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Space Weather: Glossary Space Weather: A Research Perspective Glossary AC: Alternating-current, referring (as an adjective) to electric phenomena that oscillate regularly in time. active regions: Complex groups of sunspots that show signs of rapid evolution. Radio, ultraviolet, and x-radiation usually come from the vicinity of these regions. ALEXIS: A small x-ray astronomy satellite built by the Los Alamos National Laboratory. ANIK: A series of Canadian communications satellites. AU: Astronomical unit, equal to the mean distance between the Sun and the Earth (150,000,000 km). aurora: Light emitted from the polar upper atmosphere as energetic electrons bombard it from space. Aurora is another name for polar lights. The aurora borealis (northern lights) and aurora australis (southern lights) occur most frequently in the auroral oval. auroral oval or auroral zone: The approximately circular band in the northern or southern hemisphere where aurora are most intense at any given time. The near-midnight portion of the oval, where some of the brightest emissions occur, is located at about 65 degrees latitude. The mean diameter of the oval is roughly 4000 km, but it expands toward the equator during magnetically disturbed periods, when the aurora also becomes brighter, and it contracts poleward during magnetically quiet periods. charge neutrality: A condition that within a region of space the number of positive charges is equal to the number of electrons. file:///S|/SSB/1swGlossary.htm (1 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary corona: The tenuous outer atmosphere of the Sun whose structure is controlled by solar magnetic fields. The corona has a temperature between 1 and 3 million degrees. It merges into the solar wind at its upper boundary about 1-2 solar radii above the visible surface or photosphere. coronal holes: Regions of the Sun's corona where the density is lower than average, and the temperature and associated solar wind expansion velocity are higher than average. Their name reflects the fact that they appear dark in x-ray images of the corona due to their low density. coronal mass ejection (CME): A disturbance of the Sun's corona involving eruptions from the lower part of the corona and ejection of large quantities of matter into the solar wind. These ejecta sometimes have higher speed, density, and magnetic field strength than is typical for the solar wind. If their speeds relative to the background solar wind are high, they can produce shocks in the plasma that precede them as they move outward. coronal streamers: The dense and bright regions of the solar corona seen in eclipse photographs. These are shaped by the coronal magnetic field structure. cosmic rays: Energetic particles with very high energies. These may come from the Sun, from shocks in the interplanetary medium or near the edge of the heliosphere, and from other parts of the galaxy. DC: Direct-current, referring (as an adjective) to electric phenomena that do not oscillate in time. deep dielectric charging: The addition of electric charge deep within a dielectric component of an electronic system by the impact of an energetic charged particle. This charging can disrupt the electronic signals of the system. dielectric: A material that does not conduct electricity. file:///S|/SSB/1swGlossary.htm (2 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary dipole magnetic field: A particular magnetic field structure having "north" and "south" poles from which fields of opposite sign appear to emerge. Dynamics Explorer: A NASA spacecraft used to study Earth's upper atmosphere. electrical transient: An impulsive current or voltage increase in an electrical system. electromagnetic pulse: A strong pulse of electromagnetic radiation, generated by lightning or other effects, that can propagate like a radio wave throughout the atmosphere and space. electromagnetic radiation: Radiation carried by combined electric and magnetic fields that propagate at about the speed of light. Radio waves, infrared radiation, light, ultraviolet radiation, x- rays, and gamma rays are all forms of electromagnetic radiation. The basic element of electromagnetic radiation is the photon. electron: A negatively charged particle that when combined with a positively charged nucleus in numbers equal to its charge forms a neutral atom. electron volt: A measurement unit for energy, equal to the energy an electron (or proton) would gain when accelerated by an electric voltage of 1 volt. energetic particles: Electrons, ions, or atoms that have much higher energies than expected for the temperature of the gas from which they arise. In space physics, "energetic" usually means kilo- to giga-electron-volt energies. filaments: Structures seen on the solar photosphere in the H- alpha emission line of hydrogen. Filaments are most likely to occur in active regions and so are most common near solar maximum. gamma ray: Electromagnetic radiation with wavelengths less than 0.00001 micron. file:///S|/SSB/1swGlossary.htm (3 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary geomagnetic disturbance: Any type of rapidly varying perturbation to Earth's magnetic field caused by electric current flowing in the magnetosphere and ionosphere. geomagnetic tail: The long region extending from Earth, directed away from the Sun, filled by magnetic-field lines that connect to either the northern or southern polar region of the Earth. The geomagnetic tail is created by the action of the solar wind on Earth's magnetic