The Sun and Us
Nothing is more important to us on Earth than the Sun. Without the Sun’s heat and light, the Earth would be a lifeless ball of ice-coated rock. The Sun warms our seas, stirs our atmosphere, generates our weather patterns, and gives energy to the growing green plants that provide the food and oxygen for life on Earth.
We know the Sun through its heat and light, but other, less obvious aspects of the Sun affect Earth and society. Energetic atomic particles and X-rays from solar flares and other disturbances on the Sun often affect radio waves traveling the Earth’s ionosphere, causing interference and even blackouts of long-distance radio communications. Disturbances of the Earth’s magnetic field by solar phenomena sometimes induce huge voltage fluctuations in power lines, threatening to blackout cities. Even such seemingly unrelated activities as the flight of homing pigeons, transatlantic cable traffic, and the control of oil flow in the Alaska pipeline apparently are interfered with by magnetic disturbances caused by events on the Sun. Thus, understanding these changes — and the solar events that cause them — is important for scientific, social, and economic reasons.
We have long recognized the importance of the Sun and watched it closely. Primitive people worshiped the Sun and were afraid when it would disappear during an eclipse. Since the early seventeenth century, scientists have studied it with telescopes, analyzing the light and heat that manage to penetrate our absorbing, turbulent atmosphere. Finally, we have launched solar instruments and ourselves-into space to view the Sun and its awesome eruptions in every aspect.
Once we looked at the Sun by the visible light that reached the ground, it seemed an average, rather stable star. It was not exactly constant, but it seemed to vary in a fairly regular fashion, with a cycle of sunspots that comes and goes in about eleven years. Now the Space Age has given us an entirely different picture of the Sun. We have seen the Sun in other forms of light-ultra violet, X-rays, and gamma rays that never reach the ground from space. This radiation turns out to be far more responsive to flare eruptions and other so-called solar activity.
We now see the Sun as a place of violent disturbances, with wild and sudden movements above and below its visible surface. Besides, solar activity's influence seems to extend to much greater distances than we had believed possible. New studies of long series of historical records reveal that the Sun has varied in the past in strange and unexplained ways. Scientists wonder how such variations might affect the future climate on Earth.
We have obtained a clearer picture of the scope of the Sun’s effects. Its magnetic field stretches through interplanetary space to the outer limits of the solar system. Steady streams and intense storms of atomic particles blow outward from the Sun, often encountering our Earth's atmospheres and the other planets. The spectacular photos of the Earth from space show only part of the picture. Instruments carried on satellites reveal a wide variety of invisible phenomena — lines of magnetic force, atomic particles, electric currents, and a huge geocorona of hydrogen atoms — surrounding the Earth. Each is as complex and changing as the visible face of the globe. The Earth’s magnetic field extends tens of thousands of miles into space, and many different streams of electrons and protons circulate within it. Huge electric currents flow around the Earth, affecting their high-altitude surroundings as well as our environment at ground level.
Space observations have greatly expanded our ability to look at the Sun, interplanetary space, and the Earth's immediate surroundings. We can now “see” many phenomena that are completely undetectable from the Earth’s surface, and we now have a much better, more complete, and more coherent picture of how events in one part of our solar system relate to activity in another.
The Sun as a Star
We sometimes forget that there is one star that is easily visible in the day time: our Sun. The Sun is the only star close enough to be studied in detail, but we are confident that all the processes in the Sun must also occur in billions of distant stars throughout the universe. To understand the nature and behavior of other stars, we must first understand our own. At the same time, observations of other kinds of stars help put the Sun in perspective.
The Sun is a relatively typical star among the approximately 100 billion stars in our Milky Way galaxy. The masses of most other stars that we see range from approximately one-tenth of the mass of the Sun to about 30 solar masses. The surface temperatures of most stars range from about 2000° C to 40,000° C. Although the Sun is somewhat on the cool side at about 6000° C, hot stars are rare, and most normal stars are cooler than the Sun. Compared to some of the explosive stars — novae, and supernovae — which sometimes appear in the sky, the Sun is stable and ordinary.
This long-term stability of our Sun probably was crucial for the development of life on Earth. Biologists believe that a relatively stable average temperature had to prevail on Earth during the past 3 billion years for life to evolve to its present state. The relative stability of the Sun is also important to astronomers trying to understand the basic nature of it and other stars. Violent activity in the Sun could mask the more subtle and long-enduring processes, which are the basic energy transport mechanisms of our star. Fortunately, they are not hidden, and we have been able to map the trend in solar properties with height above the visible surface.
Above the minimum temperature region in the photosphere, we have measured how the gas gets hotter as it thins out with height. The chromosphere and corona, each hotter than the layer below, are warmed by the transfer of energy from below through processes that are still not well understood.
