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Space Weather: A Research Perspective Space Weather: A Research Perspective The Elements of Near-Earth Space The Sun The Sun is a typical yellow star that, together with our solar system, formed from an interstellar cloud some 5 billion years ago. Image of the Sun in ultraviolet light showing a large prominence at the limb (from: The Space Physics Group at Oulu, Finland, Space Physics Textbook home page). Solar energy is produced deep within the Sun by nuclear reactions. Most of this energy emerges as sunlight, at a remarkably constant rate, from the visible surface or photosphere. The Sun also gives off ultraviolet, x-ray, gamma-ray, and radio emissions that are much more variable than its visible emissions. The most extreme variability occurs in the localized explosive phenomenon known as a solar flare. Cutaway diagram showing what is thought to be inside the Sun (from: The Space Science Institute home page). The hot ionized gases in the interior of the Sun are constantly in motion as a result of the heat generated within, coupled with the Sun's rotation (one rotation every 27 days, approximately). Solar magnetic fields are generated below the photosphere by a still poorly understood process related to this circulation. These fields are sometimes concentrated in sunspots or complexes of sunspots forming active regions. Active regions are the usual sites of flares, which may occur when their complicated magnetic fields are suddenly rearranged. The surface magnetic field of the sun is concentrated in sunspots, which appear dark because the material in the sunspot is much cooler than the surrounding material. Sunspots often appear in groups called active regions (courtesy of C.J. Hamilton, Views of the Solar System home page) file:///S|/SSB/1swElements.htm (1 of 9) [6/25/2003 4:36:20 PM]

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Space Weather: A Research Perspective Above the solar surface stretches the extended solar atmosphere, known as the solar corona. Propagating waves and/or processes associated with the constant rearrangement of the magnetic fields close to the Sun raise the temperature of the corona (to over 1,000,000 Kelvin), far above that of the solar surface (at about 6000 degrees Kelvin). Because of its temperature, the coronal gas is highly ionized and so its structure is affected by the coronal magnetic field. The picture of the corona below is in effect a picture of the coronal magnetic field structure. A photograph taken during a solar eclipse on February 2, 1980. Concentrations of plasma, organized by solar magnetic fields, are clearly visible (from: the High Altitude Observatory public archives). The solar magnetic field evolves over the solar cycle along with the sunspot number. Driven by the motions of the ionized gases beneath the visible surface, the North and South poles of the solar magnetic field reverse approximately every eleven years around sunspot number maximum. As a result, there is a roughly 22- year cycle in the Sun's magnetic "polarity," with the North or South magnetic pole in the northern hemisphere of the Sun during alternate sunspot maxima. The field is more complicated at solar maximum when the simple solar minimum structure, which resembles Earth's field or that of a bar magnet, is disrupted by the strong fields of many active regions. Magnetic field structures in the corona at solar minimum (left) and solar maximum (right) inferred from ground-based observations. The structures are shown by "field lines" that track the direction of the field at the points they thread. The solar minimum coronal field resembles that of a bar magnet except at high latitudes, while the solar maximum field is much more complicated (derived from Wilcox Solar Observatory magnetograph data, courtesy T. Hoeksema). Processes related to this evolution of the solar magnetic field are the ultimate causes of space weather. Effects of the 22-year magnetic cycle can be seen in some space weather records, but it is primarily the 11-year cycle of activity that is of concern in space weather. The Solar Wind The high temperature of the solar upper atmosphere generates an outward flow of the ionized coronal gas or plasma away from the Sun at typical speeds ranging from 400 to 800 kilometers per second. This outflow is known as the "solar wind." At the Earth (1 astronomical unit (AU) or 150 million kilometers away from the Sun), 1 cubic centimeter of solar wind contains about 8 protons and an equal number of file:///S|/SSB/1swElements.htm (2 of 9) [6/25/2003 4:36:20 PM]