field. geosynchronous orbit: A circular satellite orbit at the distance from Earth at which the orbital period of a satellite is 1 day, i.e., equal to the rotational period of Earth. Because a satellite in this orbit remains above the same point on the ground, it is the location of many communications and weather satellites. GGS: Global Geospace Science mission, includes the WIND solar wind spacecraft and the POLAR magnetospheric spacecraft that together allow studies of the solar wind-magnetosphere interaction. GGS is NASA's contribution to the ISTP. global electrical circuit: The electrical circuit composed of phenomena that generate and dissipate electrical currents in Earth's atmosphere. The global electrical circuit includes regions of thunderclouds, which tend to drive current upward and charge the ionosphere positively, as well as fair-weather regions where the positive charge leaks back to the ground. Electrodynamic processes in the ionosphere and magnetosphere also influence the circuit. Global Positioning System (GPS) satellites: A system of satellites orbiting Earth in a known configuration. By measuring the travel time for radio signals between the satellites and ground receivers, accurate information can be obtained on a user's location. The travel time between a GPS satellite and ground receiver is influenced by the ionosphere. GOES: A series of geosynchronous orbiting weather satellites operated by NOAA. file:///S|/SSB/1swGlossary.htm (4 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary GSFC: Goddard Space Flight Center, Greenbelt, Maryland. H-alpha: Light at 656.3 nm emitted or absorbed preferentially by hydrogen atoms. When viewed through a filter at this wavelength, the details of surface features on the Sun such as filaments and plages become more visible. heliosphere: The region of space dominated by matter from the Sun. It includes the Sun itself as well as the corona and solar wind, and extends beyond the orbit of Pluto to distances in excess of 50 AU. heliospheric current sheet: A surface in the solar wind where there is a flow of electrical current that separates regions of oppositely directed interplanetary magnetic field. The origin of the current is the stretching out of the solar magnetic dipole field by the expanding solar wind. HF: High-frequency, referring (as an adjective) to radio frequencies in the range of 3-30 megahertz. IMF: Interplanetary magnetic field (see definition below). IMP-8: One of the series of NASA's Interplanetary Monitoring Platforms. infrared: Electromagnetic radiation with wavelengths in the range of 1 to 100 microns (micron = millionth of a meter). injection: The sudden appearance in the near-Earth magnetosphere of greatly enhanced fluxes of energetic charged particles. Injections occur during magnetospheric substorms and appear to come from the magnetotail. interplanetary magnetic field: The magnetic field carried out from the Sun by the solar wind, which permeates the entire heliosphere. interplanetary space: A term referring to those portions of the file:///S|/SSB/1swGlossary.htm (5 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary heliosphere in the vicinity of the solar ecliptic plane (the approximate plane of the planetary orbits), inside the orbit of the outermost planet of the solar system, but excluding the regions near the planets where the solar wind is influenced by the planet or its magnetosphere. Often, the term "interplanetary space" is loosely used as a synonym for "heliosphere." ion: A positively charged particle produced when one or more electrons are stripped from an atom. ionization: The process by which one or more electrons are stripped from an atom or molecule, leaving a positively charged ion and free electrons. Ionization can be caused by the absorption of electromagnetic radiation or by a "collision" with a sufficiently energetic particle. ionosphere: The layer of Earth's upper atmosphere that is partially ionized by solar x-rays and ultraviolet radiation and energetic particles from space. ionospheric currents: Electrical currents produced in the ionosphere by the relative motions of the ions and electrons. ionospheric disturbances: Transient changes in ionospheric densities or currents, or the appearance of electron density irregularities, usually in association with the arrival of x-rays or ultraviolet bursts from solar flares or in association with aurora or geomagnetic activity. ISTP: International Solar-Terrestrial Program. An international collaboration to study the solar-terrestrial connection. It includes an armada of Earth-orbiting spacecraft from the United States, Russia, Europe, and Japan, as well as ground-based observations and theory and modeling efforts. jet, blue: A sporadic region of blue light that appears to move rapidly upward in the stratosphere above some thunderclouds. JHUAPL: Johns Hopkins University Applied Physics Laboratory. file:///S|/SSB/1swGlossary.htm (6 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary limb (solar): The edge of the Sun when viewed from a distance. magnetic field: A force field whose direction can be visualized with a bar magnet and iron filings. A magnetic field can be created by a "permanent" magnet or by electric currents. Charged particles can move freely along magnetic fields, but may circulate around them rather than crossing them. magnetic or geomagnetic storms: Periods when the magnetic field measured on Earth is highly disturbed, radiation environments intensify, auroras are produced, and electrical currents are enhanced in the