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Space Weather: A Research Perspective Space Weather: A Research Perspective What Is Space Weather? "Space weather" describes the conditions in space that affect Earth and its technological systems. Our space weather is a consequence of the behavior of the sun, the nature of Earth's magnetic field and atmosphere, and our location in the solar system. Space Is Not Empty Earth is immersed in the escaping ionized outer atmosphere of the Sun. This "solar wind," flowing against Earth's magnetic field, shapes the near-Earth space environment. The magnetic bubble of the "magnetosphere," carved out by Earth's field, shields our upper atmosphere with its ionized region, the ionosphere, from the direct effects of the solar wind. Cutaway illustration of Earth's magnetosphere, where the surfaces illustrate the internal magnetic field configuration and the boundary between the magnetosphere and solar wind. Key regions are labeled. The orbits of several spacecraft of the ISTP armada are shown to scale (from: P. Reiff, Rice University). The magnetosphere is home to research, telecommunications, navigation, and weather satellites that are surrounded by the energetic particles of the Van Allen radiation belts and the thin gases of the upper atmosphere. The space shuttle and the future space station fly in the upper atmosphere, where influences from both above and below determine the local conditions. The Global Positioning System (GPS) satellites, used for navigation, surveying, and geophysical research, pass through the radiation belts, although they orbit Earth at altitudes above most of the atmosphere. Geosynchronous communication and weather satellites reside on the outer edges of the radiation belts, where disturbed space weather causes increases in the intensities of hazardous energetic particles. All spacecraft send and receive their signals through the ionosphere, which is sometimes dramatically altered by space weather events. The Sun Is Not Steady Monthly averaged sunspot numbers in the visible hemisphere of the Sun. The numbers of spots wax and wane with the 11-year solar activity cycle. Larger maxima usually mean more solar "storms" (from: The National Solar Observatory/Sacramento Peak). The Sun and its atmosphere are always changing, in a sense having weather of file:///S|/SSB/1swWhat.htm (1 of 4) [6/25/2003 4:35:19 PM]

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Space Weather: A Research Perspective their own. The Sun undergoes long-term (decade or more) "climate-like" variations such as the roughly 11-year solar cycle. This cycle first showed itself in the number of sunspots counted on the solar surface (above), but it is also seen in the appearance and disappearance of other less obvious features, like the filaments in the pictures below that can be seen with a special filter. Images of the Sun obtained through filters that isolate the H-alpha emission line of hydrogen. Features on the photosphere stand out at this wavelength, in particular around solar maximum. The dark filaments are produced by cooler material that arches above the main surface, while bright "plages" are concentrated around active regions (from: The John W. Evans Solar Facility, National Solar Observatory/Sacramento Peak). Space weather disturbances are generally caused by what are effectively solar "storms." One type of solar storm is called a solar flare because the brightening of a small area on the Sun heralds its occurrence. A flare on the photosphere brightens the emissions of light from an active region (from: The Space Environment Center, National Oceanic and Atmospheric Administration). The other type of common storm is called a "coronal mass ejection." These explosions of material from the Sun's upper atmosphere are hard to see by eye except during total solar eclipses. Because of this, they were recognized only in modern times. An eruption in the Sun's corona made visible by blocking the bright solar disk (from: The Solar Data Analysis Center, Goddard Space Flight Center). The solar disturbances shown in the figures above are most frequent around "solar maximum," defined by the high points in the sunspot cycle. The "solar minimum" periods, however, are not without their own characteristic space weather. The Response of Our Space Environment to the Constantly Changing Sun Is Known as "Space Weather" "Storms" in our space weather generally follow severe solar disturbances. Dramatic changes in the particle populations and the electrical currents flowing within and through interplanetary space and near-Earth space are the result. file:///S|/SSB/1swWhat.htm (2 of 4) [6/25/2003 4:35:19 PM]

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Space Weather: A Research Perspective The details of what causes various types of space weather are subjects of active investigation. Sometimes cause-and-effect chains can be easily recognized; for example, the ionizing effects of flares' emissions in our atmosphere. At other times, the connections are more complicated and subtle. For example, the aurora is a hallmark of disturbed space weather, but auroral displays do not always follow a specific solar event. Radiation experienced by satellites in orbit may depend as much or more on the location of the orbit as on the level of solar activity. The radiation intensities from energetic charged particles in the magnetosphere can reach close to Earth, to ionospheric altitudes, as is often observed by low-altitude satellites. An example of such a low-altitude influence is shown here, over a day in the life of a satellite called SAMPEX as it circles the globe at about 600 kilometers altitude. In this view, the energetic particle fluxes are most intense in the region of the red and yellow ring around the north pole (from: D.N. Baker and S.G. Kanekal, Goddard Space Flight Center). The aurora occurs when energetic particles, mostly electrons, "rain" down from the magnetosphere during episodes of disturbed space weather. The auroral light is emitted by atmospheric atoms and molecules that become excited by the close passage of the electrons. The auroral electrons can also ionize the atmospheric particles, thus contributing to the local ionosphere (from: The Straight Scoop). Space Weather Affects How We Live and Work Today Most of the time space weather is of little concern in our everyday lives. However, when the space environment is disturbed by the variable outputs of the Sun, technologies that we depend on both in orbit and on the ground can be affected. Illustration of the constellation of GPS satellites encountering the radiation belts (shown in cross section) as they orbit Earth (from: U.S. Air Force Phillips Laboratory home pages). Forty years ago the first artificial satellite was launched. Today, we have a very large number of satellites in orbit and frequent manned presence in space. Increasingly, communication and navigation enterprises are replacing cables and repeater stations with "permanent" space-based networks. file:///S|/SSB/1swWhat.htm (3 of 4) [6/25/2003 4:35:19 PM]

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Space Weather: A Research Perspective One configuration proposed for the International Space Station home page (from: The International Space Station home page) Fifteen years from now there will be many hundreds of active satellites and a nearly continuous manned presence on the space station. Space weather will be something that we will hear more and more about, especially during periods when the Sun is active. How does space weather figure into this vision of the present and future? Corrective or preventative measures can be taken to ensure that radiation doses and electrical charging of satellites in the Earth-space environment do not damage critical spacecraft components and functions, that disturbances of the ionosphere do not degrade the performance of navigation and communications systems, and that the ionospheric currents related to those disturbances which generate magnetic fields on the ground do not interfere with the operations of our large-scale power distribution grids. Precautions can also be taken to minimize exposure of astronauts and passengers in aircraft flying at high altitudes along polar routes to hazardous radiation levels if these conditions can be foreseen. Can Space Weather Be Predicted? Our understanding of space weather is at a primitive level, perhaps analogous to that of traditional meteorology in the early 1950s. Still, much progress has been made since the 1960s as a result of experiments carried out on spacecraft. Today, realistic space environment models and enhanced observational capabilities for detecting disturbed space weather conditions or their precursors have the potential to be applied to space weather prediction and management. This requires a framework whereby both expertise and facilities at a broad range of institutions are brought to bear on the task. Such an endeavor is the ultimate test of our knowledge of the Sun-Earth connections. file:///S|/SSB/1swWhat.htm (4 of 4) [6/25/2003 4:35:19 PM]

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