Dr. James L. Green
Interplanetary space seems dark, still, and empty, a transparent void through which we see only distant points of starlight. However, discoveries made since the beginning of the space program in 1958 reveal that the Earth's near-space environment is not empty. It is filled with magnetic fields, electric fields, matter, energy, and activity invisible to the naked eye but readily apparent to more sensitive scientific instruments. In space, the region close to Earth, just above the ionosphere, is called the magnetosphere.
Here on Earth , when we navigate with a compass we are using the magnetic field of the Earth. The iron-nickel core of the Earth acts as a giant magnet, comparable to a dipole bar magnet. Without instruments the magnetic field is invisible but its effect on a compass needle has saved many lives. The Earth's magnetic field does not stop at the surface of the Earth. We now know that the magnetosphere is the vast area surrounding the Earth that is controlled by the Earth's magnetism. Many spacecraft launched over the years have carried sophisticated devices called magnetometers and have mapped a great deal of the magnetosphere.
One way to visualize a magnetic field, much like that of the Earth's, is by spreading iron filings on a piece of paper that is placed directly over a dipole bar magnet. Once spread, the iron filings seem to line up in a squashed multiple figure eight (8) pattern with the poles of the magnet located where the two symmetric ovals (halfs of the figure eight) meet. The iron filings indicate the contours of a dipole magnetic field along what are termed magnetic field lines.
The Earth's magnetic field is like a dipole magnet only close to the surface. The Earth's magnetic field extends far out into space for thousands of miles. In order to provide some basic understanding of the shape of the Earth's magnetosphere, we must first discuss phenomena caused by the Sun.
The extremely hot atmosphere of the Sun is a plasma; that is, it is a gas consisting of charged particles; mostly electrons and protons. Solar plasma streams radially into space at high speed and pulls the Sun's magnetic field with it as illustrated in Figure 1. The electrified particles and the solar magnetic field, that they pull along for a ride, is called the solar wind. These bits of the Sun come streaming at us at velocities of 450 km/second or more. While light travels from Sun to Earth in about 8 minutes, the solar wind usually reaches us in 2 or 3 days and then passes on through the solar system.
If we could see the invisible solar magnetic field, we would notice that it assumes a spiral configuration from the influence of the Sun's rotation. This has been called the "garden hose effect" because it resembles the spiral path of water droplets from a spinning lawn sprinkler. In Figure 1, notice the magnetosphere that surrounds the Earth and extends far beyond the atmosphere into space. This region is the scene of dynamic interactions between the Sun and Earth plasmas. The bullet shape of the magnetosphere is the direct result when it is blasted by the solar wind. The solar wind compresses the Sunward side of the magnetosphere to a distance of 6-10 Earth radii, and it drags out the night-side magnetosphere (into a magnetotail) to perhaps 1000 Earth radii; no one knows quite how far. This overall shape of the magnetosphere is illustrated in Figure 1.
The solar wind particles flowing directly from the Sun toward the Earth encounter the magnetosphere much as water in a swift stream comes upon a large rock. The solar wind particles must go around the magnetosphere obstacle, but at their high speed there is no time for an orderly detour. Instead, their direction is changed abruptly in the bow shock region just outside the Sunward magnetic field as illustrated in Figure 2.
Passage through the bow shock region reduces the speed and changes the motion of the particles. Most of the shocked solar wind particles are deflected around the magnetosphere through an area called the magnetosheath. The magnetosphere effectively shields the Earth from most of the direct solar wind because charged particles do not readily travel across a magnetic field but are deflected at angles to the magnetic field.
Some solar wind plasma can, however, travel along the Earth's magnetic field lines, leaking through the Earth's magnetic screen. Between the Sunward magnetic field and the tailward magnetic field are two funnel-shaped areas, called the polar cusps. When the solar wind enters the polar cusp, it follows the magnetic field lines toward Earth. Through the polar cusps, high-speed charged particles from the solar wind bombard our upper atmosphere. The polar cusp regions are shown as a dashed pattern in Figure 3.
