Click for next page ( 60


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 59
Space Weather: A Research Perspective Space Weather: A Research Perspective Practical Consequences of Space Weather Effects on Satellites Illustration of a spacecraft in an orbit that traverses the radiation belts (courtesy of Lund Space Weather and AI Center, Lund University, Sweden). Satellites often operate in the space environment for many years. As a result, they can sustain long-term exposure effects in addition to special "storm-time" problems. Depending upon their altitude, satellite electronic components, solar cells, and materials degrade from the accumulated radiation dose caused by repeated traversals of the Van Allen radiation belts. Similarly, the bombardment by atoms in the thin upper atmosphere can alter orbits and wear surfaces away. Some materials become brittle from long-term exposure to solar ultraviolet light above the protective absorbing atmosphere. Single penetrating cosmic rays can change the state in electronics components such as spacecraft memory chips. Newspaper headlines announce the failure of systems aboard the Canadian ANIK E-1 and E-2 communications satellites due to elevated intensity of activity of high-energy electrons in Earth's outer magnetosphere (courtesy of the Solar Data Analysis Center, Goddard Space Flight Center). It is thought that the trapped energetic electron radiation whose intensity increases with solar wind velocity produces deep dielectric charging in the unshielded parts of file:///S|/SSB/1swConsequences.htm (1 of 8) [6/25/2003 4:38:49 PM]

OCR for page 59
Space Weather: A Research Perspective a satellite. The figure below shows when the ANIK problems occurred compared to a record, from the SAMPEX satellite, of the energetic electrons near the geosynchronous satellite orbit (note that the 27-day rotation period of the Sun is also visible here). History of energetic electron intensities in the radiation belts during the ANIK satellite failures (courtesy of D.N. Baker, University of Colorado) Space weather "storms" add new problems while exacerbating the above cumulative effects. Some satellites charge up when they are suddenly immersed in enhanced radiation environments in the Van Allen belts, the auroral zone, or interplanetary space. Dielectric surfaces can charge to very high potential compared to the metallic surfaces of the satellite, leading to discharges between the two. Such discharges cause both material damage and electrical transients on the spacecraft. Electrical transients from surface discharging, or from internal charging (caused by the above-mentioned energetic electrons producing charges deep inside electronics components), can masquerade as "phantom commands" appearing to spacecraft systems as directions from the ground. These events can cause loss of control of instruments and power or propulsion systems. Electrical transients often occur in the local time period between midnight and dawn following what appears to be an injection of electrons toward Earth from the magnetotail during geomagnetic disturbances. Locations of spacecraft at times when they experienced anomalous discharges, presumably due to enhancements in the radiation environment accompanying magnetic storms and substorms (courtesy of the NOAA National Geophysical Data Center, Boulder, CO). Another "danger zone" for spacecraft is in the region of the South Atlantic, where the energetic particle populations in the radiation belts are found at unusually low altitudes due to a local weakness in Earth's magnetic field. Locations of spacecraft in low-Earth orbit when they experienced memory upsets. The concentration near South America arises because the Van Allen radiation belts are closest to Earth in this region (courtesy of M.A. Shea, Phillips Laboratory). file:///S|/SSB/1swConsequences.htm (2 of 8) [6/25/2003 4:38:49 PM]

OCR for page 59
Space Weather: A Research Perspective The upper atmosphere becomes inflated if it is heated by extra energy sources such as auroral particles and enhanced resistive ionospheric currents. The resulting increased atmospheric densities at 300-500-kilometers altitudes significantly increase the number of microscopic collisions between the satellite and the surrounding gas particles. This increased "satellite drag" can alter an orbit enough that the satellite is temporarily "lost" to communications links. It also causes the premature decay of the orbit (and necessitating shuttle "boosts" for some, like the Hubble Space Telescope). The Hubble Space Telescope orbit is always decaying due to atmospheric drag. Increased atmospheric densities in low-Earth orbit from solar-cycle- enhanced solar ultraviolet emissions and geomagnetic disturbances hasten the decay for this and other satellites (courtesy of Students for the Exploration and Development of Space). Effects on Power Systems Electric power systems on the ground can be affected by the enhanced currents that flow in the magnetosphere-ionosphere system during geomagnetic disturbances. These currents cause magnetic field perturbations on the ground that in turn induce other currents in long transmission lines, especially those located at high latitudes. The slowly varying "DC" part of the currents can be large enough to cause overheating and damage to systems designed for AC. Disruption of power distribution systems can adversely affect many aspects of our daily lives, for example, should a blackout result. Map showing the sites of power blackouts and other malfunctions of power grids during the large magnetic storm of March 13, 1989 (courtesy of Rice University). Effects on Pipelines Space weather-induced currents similarly flow in long conductors on the ground such as oil pipelines. These currents create galvanic effects that lead to rapid corrosion at the pipeline joints if they are not properly grounded. Such corrosion file:///S|/SSB/1swConsequences.htm (3 of 8) [6/25/2003 4:38:49 PM]