ionosphere and induced in the ground. Magnetic storms are usually the magnetosphere's response to the passage of a coronal mass ejection. They generally last several hours to several days. magnetosphere: The region surrounding Earth or another planet where the magnetic field of that planet tends to exclude the solar wind. Inside the magnetosphere the planetary magnetic field has a strong influence on charged particles. magnetotail: The geomagnetic tail (see definition above). MeV: Mega-electron-volt, or 1 million electron volts of energy. molecule: A tightly bound combination of two or more atoms of any type. MSFC: Marshall Space Flight Center, Huntsville, Alabama. NASA: National Aeronautics and Space Administration. neutron: An uncharged subatomic particle, similar in size and mass to a proton. neutron monitor: Detectors operated on the ground to detect neutrons produced in collisions of energetic cosmic rays with the nuclei of atmospheric atoms. The neutron count rate is typically used as a measure of the intensity of galactic cosmic rays in Earth's neighborhood. file:///S|/SSB/1swGlossary.htm (7 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary NOAA: National Oceanic and Atmospheric Administration. NRL: Naval Research Laboratory. NSF: National Science Foundation. NSSDC: National Space Science Data Center. nuclear reactions: Solar energy is generated in the central region of the Sun by nuclear transformation of hydrogen to helium, called fusion. nucleus: The tightly bound protons and neutrons that form the core of an atom. ozone: A gas whose molecules are composed of three oxygen atoms. Ozone strongly absorbs ultraviolet radiation. ozone layer: The atmospheric region between about 10- and 50- km altitude where ozone molecules are most numerous, though their numbers are always much smaller than the numbers of other air molecules (mostly molecules of nitrogen and oxygen that have two atoms each). This layer approximately coincides with the stratosphere. It absorbs most of the solar ultraviolet radiation incident on the atmosphere, and thus protects plants and animals (including humans) from excessive ultraviolet radiation damage. particle precipitation: The release of charged particles stored in Earth's magnetosphere into the atmosphere. The particles follow magnetic field lines, and when they strike the atoms of the upper atmosphere, they cause them to glow like a giant television screen, creating the aurora (northern and southern lights). photon: A particle description of electromagnetic radiation, which can exhibit the behavior of either waves or particles. photosphere: The thin layer of the Sun from which its visible light is emitted into space. When we look at the Sun we see the photosphere as its surface. file:///S|/SSB/1swGlossary.htm (8 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary plages: Bright patches on the solar photosphere that are often associated with active regions and are primary emitters of solar ultraviolet radiation. plasma: Ionized gas in which some electrons have been separated from their atoms or molecules. Plasma is electrically conducting and so is affected by magnetic fields. A plasma can behave in many ways like a fluid with any weak magnetic fields appearing to be "frozen in" or moving with the "fluid." The solar wind is an example of this type of plasma. POLAR: A polar-orbiting spacecraft to observe the auroral zone particles, fields, and auroral emissions. One of NASA's contributions to the ISTP. polar cap: The region inside the auroral oval where magnetic field lines from Earth extend into the distant magnetotail and sometimes into interplanetary space. potential, electric: Another term for voltage. proton: A positively charged subatomic particle. Together with neutrons, protons make up the nuclei of atoms. A single proton forms the nucleus of a hydrogen atom. The solar wind is made up primarily of protons and electrons. radiation: A form of rapid energy transport, either by particles like fast-moving protons and electrons or by photons (electromagnetic radiation). radiation belt: The Van Allen radiation belt of Earth or other similar trapped energetic particle regions at other planets having magnetic fields. radiation dose: The amount of exposure to energetic electromagnetic or particle radiation. Radiation can cause microscopic damage by depositing energy deep in materials. radio waves: Electromagnetic radiation with wavelengths above about 100 microns (micron = millionth of a meter). file:///S|/SSB/1swGlossary.htm (9 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary range finding: The process of finding the distance (range) to some object, often using radio waves. relativistic electrons: Electrons with velocities approaching the speed of light. ring current: A current carried around Earth by charged particles trapped in Earth's magnetic field. The ring current intensifies when a magnetic storm or substorm occurs. SAMPEX: Solar and Magnetospheric Particle Explorer spacecraft. scintillation (radio): A fluctuation of the amplitude or phase of a radio signal caused by irregular structure of the medium through which it is propagating, like the ionosphere. secondary particle: An energetic particle, like an electron or proton, that is produced when a cosmic ray or other highly energetic particle collides with an atom. shock wave: A structure that stands in a medium (like the solar wind) ahead of an obstacle, when the obstacle speed is faster than the speed of waves in the medium. The "bow shock" stands in front of the magnetosphere, and