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Space Weather: A Research Perspective Space Weather: A Research Perspective Solar Origins of Space Weather Solar Activity Most of the effects we classify as space weather can ultimately be traced to changes occurring at the Sun. These include variations in both the solar electromagnetic radiation and the production of solar wind, plasma, and energetic particles. All of these are ultimately related to the evolution of the solar magnetic field. The figure below summarizes the different solar phenomena affecting our space weather, which are described here. Their relative importance in influencing space weather depends on where we are in the 11-year solar activity cycle. Illustration of the different solar and coronal structures that are the ultimate causes of space weather disturbances (courtesy of Solar-Terrestrial Environment Laboratory, Nagoya University). Solar Wind Variability Coronal Mass Ejections Some of the most dramatic space weather effects occur in association with eruptions of material from the solar atmosphere into interplanetary space. These eruptions are known as coronal mass ejections, or CMEs. The sequence of coronal images below shows the release of a CME at the Sun. Sequence of white light coronagraph images from the Solar Maximum Mission spacecraft showing the release of a coronal mass ejection, a huge bubble of coronal plasma and magnetic field, from the Sun (courtesy of High Altitude Observatory). Near solar activity maximum, the sun produces about 3 CMEs every day, whereas near solar minimum it produces only about 1 CME every 5 days. The faster CMEs have outward speeds of up to 2000 kilometers per second, considerably greater than the normal solar wind speeds of about 400 kilometers per second. These produce large shock waves in the solar wind as they plow through it. Some of the solar wind ions are accelerated by the shock, which then becomes a source of intense and long-lasting energetic particle enhancements in interplanetary space. file:///S|/SSB/1swSolar.htm (1 of 5) [6/25/2003 4:37:28 PM]

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Space Weather: A Research Perspective Illustration of a coronal mass ejection plowing through the solar wind at high enough relative speed to produce a leading shock wave. The shock wave accelerates some of the underlying solar wind particles to cosmic-ray energies (adapted from Hundhausen, p. 395 in Solar Wind, eds. C.P. Sonett, P.J. Coleman Jr., and J.M. Wilcox, NASA SP-308, Washington D.C.). Solar Wind Stream Structure The nonuniform source of the solar wind is responsible for other interplanetary disturbances. Solar rotation causes the high-speed flows from the coronal hole regions to run into the slower flows as they propagate out from the Sun, producing compressive interaction regions. Because the solar source regions for the flows are relatively long lived, these stream interaction regions appear to rotate with the Sun in spiral configurations like that illustrated below. Enhanced interplanetary magnetic field strengths and solar wind densities are associated with these regions. These structures are most pronounced on the approach to, and during the minimum of, the 11-year solar activity cycle when the coronal holes are largest. At these times, 27-day patterns are clearly evident in the solar wind properties near Earth. Illustration of the shapes of the compressive structures in the solar wind produced by interaction of the fast streams from the polar regions with slow solar wind from low latitudes. In this case the interactions occur because the magnetic dipole axis defining the coronal hole region is tilted with respect to the solar rotation axis (from V.J. Pizzo, 1994, J Geophys Res 99:4175). Magnetic Field Variations In addition to the striking solar-cycle-dependent changes, less dramatic variations in the solar wind are constantly occurring as processes in both the corona and interplanetary space produce waves and turbulence. To a large extent, it is these variations, superimposed on the underlying average conditions, that determine our typical space weather. The direction of the interplanetary magnetic field is particularly important in this respect since any southward inclination enables more efficient transfer of energy from the solar wind into the magnetosphere than a file:///S|/SSB/1swSolar.htm (2 of 5) [6/25/2003 4:37:28 PM]