OCR for page 59
Space Weather: A Research Perspective requires expensive repairs or can lead to permanent damage. Voltage fluctuations observed on a long electrical cable caused by changes in the magnetic field on Earth during a magnetic storm. These changes are caused by a combination of magnetospheric, ionospheric, and induced ground currents (courtesy of Lund Space Weather and AI Center, Lund University, Sweden). Effects on Communications Systems Shortwave radio communication at HF frequencies (3-30 megahertz), which is still extensively used by the military and for overseas broadcasting in various countries, depends upon the reflection of signals from Earth's ionosphere. These electromagnetic waves are attenuated as they pass through the lower ionosphere (below 100 km), where collisions between the electrons and air molecules are frequent. Ionosphere attenuation affects the usable radio communication frequencies. If it becomes especially strong due to an increase in the local electron density, it can cause a total communications blackout. Solar flare ultraviolet and x- ray bursts, solar energetic particles, or intense aurora can all bring on this condition. Solar energetic particle events produce a particular type of disturbance called Polar Cap Absorption (PCA) that lasts up to days. The deep ionization produced by the solar protons also alters the path taken by the waves reflecting from the ionosphere. Illustration of the communications effects of ionospheric changes caused by solar proton events, known as Polar Cap Absorption (PCA) events (courtesy of M.A. Shea, Phillips Laboratory). The ionospheric changes that occur during disturbed times also increase the incidence of electron density irregularities, leading to sometimes severe variations or scintillations in the phase strength of signals sent from the ground to satellites at VHF and UHF frequencies (30 megahertz to 3 gigahertz). file:///S|/SSB/1swConsequences.htm (4 of 8) [6/25/2003 4:38:49 PM]

OCR for page 59
Space Weather: A Research Perspective Prediction of the global pattern of ionospheric scintillation intensity from a model (courtesy of Northwest Research Associates). Finally, solar radio bursts can directly interfere with communications in the frequency range between 245 MHz and 2.7 GHz, which is widely used. In summary, space weather-related disruptions to communication systems have wide- ranging effects—from social interactions to economic transactions on a global level to intelligence and surveillance activities. Effects on Geomagnetic Surveys An off-shore drilling platform (courtesy of Greenpeace International). Geomagnetic surveys are important tools in the commercial exploration of natural resources. However, space weather-related perturbations can create signals in survey data that can be mistaken for signatures of subsurface resources. Survey schedules or operations must be modified, often suddenly and with significant cost impact, to avoid this contamination of the survey data. Effects on Navigation Systems Logo showing Coast Guard use of the GPS system (courtesy of U.S. Coast Guard). file:///S|/SSB/1swConsequences.htm (5 of 8) [6/25/2003 4:38:49 PM]

