Hydrographer. Here onto are annexed certaine necessarie rules for the art of Nauigation, by the same R.N. Imprinted at London by John Kyngston, for Richard Ballard, 1581.
1.4 THE IONOSPHERE AND MAGNETOSPHERE 9 or to any kind of dynamic action taking place within the sun. Lord Kelvin also claimed "that the supposed connection between magnetic storms and sunspots is unreal, and the seeming agreement between periods has been a mere coincidence." More telling was the criticism of A. Schuster that a beam of electrons from the sun could not hold together against their mutual electrostatic repulsion. FIG. 1.5. Auroral physicists C. F. St0rmer, standing, and Olaf Birkeland, seated, in northern Norway, ca. 1910. (Photo courtesy of A. Egeland.) 1.4 THE IONOSPHERE AND MAGNETOSPHERE The electrically conducting region above about 100 km altitude that we now call the ionosphere may rightly be claimed to have been discovered by Balfour Stewart. In his 1882 Encyclopaedia Britannica article entitled "Terrestrial Magnetism" he concluded that the upper atmosphere was the most probable location of the electric currents that produce the solar-controlled variation in the magnetic field measured at the surface of the earth. He noted that "we know from our study of aurora that there are such currents in these regions - continuous near the poles and occasional in lower latitudes." He proposed that the primary causes of the daily variations in the intensity of the surface magnetic field were "convective currents established by the sun's heating influence in the upper regions of the atmosphere." These currents "are to be regarded as conductors moving across lines of magnetic force and are thus the vehicle of electric currents which act upon the magnetic field." Those statements are very close to modern atmospheric-dynamo theory. How- ever, it was left to A. Schuster to put the dynamo theory into quantita- tive form.
1.4 THE IONOSPHERE AND MAGNETOSPHERE 11 1000 500 100 50 I I \ I Ionosphere \ F Region / ./ D Region I I I i - \ J F2 Layer F, Layer E Region I Electron Density [cm3] ever, identification of the yellow-green line at 557.7 nm was elusive. Finally, H. Babcock's precise measurements in 1923 allowed John McLennan to identify it as a metastable transition of atomic oxygen. At atmospheric pressures close to that at the surface of the earth, collisions between molecules de-excite the molecules before they have a chance to radiate if they happen to become excited into one of the metastable states. However, at the altitude of the aurora, collisions are so rare that the time between collisions is longer than the lifetimes of the metastable states, and the excitation energy of the state can be released by radia- tion. A similar line in the auroral spectrum is the 630.0-nm red line of atomic oxygen. This metastable transition has a lifetime of 110 s and can radiate only above some 250 km. Those discoveries led to the realization that the varied colors of the aurora were simply related to height. In low-altitude auroras, below 100 km, where collisions quench even the oxygen green line, the blue and red nitrogen bands predominate. From 100 to 250 km, the oxygen green line is strongest. Above 250 km, the red line is most important. Although most of the auroral forms are associated with electrons, some aurora are due to precipitating protons. The first observations of the proton aurora were made in 1939. Measurements of the Doppler shifts of the proton emissions permitted estimates of the energy of the precipitating particles from the ground. Chapter 14 contains more de- tailed discussion of the aurora and auroral ionosphere. With the concept of the ionosphere firmly established, scientists be- gan to wonder about the upper extension of the ionosphere, linked magnetically to the earth, which region we today call the magneto- sphere. In 1918, Sydney Chapman postulated a singly-charged beam FIG. 1.7. Electron density of the earth's ionosphere as a function of altitude.