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Space Weather: A Research Perspective northward inclination. Because of this effect, even the orientation of Earth's magnetic axis relative to the solar direction, which changes throughout the year, modifies the magnetophere's response to a particular solar wind state. Sample of interplanetary magnetic field measurements from the WIND spacecraft when it was located more than 200 Earth radii upstream of Earth. The north-south magnetic field component is called "Bz." ("Bx" points toward the Sun and "By" lies roughly parallel to the equatorial plane, while "B total" is the field strength including all of these contributions.) Times of negative (southward) Bz are times of increased interconnection between the interplanetary and geomagnetic fields (courtesy of the WIND magnetometer team). Solar Electromagnetic Variability The Sun is not only the source of light and heat, it is also a powerful and highly variable source of radio waves, ultraviolet rays, and x-rays. The latter emissions are the primary reason for the existence of our ionosphere. Almost all of the variability of the Sun at these wavelengths is connected with solar activity. Active Regions For well over a century, dark concentrations of intense magnetic fields called sunspots have been observed emerging from below the Sun's surface in an 11-year cycle. Recent space observations have revealed that the complexes of sunspots called active regions are the main source of long-lived solar ultraviolet and x-ray emissions. Solar gas, confined by the strong active-region magnetic fields into loop- like structures, is heated to temperatures of millions of degrees. During times of maximum solar activity, the average level of solar ultraviolet emission can increase to several times the quiet Sun level, while the x-ray intensity shows even greater enhancements. Since active regions usually last longer than the 27-day solar rotation period, the radiations they emit also vary periodically on this time scale. The changing appearance of the sun in low-energy x-rays from times of high to low solar activity is evident in the sequence of images below. file:///S|/SSB/1swSolar.htm (3 of 5) [6/25/2003 4:37:28 PM]

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Space Weather: A Research Perspective Collage of x-ray images of the Sun obtained on the Yohkoh satellite between 1991 and 1995 at 120-day intervals, showing the evolution in the appearance of the Sun in these emissions from active to quiet times. The dark regions are locations of low coronal density, which are the sources of the faster solar wind streams. The brightest regions are coronal loops heated above active regions (courtesy of Lockheed-Martin Solar and Astrophysics Laboratory). Flares Short periods of explosive energy release, known as solar flares, frequently occur in active regions during the period around solar maximum. An example of a flare observed on the limb of the sun is shown below. Flares have lifetimes ranging from hours for large gradual events down to tens of seconds for the most impulsive events. During a very strong flare, the solar ultraviolet and x-ray emissions can increase by as much as 100 times above even active-region levels. During solar maximum, approximately one such flare is observed every week. Flares heat the solar gas to tens of millions of degrees. The heated gas then radiates strongly across the whole electromagnetic spectrum from radio to gamma rays. The largest of these explosions are so bright that they can even be seen from Earth in visible light. The Sun as seen in x-rays (left panel). The upper atmosphere or corona of the Sun emits x- rays because it is very hot, with temperatures of a few million degrees. The Sun's magnetic field traps the ionized gas (plasma) in loops. On the right limb of the Sun is a loop that has been illuminated by the extraordinary heating associated with a solar flare (enlargement in right panel). Flares are powerful explosions, lasting minutes to hours, that produce strong heating and acceleration of particles (courtesy of Solar Data Analysis Center, Goddard Space Flight Center). Flares can accelerate protons and electrons that travel to Earth directly from the Sun along the interplanetary magnetic field (which "channels" the charged particles). These contribute to the high-energy particle environment in the vicinity of the magnetosphere if Earth's location is magnetically connected to the flaring region by the interplanetary magnetic field. file:///S|/SSB/1swSolar.htm (4 of 5) [6/25/2003 4:37:28 PM]

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Space Weather: A Research Perspective Illustration of the importance of magnetic connection to a flare site for determining the intensity of flare-accelerated energetic particle radiation at the Earth. Note that because of the spiral configuration of the interplanetary magnetic field, the best connections occur for flares on the right side of the Sun as viewed from Earth (courtesy of M.A. Shea, Phillips Laboratory). Radio Bursts Unlike the case of solar ultraviolet and x-ray radiation for which we know the basic emission processes, the sources of solar radio bursts are poorly understood. Almost all manifestations of solar activity have some signature in radio waves, and the radio bursts themselves appear in many forms. The most notable radio emissions are intense bursts associated with flares or CMEs and long-lived noise storms associated with active regions. During such storms the emissions are strong for several days. The figure below shows an example of a strong radio burst detected over a range of frequencies. Time history of the intensity of solar radio waves at various frequencies during a radio burst occurring in June 1992. The color reflects the intensity of the emissions (from: Culgoora Radio Observatory, Narrabri, New South Wales, Australia). file:///S|/SSB/1swSolar.htm (5 of 5) [6/25/2003 4:37:28 PM]

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