Until space observations became possible, we knew nothing about coronae in any other stars and had only marginal information about stellar chromospheres' properties. Now, space observations have shown us that a large fraction of the stars in the sky have chromospheres and coronae.
On several dozen stars, we have even detected activity that may be connected with sunspot (or “starspot”) cycles like those of our own Sun. X-ray telescopes carried on satellites have recorded flares in other stars that are far more powerful than the already impressive flares of the Sun. By observing the strength and frequency of these events on stars with masses, ages, and rotation rates which differ from those of the Sun, we search for answers to such basic questions as: “How does the sunspot cycle period depend on the star’s rotation rate?” or “What is the relation between the temperature of a star’s corona and the strength of its magnetic field?” By deciphering the general pattern of stellar properties, we can better understand what makes things happen on the Sun.
The Sun presents us with a bewildering variety of surface features, atmospheric structures, and active phenomena. Sunspots come and go. The entire Sun shakes and oscillates in several different ways at the same time. Great eruptions called prominences hang high above the Sun’s surface for weeks, suspended by magnetic force, and sometimes shoot abruptly into space from the corona. The explosions called solar flares emit vast amounts of radiation and atomic particles in short periods of time, often with little or no warning.
Space observations have discovered many new aspects of solar events hidden from ground-based observatories—the Sunshine's hottest spots primarily in ultraviolet and X-rays, rather than in visible light. Thus, only from space can we map high-temperature solar flares' true structure and determine their physical conditions. Space observatories have shown us the higher, hotter layers of the Sun’s atmosphere that normally are invisible from the ground. Instruments on satellites revealed that in flares and other violent disturbances, the Sun acts like an atomic accelerator, driving electrons and protons to velocities approaching the speed of light. At such high speeds, the particles emit the high-energy X-rays and gamma rays measured by our satellites. Sometimes they even induce nuclear reactions on the surface of the Sun.
Two aspects of our improved knowledge of the Sun deserve special attention. One is the role of magnetic fields in determining virtually all aspects of the Sun’s upper atmosphere's structure and behavior. The other is discovering the solar wind, a stream of atomic particles that constantly evaporate from the Sun’s atmosphere and are accelerated to speeds of hundreds of kilometers per second, escaping into space in all directions.
The Earth-Sun Battle
For any solar particle to reach the Earth, it must first pass through the Earth’s magnetic field. Before the solar wind was discovered, the Earth’s field was thought to be symmetrical, resembling a huge bar magnet, fading off indefinitely into space. However, we now know that the solar wind shapes the Earth’s magnetic field's outer regions and is sharply bounded. Outside the boundary, space is dominated by the solar wind and the interplanetary magnetic field. Inside the boundary is the region or magnetosphere dominated by the Earth’s magnetic field. The measurements from many space missions have been combined to reveal that the solar wind blows out the Earth’s magnetosphere into a teardrop shape. The head of the drop extends only about 10 Earth radii, or about 65,000 kilometers (40,000 miles) “upwind” toward the Sun. The tail of the drop stretches away in the direction opposite the Sun, actually reaching beyond the Moon’s orbit. This long magnetotail extends more than 600,000 kilometers (370,000 miles) from the Earth.
At the boundary of the magnetosphere, there is a constant struggle between the Earth's magnetic field and the forces of the Sun. Buffeted by fluctuations in the solar wind velocity and density, the magnetosphere’s size and shape are continuously changing. When the solar wind strikes the magnetosphere, shock waveforms are analogous to the sonic boom preceding a supersonic airplane. Inside the boundary with the solar wind, the magnetosphere remains an active region. It contains two belts of very energetic charged atomic particles trapped in the Earth’s magnetic field hundreds of miles above the atmosphere. These belts were discovered by Professor James Van Allen of the University of Iowa and his colleagues in 1958, using simple radiation detectors carried by Explorer 1, the first U.S. satellite.
The Northern and Southern Lights: Gifts from the Sun
The structure of the Earth’s magnetosphere also controls aurorae's behavior, seen in our night skies. Pre-Space Age textbooks stated that aurorae are produced by photons emitted from the Sun and reach the Earth’s upper atmosphere through gaps in the Earth’s magnetic field at the north and south magnetic poles. According to the theory, these protons strike oxygen atoms in the atmosphere, and the collisions cause the glow, which we call the Northern Lights.
This view has changed in the Space Age. The data collected by many spacecraft showed that the situation is more complicated. Particles from both the solar wind and from the Earth’s atmosphere apparently are stored in the magnetotail. From there, they periodically are violently ejected into the northern and southern polar regions of the atmosphere along the Earth’s magnetic field. They are accelerated to high speeds by a process not yet fully explained. The magnetotail is, in effect, a reservoir of particles that is periodically refilled. When the Sun is active during maximum sunspot years, this process is especially intense and frequent, and the aurorae are brighter and move closer to the equator.