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Space Weather: A Research Perspective electrons (so there is no net electrical charge in the gas). Helium and heavier ions are also present in the solar wind but in smaller numbers. Since the Skylab mission in 1973, it has been realized that the solar wind does not flow uniformly from everywhere on the Sun. Measurements have shown that it comes mainly from low-density regions in the corona called "coronal holes." Coronal holes are so-named because they appear dark in x-ray images of the Sun (see the example below). They are usually located in the Sun's polar regions, but their irregular boundaries can dip into low solar latitudes where they affect conditions in interplanetary space. In general, the sizes, shapes, and distributions of coronal holes change over the solar cycle, with the largest coronal holes appearing around sunspot minimum when the coronal magnetic field has a simple configuration. The highest-speed solar wind streams observed at the Earth are often associated with these large coronal hole sources. Illustration showing Ulysses spacecraft solar wind speed measurements referred to their source regions seen on an x-ray image from the Yohkoh satellite. The high-speed solar wind from the polar coronal holes (dark areas) was measured all of the time when Ulysses was at high solar latitudes (from: The Ulysses Mission home page). Coronal magnetic fields are constantly being carried with the solar wind into interplanetary space. The solar rotation winds up the field into a spiral resembling the water streams from a rotating garden sprinkler because the source of the field keeps moving with the Sun. Illustration of the "winding up" of the magnetic field in the solar wind (courtesy of J. Luhmann, University of California at Berkeley). At the Earth's distance from the Sun, the typical interplanetary magnetic field strength is about 5 nano teslas, or about 1/10,000 the strength of the Earth's magnetic field at the surface. The "polarity" of the interplanetary magnetic field file:///S|/SSB/1swElements.htm (3 of 9) [6/25/2003 4:36:20 PM]

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Space Weather: A Research Perspective depends on the direction of the coronal field at its roots. As illustrated below, the interplanetary field is typically organized into hemispheres of inward and outward field corresponding to the North and South magnetic poles of the solar field. The two hemispheres are separated by a sheet-like boundary carrying an electrical current. The interplanetary magnetic field directions reverse with the Sun's magnetic polarity near sunspot maxima. Sketch showing the solar connections of the interplanetary current sheet. The directions of the magnetic fields (arrows) define their "polarity" (from: The Solar Data Analysis Center, NASA Goddard Space Flight Center). Since the solar field is not a perfect configuration with simple North and South poles aligned along the Sun's rotation axis, the current sheet often has ripples like a ballerina's skirt. The interplanetary current sheet shape appears like a twirling ballerina's skirt when the solar dipole magnetic field has an axis different from the solar rotation axis (from: Lund Space Weather and AI Center, Lund University, Sweden). The passage of this current sheet is an important marker for space weather. During solar maximum, the structure of the current sheet can become quite complicated as the magnetic fields of active regions on the Sun disrupt this simple picture. The Magnetosphere Processes analogous to those in the Sun's interior work in the molten core of the Earth to generate our own magnetic field. The Earth's field is even more like that of a bar magnet than the solar field, with oppositely located North and South poles characterizing its very "dipolar" (two-pole) structure. file:///S|/SSB/1swElements.htm (4 of 9) [6/25/2003 4:36:20 PM]