Another region in which the solar wind plasma is found is the magnetotail, but its method of entry into these regions is still subject to debate. Perhaps by some interaction of the interplanetary and terrestrial magnetic fields, charged solar particles penetrate the magnetotail and populate the plasma sheet , a hot region of variable thickness and density. The plasma sheet is divided by a thin neutral sheet . The neutral sheet gets its name from the fact that magnetic fields from the northern and southern hemispheres of the Earth nearly cancel themselves out, making the region magnetically "neutral".
In the upper (northern) half of the plasma sheet, the magnetic field is directed toward Earth; in the lower (southern) half, the field is directed away from the Earth. As long as the impact of the solar wind on the magnetosphere remains fairly steady, the plasma sheet exists in equilibrium. When the plasma sheet balance is disturbed, its dimensions are altered radically, with consequences throughout the magnetosphere.
The region of the magnetosphere directly adjacent to the plasma sheet is the northern and southern tail lobes. The lobes are nearly devoid of plasma except for the very cold ionospheric plasma evaporating from the north or south pole which move tailward and eventually stream into the plasma sheet. The very thin boundary between the plasma sheet and the tail lobes is called the plasma sheet boundary layer.
Just beyond our atmosphere lies the ionosphere, a region of variable height that is populated by charged particles. Some of the cold ionospheric electrons and ions evaporate from this area into the plasmasphere, a region of dense, cold, low-energy plasma. Both the plasmasphere and the ionosphere shift in size and density in response to disturbances elsewhere in the magnetosphere.
A three-dimensional view of the Earth's magnetosphere is shown in Figure 4. In Figure 4 the plasma sheet is seen to extend all across the width of the magnetotail. This same dot pattern is shown to extend from the outer flanks of the magnetotail and over the northern tail lobes and into the cusp region. At the very edges of the magnetosphere is a region called the high latitude boundary layer of the plasma sheet or shorten to just the high latitude boundary layer. It is believed that some magnetic field lines in the high latitude boundary layer are connected to interplanetary magnetic field lines, as shown in Figure 4. These field lines are referred to as "open" since they do not close back on the Earth. This magnetic coupling with the Sun's magnetic field enables the direct transfer of energy from the solar wind into the magnetosphere and probably drives the complex circulation of magnetospheric plasmas and electric currents such as the ring current and field aligned currents.
Each of the basic regions of the magnetosphere discussed above have unique magnetic field topologies, particle populations, and electron and ion flow velocities. Table 1 shows the characteristics of plasmas measured from over 20 years of spacecraft observations in these magnetosphere regions.
Up until this point we have not discussed the first major spacecraft observations which lead to the discovery of the magnetosphere. In 1958 a Geiger counter mounted onboard the United States' first satellite, Explorer 1 , provided surprising evidence that the Earth is surrounded by intense particle radiation. Subsequent missions and experiments collected data on this particle population and found that two huge zones of trapped electrons and protons encircle the Earth like donuts. These belts lie approximately within the plasmasphere and bear the name of the discoverer, the Van Allen radiation belts .
The two Van Allen radiation belts contain particles trapped in the Earth's magnetic field. The chief constituent of the inner belt is high-energy protons, produced when cosmic rays blast particles out of the upper atmosphere. The outer belt is populated chiefly with high-energy electrons produced by cosmic rays and magnetospheric acceleration processes. During steady state conditions in the magnetosphere, particles neither enter nor escape these trapped orbits. During magnetospheric disturbances (to be explained below), however, accelerated particles may enter and leave the Van Allen belts.
When charged particles enter a magnetic field, they may be trapped along field lines in three superimposed motions: spiral, bounce, and drift, as shown in Figure 5. The spiral trajectory resembles a coil wound around a magnetic field line. Near the poles, field lines converge and alter the forces on particles, reversing their motion along the magnetic field lines. The particles bounce toward the opposite pole, where they are reflected again. Finally, as they spiral and bounce from pole to pole, particles drift transversely through the magnetic field. In this way, charged particles become trapped in the magnetosphere, like those in the Van Allen radiation belts.
