THE SUN IN A NEW LIGHT|
Since the dawn of history, the Sun has occupied a central role in the human mind and imagination. Yet it has only been within the past few hundred years since Copernicus and Galileo, and 19TH Century inventors like Fraunhofer and George Ellery Hale that we have been able to closely examine the changing face of the Sun. The Sun's role in driving space weather around Earth has only been appreciated in the last 150 years, and in detail only within the last forty.
As early as 28 B.C.E. (Before the Current Epoch), astronomers in China observed changing dark patches on the surface of the Sun. The writings of Greek philosophers from the fourth century B.C.E. also include references to what seem to be sunspots. However, none of the early observers could really explain what they were seeing.
In Western culture, sunspots were discovered by Johannes Fabricius of Holland, Galileo Galilei of Italy, Christopher Schiener of Germany, and Thomas Herriot of England, all of whom claimed to have discerned sunspots sometime in 1611. All four observed sunspots through telescopes, and made drawings of the changing shapes by hand, watching them traverse the visible disk of the Sun. By 1749, astronomers were making daily sunspot logs at the Zurich Observatory in Switzerland, and with the addition of other facilities, continuous record-keeping started in 1849. An amateur astronomer, Heinrich Schwabe, was the first to note in 1859 that there was actually a cycle, a rhythm, to the appearance of sunspots.
In the late nineteenth and early twentieth centuries, scientists began to notice other aspects of the Sun-Earth relationship. In 1859, Richard Carrington used his telescopes to make out solar flares. On February 4, 1872, unrecognized by scientists and engineers on Earth, a great magnetic storm began producing exceptional auroral displays and started fouling up telegraph messages with fluctuating electric currents... currents that were, in fact, caused by Earth's disturbed magnetic field. In 1920s and 1930s, Karl Jansky of Bell Labs was busy developing telephone and radio equipment when he found that Trans-Atlantic telephone connections suffered from interference and a mysterious phenomenon he called "magnetic storms." By 1942, J. S. Hey found the clue to those disturbances when he discovered that the Sun emits its own radio frequencies.
But it was not until James Van Allen and other scientists helped launch the Space Age with NASA's Explorer I satellite on January 31, 1958 that humans could really sense the "electric space" around Earth. Data from a single Geiger-Mueller tube on Explorer I shot off the top of the scale and resulted in the discovery of a radiation belt around Earth a huge region of space populated by energetic charged particles trapped within our planet's magnetic field.
HOW DO YOU SEE THE INVISIBLE?
Sunspots on our star, and auroras here at Earth are visible signs of magnetic mayhem in the Sun-Earth system, but beyond that, the human eye can't detect much of what we call space weather. Telescopes see more keenly, but only as clearly as Earth's atmosphere will allow. Because of pressure systems, gases, and winds flowing above us like a river of air, observing the fine details of our star or any other is like looking at an object through water. Murky water, at that, which blocks out some of the most interesting light, such as X-rays and gamma rays. To top it off, most of the material flowing from Sun to Earth is too small, too diffuse, or too dim when measured against the background of space or the brightness of the Sun to register in the visible portion of the spectrum, and certainly not in daylight.
In order to study and understand the invisible fabric of the Sun-Earth system, scientists needed to get above the distractions and distortion of the atmosphere. And once there, they needed more than just a telescope. Over the past 40 years, space physicists have developed and refined instruments in support of dozens of satellite missions, including Skylab, the Interplanetary Monitoring Platform, the Solar Maximum Mission, Dynamics Explorer, Helios, and the International Sun-Earth Explorer (ISEE).
They have come to rely on telescopes that detect visible light, ultraviolet light, gamma rays, and X rays utilizing virtually the entire electromagnetic spectrum. They use receivers and transmitters that detect radio shock waves created when blasts from the Sun smash into slower ebbs of solar wind (the equivalent of a sonic boom in space). They employ particle detectors to count ions and electrons, magnetometers to record changes in magnetic fields, and CCD cameras to observe the auroral patterns over the whole Earth.
For instance, since the corona is only visible to the naked eye during an eclipse, scientists must use an occulting disk which blocks out the light from the solar surface to create an artificial eclipse to detect what the Sun is spitting into space. Some of the most important recent advances in understanding and tracking coronal mass ejections have come from cameras that photograph the corona and detect the tell-tale halo signature of a CME as it heads toward Earth.
MANY EYES, ONE VISION
Since Explorer 1, Earth's space environment traditionally has been studied as a set of independent parts the Sun, the interplanetary region (or heliosphere), the magnetosphere, the ionosphere, and Earth's upper atmosphere. Consequently, past missions have understood these phenomena only individually.
Yet even from the earliest studies, scientists have known that the Earth-Sun system is composed of highly interactive elements, and so have come to call it "geospace". To understand the system as a whole, scientists needed to plan a program of simultaneous space and ground-based observations and theoretical studies. The idea was to assess the production, transfer, storage, and dissipation of energy across the entire solar-terrestrial system. In essence, they needed a comprehensive, quantitative study of the energy chain all the way from the Sun's interior right out to Earth's magnetic tail. Supported by the scientific community, NASA's answer was the International Solar-Terrestrial Physics (ISTP) program.