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Space Weather: A Research Perspective Illustration of the dipolar magnetic field structure generated in the Earth's interior (from NASA Goddard Space Flight Center Space Physics Education home page). The highly conducting solar wind gas is not able to penetrate Earth's magnetic field at most locations, instead flowing around it. Before it is diverted, however, it slows down at a (shock) wave called the "bow shock" that stands upstream of Earth in the solar wind. A bow shock stands upstream of the Earth in the solar wind plasma. It serves to slow the flowing ionized gas before it encounters the obstacle presented by the Earth's magnetic field, analogous to air flow around a supersonic aircraft (from: Rice University). The diversion of the solar wind flow generally occurs at about 6 to 15 Earth radii above Earth's sunlit surface, where the variable solar wind plasma "ram" pressure is balanced by the pressure of the compressed dipolar field of Earth. The cavity that Earth's field makes in the solar wind stretches out into a long "magnetotail" on the nightside. This cavity and everything inside it make up the "magnetosphere." The size of the magnetosphere is smallest when the solar wind is strong, sometimes pushing the dayside boundary inside of the orbit of geosynchronous satellites. The magnetosphere is the region of space above the atmosphere that is dominated by the Earth's magnetic field. This cutaway illustration shows the major structural features of this complex, dynamical system derived from spacecraft observations in many different orbits (from: Rice University). Earth's magnetic field connects with the interplanetary magnetic field in the polar caps. This interconnection allows transfer of energy from the solar wind to the magnetosphere and ionosphere, as well as entry of charged particles from interplanetary space. The amount of interconnection is greatest when the interplanetary magnetic field has a southward direction. Illustration showing why southward-oriented interplanetary fields are more effective at transferring energy from the solar wind to the magnetosphere. The interconnection between the interplanetary and geomagnetic field is enabled at the "nose" of the magnetosphere because the fields are oppositely directed there. Once connected, the fields move roughly as indicated by the sequence of numbers (from: NASA Goddard Space Flight Center Space Physics Education home page). file:///S|/SSB/1swElements.htm (5 of 9) [6/25/2003 4:36:20 PM]

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Space Weather: A Research Perspective The auroral ovals are located in the polar regions bordering the area threaded by the interconnected fields. Here, energetic charged particles from both the solar wind and the magnetosphere precipitate or "rain" into the atmosphere from above. This rain of typically kilovolt-energy particles is responsible for the aurora borealis (northern lights) and aurora australis (southern lights), produced when atmospheric atoms and molecules energized by collisions with the incoming particles emit light. This image of the auroral oval shows the bright emissions that occur in the atmosphere at altitudes between 80 and 300 kilometers when atoms are excited by energetic electrons traveling along magnetic field lines. This image was obtained from the Dynamics Explorer spacecraft over the north polar region during winter, when the entire aurora borealis was over the night hemisphere of the Earth (from: Dynamics Explorer University of Iowa Imaging Experiment, L.A. Frank principal investigator). Several concentrations of charged-particle populations with different origins, densities, and energies are found in the magnetosphere. The Van Allen radiation belts are one such population with sources as diverse as the decay products of cosmic-ray collisions with atmospheric atoms and ions that originated in the interstellar gas. The electrons and ions in the Van Allen belts are effectively trapped in Earth's magnetic dipole field. Bouncing between hemispheres, they reverse their motion at their closest approach to Earth, while at the same time drifting around it. If their trajectories take them too deeply into the atmosphere where they collide with the ambient particles and lose their energy, they are lost from the radiation belts. In this figure, "trapped ACR" refers to ions that originate in the interstellar gas, while energetic secondary ions originate from collisions of cosmic rays with atmospheric gases (courtesy of J.B. Blake, The Aerospace Corporation). Earth's upper atmosphere is another source of particles for the magnetosphere. Ionized atmospheric gases at high altitudes can be energized and transported around by a variety of processes that are still under study. The ions and electrons in the magnetosphere carry electrical currents that produce deviations in the magnetic field measured on the ground as well as in space. The Upper Atmosphere and Ionosphere At the base of the magnetosphere lie the upper reaches of Earth's atmosphere and ionosphere. The upper atmosphere is composed of atoms and molecules of gases such as oxygen and nitrogen, which become increasingly sparse with increasing altitude. At the 300- to 400-kilometer altitude of the shuttle orbit, the atmospheric density is on the order of 300 million (300,000,000) particles per cubic centimeter or file:///S|/SSB/1swElements.htm (6 of 9) [6/25/2003 4:36:20 PM]