In a pure vacuum devoid of any other matter or magnetic fields a charged particle such as an electron or a proton will emit light only if it is accelerated. Acceleration is defined as a process by which there occurs a change in velocity over a unit time or a change in the direction of motion. It has been known for some time that charged particles spiraling in magnetic fields will give off electromagnetic waves. The frequency of the emitted waves is usually related to the strength of the magnetic field. This process and others like it are responsible for the generation of a whole host of electromagnetic and electrostatic waves in the Earth's magnetosphere.
Figure 4 shows the generation location of many of the observed magnetospheric emissions. Some of these waves are trapped within the magnetosphere, such as ion cyclotron and electron whistlers, ELF and VLF hiss, and chorus just to name a few. Other magnetospheric electromagnetic waves can travel to great distances such as the non-thermal continuum radiation and auroral kilometric radiation. They can easily be observed in interplanetary space. Table 2 (pages 1 and 2 ) gives an extensive list of magnetospheric plasma waves, their frequency ranges, source regions, and proposed source mechanisms. Even after more than a decade of spacecraft observations of magnetospheric plasma waves, we understand very little about how they are generated.
The magnetosphere is a complex configuration of plasma regions, particles, and electric currents. In reality, regional boundaries are not as well defined as implied above since the magnetosphere is a dynamic, fluctuating system. Though we do not yet fully understand many of the physical mechanisms and processes that drive the magnetosphere, we do know that the space around Earth is a highly responsive buffer zone against the high speed solar wind. Invisibly, sometimes violently, the magnetosphere reacts to events on the Sun as they are communicated through the Earth's magnetosphere by the solar wind.
When the magnetic field coupling of the solar wind with the Earth's magnetosphere is dramatically enhanced periods of instability, called substorms, occur in the magnetosphere. A rapid sequence of substorms constitutes a full-scale magnetic storm, a spasmodic contortion of the Earth's entire near-space environment. Magnetic storms are associated with powerful solar eruptions that alter the velocity and composition of the solar wind. At the beginning of a substorm, the enhanced connection with interplanetary magnetic field lines occurs with the outermost Sunward loops of the Earth's magnetic field which are broken and swept into the magnetotail, as shown in Figure 6 . The increase in magnetic pressure in the magnetotail lobes literally pinches the plasma sheet until a neutral point forms, as shown in Figure 7 .
It is believed that once the neutral point forms, energy stored in the magnetotail is released explosively. Within the neutral point a process called reconnection occurs which "snaps" magnetic field lines toward the Earth like rubber bands, accelerating plasma particles with enough force to inject them into the upper atmosphere. On the far side of the pinch, a bubble of plasma is forced out of the magnetotail.
When the plasma sheet is disturbed in this manner, accelerated particles move along the Earth's magnetic field and bombard the upper atmosphere around the poles in regions known as auroral ovals . The ovals are "windows" into the magnetosphere, for there occurs the only visible sign of magnetic activity, the aurora. During a typical two-hour substorm, energy comparable to that of a very strong earthquake (about 10^9 watts) is released into the upper atmosphere. Though substorms occur, on the average four times a day, people living at relatively low latitudes on the Earth are seldom aware of them since they are not able to view the auroral. Only rarely, during the most violent magnetic storms, can the aurora be seen by United States residents outside Alaska, though in the auroral zones the lights appear nightly. Actually, the aurora occurs on the Earth's day-side as well, but it is masked by sunlight.
People who live in far northern and southern latitudes, under the auroral ovals, witness the only visible sign of magnetic storms, the ghostly movement of beautiful, multicolored forms across the night sky. Alaskans and Canadians call the brilliant spectacles the "Northern Lights" or "Aurora Borealis." In the southern hemisphere, the displays are known as the "Aurora Australis." During a moonless night the auroral is regularly so bright that one can easily read a book by this natural "candle light".
The auroral lights themselves are produced when energetic charged particles that have entered the magnetosphere through the polar cusps or in the auroral oval during magnetic substorms collide and excite neutral atoms and molecules in the upper atmosphere. Auroras appear in a variety of forms and colors, according to the degree of turbulence in the magnetosphere. When conditions are relatively quiet, the aurora drifts across the sky as a green or white curtain.