ISTP was conceived in the 1970s, planned in the 1980s, and launched in the 1990s. The mission is intended to be a global effort to observe and understand our star and its effects on our environment. An armada of more than 25 satellites, working together with ground-based observatories and computer simulators and theory centers, allows scientists to study the Sun, the Earth, and the space between them from many perspectives and in many different ways. Individually, the spacecraft contributing to ISTP act as microscopes, studying the fine detail of the Sun, the solar wind, and the boundaries and internal workings of Earth's magnetic shell. When linked together with each other and the resources on the ground, they act as a wide-field telescope that sees the entire Sun-Earth environment.
The primary spacecraft of ISTP Geotail, Wind, Polar, and the Solar and Heliospheric Observatory (SOHO) allow physicists to observe all the key regions of Earth's space. The first, Geotail, was launched in 1992 by Japan's Institute of Astronautical Science (ISAS) and by NASA to study the distant reaches of Earth's magnetic tail, or the region downwind of Earth. In 1994, NASA launched the Wind mission into an orbit that placed in on the sunny side of Earth, where it could sample the solar wind. In 1996, NASA launched the Polar satellite to swing over Earth's north and south poles and look down to monitor the aurora and other physical activity. Finally, NASA and the European Space Agency launched the SOHO to watch the Sun full-time from space, without the intrusion of Earth's shadow.
In addition to these four "flagships", ISTP relies on significant contributions from smaller spacecraft and computer and radar facilities run by: Germany's Max Planck Institute, Russia's Space Research Institute, the U.S. National Oceanic and Atmospheric Administration (NOAA), Los Alamos National Laboratory, the U.S. Air Force, the Canadian Space Agency, the British Antarctic Survey, and the U.S. National Science Foundation. In addition, several thousand scientists from 40 countries contribute observations to and analyze data from ISTP. The newest Sun-Earth spacecraft, such as the Advanced Composition Explorer (ACE), the Fast Auroral Snapshot (FAST), and the Transition Region and Coronal Explorer (TRACE) are not formal members of ISTP, but given the increasingly coordinated methods of space physics, each mission is collaborating with ISTP investigators.
Collectively, the spacecraft of ISTP and its partners are placed in orbits that allow physicists to observe the key regions of geospace. Those regions include the Sun's outer layers and atmosphere, the solar wind, and Earth's magnetosphere from the bow shock, to the auroral regions and to the magnetic tail. Orbiting as far as one million miles from Earth (SOHO) and as close as just a few hundred miles (TRACE), the spacecraft of ISTP make coordinated, simultaneous observations of the Sun and geospace over extended periods of time.
WHAT'S NEXT? HOT PLASMAS AND COOL LIGHTS
Given the many thousands of years we have waited to achieve our current understanding of the Sun, the Earth, and the space in between, we are now privileged to enjoy a most remarkable situation. Scientists and their national science agencies have put into place an amazing array of spacecraft and ground facilities for studying the Sun-Earth environment. Sensitive telescopes in space and on the ground examine the Sun's many layers in unprecedented detail. Other spacecraft sample the hot, high-speed plasmas flowing past the Earth from the expanding solar corona. Still more satellites continuously monitor the plasmas which ebb and flow within the magnetosphere as it is buffeted by the solar wind. There is even a network of ground observatories recording the signatures of Sun-Earth interaction in our atmosphere and ionosphere. Merged as they are into a unified, comprehensive mission, these many components afford an opportunity to stretch our understanding of the physics of the Sun-Earth Connection.
It is an historic occurrence that such a constellation should be operating as Solar Maximum approaches. Never before have scientists had such a complete set of tools with which to study the climax of a solar cycle. And never before have they had tools of such power and precision to study our most important star the Sun and our most important planet Earth. As the new millennium dawns, space physicists have a chance to study all aspects of this solar maximum and its consequent effects on near-Earth space. Even as some of the spacecraft of ISTP and other programs reach the end of their useful life (it's pretty tough working in space, after all), a new line of spacecraft are queuing for takeoff. Missions such as the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), Cluster II, Solar Probe, and the Solar Terrestrial Relations Observatory (STEREO) are all planned for launch between 2000 and 2004, keeping the coordinated observation program going..
What they learn will refresh and reshape our understanding of how our Sun and Earth dance with each other. It might also change our view of what lies beyond our small solar system on the edge of but one galaxy. After all, astronomical observations have shown us that our Sun is a rather common and ordinary star, akin to the many variable, magnetic stars in the universe. In essence, our Sun-Earth system is the physical prototype for solar systems throughout the cosmos. And while we may never be able to observe first-hand the physical processes that drive much of the Universe, we can study many of those phenomena right here in our own cosmic backyard. For the foreseeable future, our Sun is the only one we can study up close.
Written by Mike Carlowicz, science writer and senior outreach coordinator for the International Solar-Terrestrial Physics (ISTP) mission, based out of NASA Goddard Space Flight Center.