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Space Weather: A Research Perspective about 1/10,000,000,000 times the density of all of the gases of the air at the ground. At typical weather satellite orbit altitudes (about 850 kilometers), the density is down to 1/100 to 1/1000 times the density at the shuttle orbit. However, these densities can increase by more than ten times when the upper atmosphere is heated by strong solar ultraviolet radiation at solar maximum or by processes occurring during disturbed space weather. The ionosphere is the region of the atmosphere above 60-kilometer altitude that is slightly ionized by solar ultraviolet radiation. The UV radiation can eject negatively charged electrons from their "parent" atoms and molecules to make positively charged ions. These charged particles live for minutes to hours before recombining to again form neutral or uncharged gas particles in an ongoing cycle of ionization and recombination. On the average, the abundance of charged particles increases with altitude up to about 300 kilometers, decreasing gradually at higher altitudes (see the figure below). Ionization by energetic cosmic-ray particles contributes some of the lowest-altitude ionosphere. Illustration of the upper-atmosphere temperature variability and the regions of Earth's ionosphere, which are labeled by letters. The various ionospheric peaks are the result of the various sources of atmosphere ionization and the atmospheric chemistry at different altitudes (courtesy of J.H. Yee and associates, Applied Physics Laboratory, Johns Hopkins University). The aurora is produced at around 100-km altitude by the impact of energetic particles from above that cause the atmospheric gases to emit light in a process similar to that in a neon sign. At night, when the ionizing sunlight is absent, the particles that produce the aurora can also be the primary source of the local ionosphere. The aurora borealis as seen from the ground. Different colors arise because different atmospheric gases are excited, and the excitation occurs at different altitudes as a result of the wide energy spread of the exciting electrons (from: Rice University educational home pages). Even though there are many more neutral gas particles than ions and electrons in file:///S|/SSB/1swElements.htm (7 of 9) [6/25/2003 4:36:20 PM]

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Space Weather: A Research Perspective the ionosphere, the charged particles give it electrically conducting properties that make it an essential element of space weather. Cosmic Rays In addition to the space environment described above, Earth is immersed in an extremely tenuous bath of high-energy charged particles called cosmic rays. Most cosmic rays enter the solar system from the galaxy, but solar flares contribute on occasion and disturbances moving through the solar wind can energize particles in their path. Both solar wind particles as well as some originating from interstellar gas ionized in the vicinity of the Sun can be so energized. The cosmic rays produced within the solar system sometimes make up the dominant contribution near Earth. Although Earth's magnetic field acts like a shield, deflecting all but the fastest of these particles, even low-energy cosmic rays reach low altitudes in the polar regions where the planet's magnetic field interconnects with the interplanetary field. The most energetic cosmic rays can reach cloud levels (around 10-km altitude) where they affect cloud electrification, while a few may even reach the ground or create secondary particles in the atmosphere. Cosmic rays can penetrate into the atmosphere, producing many "secondary particles" as products of their collisions with atmospheric nuclei (courtesy of M.A. Shea, Phillips Laboratory). Particles that are produced when cosmic rays interact with the thin upper atmosphere can become part of the Van Allen belts. The local density of cosmic rays varies with the solar activity cycle because the disturbed solar wind during solar maximum sweeps these particles out of the solar system more effectively than does the quiet solar wind. Data from the Climax, Colorado, neutron monitor operated by the University of Chicago. The cosmic rays counted exhibit an inverse relationship to the solar cycle because solar wind from the active Sun sweeps more away from the Earth than does solar wind from the quiet Sun (from: The NOAA National Geophysical Data Center, Boulder, CO). The expanding solar wind forms a bubble in the interstellar material thought to extend at least 100 AU from the Sun. (For comparison, the outermost planet, Pluto, orbits the Sun at about 40 AU distance.) Space weather occurs everywhere within this bubble known as the "heliosphere." Every object in the solar system experiences the equivalent of Earth's space weather, although its detailed characteristics differ from place to place. file:///S|/SSB/1swElements.htm (8 of 9) [6/25/2003 4:36:20 PM]

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Space Weather: A Research Perspective The heliosphere is a bubble, formed by the expanding solar wind, in the material between the stars (from: NASA's Solar Connections home page). file:///S|/SSB/1swElements.htm (9 of 9) [6/25/2003 4:36:20 PM]

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