1.6 MAGNETOSPHERIC EXPLORATION 17 early years that the solar wind undergoes an abrupt transition prior to reaching the magnetopause. This transition is a shock produced as the high-speed solar wind encounters the earth as an obstacle in its flow. The existence of a shock in a collisionless plasma surprised many physi- cists. In the intervening years it has become clear that the electric and magnetic fields in the plasma can alter the motion of the particles in a manner similar to ordinary collisions. These changes provide the dissi- pation needed to form a shock. The physics of this process is discussed in Chapter 5. The shock allows the solar wind, which flows faster than the speed of compressional waves in a plasma, to be slowed, heated, and deflected around the planet. It was not until the launch of the first Orbiting Geophysical Observa- tory (OGO) in 1964 that scientists obtained time-resolution measure- ments of sufficient accuracy to study the bow shock. The OGO 1, 3, and 5 spacecraft in highly eccentric orbits mapped the locations of both boundaries as the earth orbited the sun and the orbits precessed relative to the magnetosphere, as shown in Figure 1.13. Those measurements and data from other spacecraft launched in the 1960s, such as the Inter- planetary Monitoring Platform (IMP) spacecraft and the VELA space- craft, revealed that the structure of the bow shock was very sensitive to the conditions in the plasma, the ratio of the speed of the solar wind to the speed of compressional waves (Mach number), and the ratio of the thermal pressure to the magnetic pressure (beta). It was also found to be sensitive to the direction of the interplanetary magnetic field. When the magnetic field is almost aligned with the direction of propagation of the shock, the shock normal, the shock is said to be quasi-parallel. When the field is more nearly perpendicular to the normal, it is referred to as being quasi-perpendicular. Ion beams are found upstream of the quasi- parallel shock, as illustrated in Figure 1.14, and these ion beams interact with the incoming solar-wind plasma to produce copious large-amplitude waves, called upstream waves, shown in Figure 1.15. FIG. 1.13. Mapping of the magnetopause and bow shock as the earth goes around the sun. During the course of a year, the magnetopause and bow-shock surfaces maintain their orientation about the earth—sun line, while the spacecraft orbit is fixed in inertial space. Thus the orbits appear to sweep through these boundaries in the course of a year.
20 A BRIEF HISTORY OF SOLAR-TERRESTRIAL PHYSICS FIG. 1.16. Topology of the magnetosphere for northward and southward interplanetary fields, according to J. W. Dungey in the early 1960s. In the steady state, the plasma flows as indicated by the short arrows. (From Dungey, 1963a.) t North Solar Wind I North So/or Wind Interplanetary Field Southward Interplanetary Field Northward point, a location where the magnetic field vanishes, is the site of the reconnection. It was not until the late 1970s, after the launch of the dual co-orbiting satellites ISEE 1 and 2, that the reconnection mechanism gained general acceptance. Those satellites returned plasma data of sufficiently high resolution to show the accelerated flows at the magneto- pause and in the magnetotail. Chapter 9 provides further details about these processes. However, even today there is debate as to where reconnection occurs and how important it is relative to other processes. Furthermore, it has been found that three-dimensional structures are present, and their explication will require not just two spacecraft, but a whole cluster. Thus there is much to be done in future magnetospheric exploration. Laboratory plasma measurements have also been useful in under- standing the magnetosphere. Figure 1.17 shows a wire model of the magnetosphere developed by Igor Podgorny and his colleagues at the Space Research Institute in Moscow, based on laboratory experiments undertaken in the 1960s, illustrating the development of cusp-shaped openings in the field pattern on the dayside and a long tail at night. Figure 1.18 shows a three-dimensional sketch of the magnetosphere, representing the structure that has been inferred from spacecraft obser- vations. It is deep within this magnetosphere that the radiation-belt particles bounce and drift, as illustrated in Figure 1.19. As noted earlier, the converging magnetic-field lines of the dipole magnetic field stop the forward motion of the particles and accelerate them back toward the equatorial regions. While gyrating and bouncing, these particles also drift, because their gyroradii are greater when they gyrate into weaker fields than when they are in the stronger fields on the inner part of their trajectories. The dipole magnetic field of the earth can confine particles over a wide range of energies, the more energetic of which Van Allen encountered when he and his colleagues discovered the radiation belts.