As magnetic activity increases, the auroral curtain brightens, develops folds, and moves more rapidly across the sky. The excited aurora may become a brilliant spiral that swirls from horizon to horizon, though the night seems calm and clear with no hint of turbulence except these eerie lights. From the ground, auroral displays may last only a few minutes, punctuated by brief periods of quiet sky, or they may linger for hours. During the most intense magnetic activity, the entire sky explodes in fiery red. Silently, distantly, a storm rages through the magnetosphere as it undergoes a massive reconfiguration in response to the intensified incoming solar wind.
Solar activity may affect both short-term weather patterns and long-term climate trends. Changes in the magnetosphere seem to be transmitted to the lower atmosphere where they may influence the circulation of air masses. If we can discover the physical links between these two regions of our environment that trigger weather and climate changes, we can better predict and prepare for our weather. At this time, it is believed that energy from the solar wind transmitted through the magnetosphere has only a minor effort on our climate, however, a significant amount of research needs to be completed before a better understanding is obtained.
It is important to note that magnetic storms have produced other noticeable effects on the Earth, such as:
Over the last 20 years many spacecraft have discovered and explored magnetospheres of other planets. Particularly noteworthy are the Pioneer 10 &11 spacecraft which have gone to Jupiter and Saturn while the Voyager 1 & 2 spacecraft have gone to Jupiter, Saturn, Uranus, and Neptune. We have discovered that each of these planets have extensive magnetospheres. Each one has a bullet shape like the Earth's, caused by the solar wind. Each magnetosphere of the giant planets is much larger than the Earth's. Figure 8 illustrates the relative sizes of magnetospheres. In this Figure, the insets indicate the relative size of one diagram to the next, with increasing size of the magnetospheres progressing from top to bottom.
Figure 6 also shows that magnetosphere-like systems are probably common not only in our own solar system but throughout the universe. However, the plasma conditions and other characteristics can vary over a wide range. The size of our own huge Sun is tiny compared to the size of Jupiter's magnetosphere. The size of a typical pulsar and the Earth's magnetospheres are similar but the energies involved are vastly different. For instance, the intermittent light from a pulsar is coming from near a magnetic pole. This pulsing light may be generated in a process not too different from the Earth's auroral kilometric radiation. Both emissions are believed to be magnetospheric in origin and related to the magnetic field strength in the source region and yet the pulsar light is in the visible frequency range while the auroral kilometric radiation is in the few hundreds of kilohertz in the radio frequency range. The difference in wave length is nearly 10^8 - 10^9 times! Therefore, the strength of the magnetic field in a pulsar's magnetosphere is enormous.
The Earth's magnetosphere is a dynamic region of flowing plasma guided by our magnetic field which at times connects into the Sun's magnetic field. Within the Earth's magnetosphere are found cold plasma from the Earth's ionosphere, hot plasma from the Sun's outer atmosphere, and even hotter plasma accelerated to great speeds which "rains" on our upper atmosphere causing aurora in both the northern and southern hemispheres.
By means of orbiting spacecraft, we can chart both the Sun's activity and the responses of the magnetosphere and atmosphere over long periods. Study of our own magnetosphere may also contribute to our understanding of the physical processes that may also be operating in other parts of the solar system and perhaps explain many phenomenon in magnetospheres of other planets, stars, and galaxies.
The aim of exploring the Earth's magnetospheric environment is to understand the physical processes that control this vast, dynamic system. We are also looking for the magnetic link between the Earth's atmosphere and magnetosphere, and we hope to learn more about their responses to cyclic and transient changes in solar activity as well as to the steady stream of solar wind.
The better we understand the processes that govern the solar-terrestrial environment, the better we can forecast and accommodate remote, invisible events that may drastically affect our life on Earth. The challenge we face is to solve the riddles of the solar-terrestrial environment, to learn precisely how the Sun and Earth interact through the Earth's magnetosphere, and to apply that knowledge to the benefit of humanity.