OCR for page 59
Space Weather: A Research Perspective The same disturbance-related changes in Earth's ionosphere that affect communications introduce changes in the time it takes signals to traverse the ionosphere. The abnormal time delays introduce position errors and decrease the accuracy and reliability of the Global Positioning System (GPS), which is used for many range-finding and navigational purposes. For example, phase scintillation in the ionosphere can defeat efforts on the part of surveyors who could otherwise use GPS to achieve distance measurements between separated receivers to centimeter accuracy. The changes in ionospheric attenuation and reflection of electromagnetic waves described above also affect the use of "over-the-horizon" HF radars used to detect and monitor aircraft and sea conditions. Ionospheric irregularities also produce noise or "clutter" in the radar signals. Hazards to Humans in Space View of Earth across the space shuttle Bay. An astronaut undertakes some extravehicular activity. The space shuttle program has heightened our awareness of radiation dangers to humans in space (courtesy of Goddard Space Flight Center). The principal space weather hazard to humans is radiation exposure to astronauts and passengers in high-altitude aircraft. Although the residual atmosphere above an aircraft provides a measure of protection from cosmic rays and solar energetic particles that enter the magnetosphere, there is still concern for flights on polar routes during major solar particle events. The primary means of reducing this hazard is to modify flight paths as necessary and to limit the flight time of personnel on high-altitude aircraft like the supersonic transport. It is clear that in this case early warnings of solar energetic particles are extremely desirable. While flares can be monitored at least on the visible disk of the Sun, solar indications of the shock- producing fast coronal mass ejections toward Earth are less apparent. Potential hazards from the high-altitude electrical discharges called sprites and jets are unknown. Since they seem to occur between the cloud tops at around 15-km file:///S|/SSB/1swConsequences.htm (6 of 8) [6/25/2003 4:38:49 PM]

OCR for page 59
Space Weather: A Research Perspective altitude and at the base of the ionosphere near 100-km altitude, interest in their effects will depend on the future use of this region of Earth-space. Astronaut radiation exposure is a major concern of our manned space flight program. Most manned missions occur in orbits that are below the regions where the Van Allen belt radiation is most intense. Extravehicular activities (EVAs) in the region of anomalously high radiation over the South Atlantic are also avoided. However, the MIR space station and the currently planned International Space Station (ISS) have orbits sufficiently inclined from the equator to bring them into the expanded auroral zones that occur during geomagnetically disturbed times. Ground track of the proposed International Space Station orbit. The South Atlantic anomaly region is circled (courtesy of Ron Turner, ANSER). The potential impact of excursions into these high-latitude regions on both the station itself and the planned construction activities requiring EVAs is being explored. It is known, for example, that the intensity of galactic cosmic rays that reach the atmosphere is about ten times higher inside the auroral zone than near the equator and that solar particles have increased access to this same region. The likelihood of a frequently disturbed magnetosphere and presence of solar energetic particles is considerable given the phasing of the ISS construction with the next solar maximum. One version of the International Space Station construction EVA schedule, compared to the expected phasing of the coming solar activity cycle (courtesy of Ron Turner, ANSER). For missions that leave low-Earth orbit, like the Apollo missions to the moon, the ability to rapidly traverse the radiation belts and to predict the occurrence of solar energetic particle events is essential. While envisioned manned modules for future missions to Mars are generally equipped with shielded astronaut shelters, adequate warning is necessary for these to be useful. file:///S|/SSB/1swConsequences.htm (7 of 8) [6/25/2003 4:38:49 PM]

OCR for page 59
Space Weather: A Research Perspective An astronaut on the lunar surface would be in danger of a lethal dose of radiation from solar energetic particles were a major coronal mass ejection to occur unnoticed (Figure courtesy of NASA Apollo 11 image archives). file:///S|/SSB/1swConsequences.htm (8 of 8) [6/25/2003 4:38:49 PM]

OCR for page 59
file:///S|/SSB/goes.gif file:///S|/SSB/goes.gif [6/25/2003 4:38:53 PM]

OCR for page 59
file:///S|/SSB/ps_seu_s.gif file:///S|/SSB/ps_seu_s.gif [6/25/2003 4:38:56 PM]

OCR for page 59
file:///S|/SSB/spweasto.jpg file:///S|/SSB/spweasto.jpg [6/25/2003 4:39:00 PM]

OCR for page 59
file:///S|/SSB/gic91nov.gif file:///S|/SSB/gic91nov.gif [6/25/2003 4:39:07 PM]

OCR for page 59
file:///S|/SSB/ps_pca_e.gif file:///S|/SSB/ps_pca_e.gif [6/25/2003 4:39:09 PM]

OCR for page 59
file:///S|/SSB/ssiorb.gif file:///S|/SSB/ssiorb.gif [6/25/2003 4:39:18 PM]

OCR for page 59
file:///S|/SSB/ssicon.gif file:///S|/SSB/ssicon.gif [6/25/2003 4:39:21 PM]