1.7 PLANETARY AND INTERPLANETARY EXPLORATION 21 Figure 1.20 shows the intensities of very energetic protons and electrons in the inner magnetosphere. These particles enter the radiation belts through a variety of means, including radial diffusion from more distant regions, with accompanying acceleration and the decay of neutrons from the sputtering of the atmosphere by cosmic rays. Whereas energetic protons form a single belt, as illustrated in the top panel of Figure 1.20, electrons, as illustrated in the bottom panel, form two belts separated by a region called the slot. This slot in the flux of electrons of fixed energy is formed when naturally occurring electromagnetic waves inter- act with the gyromotion of the electrons, causing them to spiral into the atmosphere, where they are lost through collisions. The inner electron belt is quite stable and the outer belt quite variable. The radiation belts and the motions of charged particles are discussed further in Chapter 10. FIG. 1.17. Three- dimensional wire model of the magnetosphere based on the laboratory experimental data of Podgorny and colleagues. (From Podgorny, 1976.) 1.7 PLANETARY AND INTERPLANETARY EXPLORATION The earth is but one test bed for the physical processes occurring in space plasmas. There is much occurring at the other planets and in the interplanetary plasma itself. The solar-wind properties evolve with heliocentric distance, and on the way through the solar system the solar wind encounters a variety of obstacles to its flow, including magnetized bodies, unmagnetized bodies, some with atmospheres and others with- out, and eventually the heliopause, where the solar wind and the inter- stellar wind meet.
1.7 PLANETARY AND INTERPLANETARY EXPLORATION 23 2x10^ Protons Energy >10 MeV Electrons Energy > 0.5 MeV Inner belt Slot ""Outer belt atmosphere that extends into the solar wind becomes ionized and adds to the solar-wind flow, further decelerating it. The slowing down of the magnetized flow around the planet leads to the draping of magnetic-field lines over the obstacle and the formation of a long tail. In this respect, the solar-wind interaction with Venus and Mars resembles that with a comet. This interaction was probed by the International Cometary Explorer (ICE) spacecraft at comet Giacobini- Zinner in 1985 and by the VEGA 1 and 2, Giotto, Suisei, and Sakigake spacecraft at the comet Halley in 1986. Chapter 8 describes in greater detail the solar-wind interaction with such unmagnetized bodies. Mariner 10, with a gravitational assist from Venus, made three passes by Mercury in 1974 and 1975. As illustrated in Figure 1.22, Mariner 10 found a minimagnetosphere very much like that of the earth. Mercury, however, has almost no atmosphere, so that the ionospheric current systems that we believe to be so important for the terrestrial magneto- sphere must be absent from Mercury. Thus we expect Mercury's magne- tosphere to be quite different in some respects from that of the earth. There has been little investigation of even the meager data from the FIG. 1.20. Earth's radiation belts. The top panel shows the contours of the omnidirectional flux (particles per square centimeter per second) of protons with energies greater than 10 MeV. The bottom panel shows the contours of the omnidirectional flux of electrons with energies greater than 0.5 MeV.
26 A BRIEF HISTORY OF SOLAR-TERRESTRIAL PHYSICS 1.8. CONCLUDING REMARKS The field of solar-terrestrial physics has advanced greatly since the earliest recorded sightings of the aurora. We now have physical models for almost all the observed phenomena. Sometimes several competing models exist. The discipline has evolved from one of remote sensing to in situ observations, theory, and computer modeling. In fact, it is now perhaps more appropriate to refer to the field as "space physics," as one of the major journals in the field does, rather than solar-terrestrial physics. ADDITIONAL READING Brekke, A., and A. Egeland. 1983. The Northern Light. Berlin: Springer-Verlag. Chapman, S., and J. Bartels. 1940. Geomagnetism. Oxford University Press. Eather, R. H. 1980. Majestic Lights. Washington, DC: American Geophysical Union. Gilbert W. 1893. De Magnete, trans. P. Fleury Mottejay. Reprinted 1958, New York: Dover. Helliwell, R. A. 1965. Whistlers and Related Ionospheric Phenomena. Stanford University Press. PROBLEMS 1.1. Discuss the role of new technology in the development of solar- terrestrial physics. 1.2. At its typical velocity of 440 km • s~', how long does it take the solar wind to arrive at Mercury, Earth, Jupiter, and Pluto?