interplanetary shocks precede fast coronal mass ejections as they travel from the Sun. At a shock, density and temperature increase, while velocity decreases to allow the medium to flow around the obstacle. shortwave: An adjective referring technically to radio waves shorter than 80 meters, corresponding to a frequency of 3.75 megahertz or more. The term is often loosely used to refer to HF frequencies. solar cycle: The regular increase and decrease in the level of solar activity, usually measured by the number of sunspots on the solar surface. The time between successive maxima or minima in the sunspot number is between 9.5 and 11 years. Periods of large sunspot number are called "solar maximum" periods, while periods of low sunspot number are called "solar minimum" periods. file:///S|/SSB/1swGlossary.htm (10 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary solar energetic particle event: A burst of energetic ions (typically protons) or electrons accelerated at a solar flare site or at the shock preceding a fast coronal mass ejection in the solar wind. solar flare: An abrupt release, from a localized region on the Sun, of large amounts of energy in ultraviolet light, x-radiation, and occasionally gamma radiation. Flares usually occur in or near complex sunspot regions and may be related to the rearrangement of the intense magnetic fields there. solar physics: The study of conditions and processes throughout the Sun and its atmosphere. solar radio burst: An impulsive intensification of radiation from the Sun at radio frequencies. Radio bursts are often used as indicators of solar disturbances such as flares and coronal mass ejections. solar wind: The ionized atmosphere above the solar surface that has such a high temperature it can overcome the Sun's gravity and expand outward at supersonic speeds. solar wind stream: High-speed solar wind originating in a coronal hole. spacecraft charging: Electrical charge obtained by a spacecraft immersed in a plasma. Different materials will be charged to different levels, making discharges on the spacecraft possible. Charging levels tend to increase with the energies of the particles making up the plasma. Also deep dielectric charging. space physics: The study of conditions and processes throughout space in the environment above the bulk of the atmosphere. Its domain includes the Sun, interplanetary medium, magnetosphere, upper atmosphere, and ionosphere. sprite: A weak reddish flash of light at heights of 60 to 90 km associated with thunderstorm electrical activity. file:///S|/SSB/1swGlossary.htm (11 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary stratosphere: The region of the atmosphere at altitudes of about 10 to 50 km, above the heights of usual meteorological phenomena. stream interaction regions: Regions of compressed solar wind plasma and magnetic field that form when a faster solar wind stream runs into a slower flow ahead of it. At heliospheric distances beyond the Earth's orbit, these compressions steepen into huge interplanetary shock waves spiraling away from the Sun. substorm: A sudden (tens of minutes) dynamic reconfiguration of the magnetosphere in which energy stored in the magnetic field is converted into charged-particle energy. The process involves enhanced auroral emissions, strong disturbances in the high- latitude magnetic field, and the sudden appearance of increased numbers of energetic particles in the magnetosphere. sudden ionospheric disturbance: A rapid increase in the electron density of the lower ionosphere produced by solar-flare x- rays that can sometimes cause, among other things, severe shortwave radiowave absorption. sunspot: A dark region on the solar surface where the magnetic field is so strong that the flow of energy from below is suppressed. Without this replenishing energy the sunspot cools below the average temperature and appears much darker than surrounding areas, although it would appear bright against a truly dark background. sunspot number: The number of sunspots on the visible solar surface is counted by many solar observatories and is averaged into a single standardized quantity called the sunspot number. This number has been determined from observations dating back to 1620. UHF: Ultrahigh frequency, referring here to radio frequencies in the range of 300 to 3000 megahertz. ultraviolet (UV): Electromagnetic radiation with wavelengths in the range of 0.01 to 0.4 microns (micron = millionth of a meter). file:///S|/SSB/1swGlossary.htm (12 of 13) [6/25/2003 4:40:03 PM]

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Space Weather: Glossary upper atmosphere: A term used with different meanings in different contexts. It can denote those portions of the atmosphere bounded from below at either 10, 50, 80, or 100 km and from above at anywhere from about 500 km out to the magnetopause. USAF: United States Air Force. Van Allen radiation belts: A donut-shaped region in Earth's magnetosphere that contains a high density of energetic charged particles trapped in the dipole field of the planet. VHF: Very high frequency, referring here to radio frequencies in the range of 30 to 300 megahertz. WIND: A spacecraft that measures the properties of the solar wind incident on the magnetosphere. One of NASA's contributions to the ISTP. x-ray: Electromagnetic radiation with wavelengths in the range of 0.00001 to 0.01 microns (micron = millionth of a meter). The National Academies Current Projects Publications Directories Search Site Map Feedback file:///S|/SSB/1swGlossary.htm (13 of 13) [6/25/2003 4:40:03 PM]