2.3 COLLECTIONS OF PARTICLES 35 Gyro Motion Bounce Motion Drift Motion If this flow or "bulk" velocity is different for ions and electrons, then electric currents will flow in the system. Notice the notation that has been used to distinguish between the velocity of an individual particle, v, and the average velocity of a collection of particles, us. Often the velocities of the individual particles are orders of magnitude larger than the flow velocity, but if the velocities are random, they contribute little to the average velocity. To obtain the average kinetic energy per particle of type s in a unit volume of the plasma, we add together the energies of all the particles by integrating over velocities and divide by the total number of particles in the volume. Evidently there is kinetic energy associated with the flow velocity ns, but more interesting is the kinetic energy of random mo- tion relative to the average. The equation for that part of the kinetic energy is <im>~usJ> = jdv , v, Q/fdvfJjr, v, t) B.18) The angle brackets are used to designate the average in the volume. The hydrostatic partial pressure of species s particles is related to their average random energy by Pslns = B/AO < i#n,(v - usJ B.19) where N is the number of independent components of velocity, normally three. In SI units, pressure is measured in pascals (Pa), with 1 Pa equal to one newton per square meter A N • m~2). For systems in equilibrium, the phase-space distribution is the Max- wellian distribution, the equilibrium distribution function for particles of type s flowing with velocity us. It is given by fs(r, v) = Asexp - L "* (Maxwellian distribution) B.20a) FIG. 2.2. Schematic of particle motion in a magnetic field. On time scales of gyroperiods, the particle spirals about the magnetic field (see illustration of gyromotion at the left). If there is a field-aligned gradient of the field strength, the component af velocity parallel to the field decreases as the particle moves into regions of increasing field magnitude, although the total velocity is conserved. Eventually, the parallel velocity reverses. The reflection of the parallel motion is called "magnetic mirroring." Motion between mirror points is called bounce motion (central panel). As the particle bounces, it also drifts about the source of the field, tracing out a drift shell (see the right-hand illustration).
2.3 COLLECTIONS OF PARTICLES 37 -2vlh (a) 0 Vv X n,/v 0 / \ Ux Vlh V!h+U, 2VU, (C) 4m,(v.-K «.23) (bi-Maxwellian distribution) Here, A's = AsTyT±sT^. For this distribution, we can specify the average random energy of motion separately for motion parallel and perpendicu- lar to the field: = 1/cTy and y± - u±sJ >=kT±s This form reflects the fact that there is one degree of freedom for motion along the magnetic field and two degrees of freedom for motion across it. The definitions of parallel and perpendicular pressures follow from these relations and the obvious generalization of equation B.19). Sometimes the Maxwellian form is followed quite closely up to some energy, but the number of particles at high energies follows a power- FIG. 2.3. Maxwellian distributions: In (a) and (b) the plasma is at rest. Part (a) shows the dependence of a three- dimensional Maxwellian distribution on one component of the velocity, here taken as vx. The dependence on the other components of velocity has been removed by integration. Part (b) shows the dependence of ?l(r/ v) on the speed, v; v,,, = tfJW/mJ'/2 's the thermal speed. In (c) the plasma is flowing toward positive x at a velocity ux. Notice that in (a) and (c), the position of the maximum is unaffected by changes of temperature. As the temperature increases, v^, increases, and the height of the peak of the distribution drops, whereas the width of the distribution increases. In (b), the position of the maximum shifts to the right as temperature increases. Its height increases.