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• Absolute Instabilities:

  A class of plasma instabilities growing exponentially with time at a point in space, in contrast to convective instabilities (see entry).

• Absorption:

  In plasma physics, the loss of (electromagnetic) energy to a medium. For instance, an electromagnetic wave which propagates through a plasma will set the electrons into motion. If the electrons make collisions with other particles, they will absorb net energy from the wave.

• Absorption Coefficient:

  Measures the degree of wave absorption (see Absorption above); defined as the fraction of wave energy lost as the wave travels a unit distance.

• AE: (Sat.)

  See Atmospheric Explorer.

• Adiabatic:

  Not involving an exchange of heat between the system said to be adiabatic and the rest of the universe.

• Adiabatic Compression:

  Compression (of a gas, plasma, etc. not accompanied by gain or loss of heat from outside the system. For a plasma in a magnetic field, a compression slow enough that the magnetic moment (and other adiabatic invariants - see entry) of the plasma particles may be taken as constant.

• Adiabatic Invariant:

  Characteristic parameters which do not change as a physical system slowly evolves; the most commonly used one in plasma physics is the magnetic moment of a charged particle spiraling around a magnetic field line.

• Air Glow:

  Amorphous, optical radiation continuously emitted by a planetary atmosphere. It extends from the far ultraviolet into the near infrared. The Earth's air glow arises mainly from discrete atomic and molecular transitions and thus has predominantly an emission-line and an emission-band spectrum.

• Alfven, Hannes:

  Pioneering Swedish plasma physicist, and winner of the Nobel Prize in Physics; now deceased.

• Alfven Ion Cyclotron instability: (AIC)

  An electromagnetic microinstability near the ion cyclotron frequency; driven by the ion loss cone in a mirror device. (See relevant entries.)

• Alfven Velocity:

  It is the limit at which information can be carried in collisionless solar wind plasma. Phase velocity of the Alfven wave; equal to the speed of light divided by the square root of (1 plus the ratio of the plasma frequency to the cyclotron frequency for a given species). i.e.,
  Va = c / SQRT(1 + plasma freq. / cyclotron freq.)

  (As defined in Stix, _Waves in Plasmas_, 2nd ed. 1992, p. 31)

• Alfven Waves:

  Transverse electromagnetic waves that are propagated along lines of magnetic force in a plasma. The waves have frequency significantly less than the ion cyclotron frequency, and are characterized by the fact that the field lines oscillate (wiggle) with the plasma. The propagation velocity depends on the particle density and the strength of the magnetic field. "[Relatively] Low frequency ion oscillation in the presence of an equilibrium magnetic field. Also called the transverse hydromagnetic wave along Bo. The torsional Alfven wave in cylindrical geometry was first measured in liquid mercury by B. Lehnert. Alfven waves were first generated and detected in plasma by Allen, Baker, Pyle, and Wilcox in Berkeley and by Jephcott in England in 1959." (quoting from Chen's book; see bibliography) - Albert Chou

• Ambipolar Diffusion:

  Diffusion process in which buildup of spatial charge creates electric fields which cause electrons and ions to leave the plasma at the same rate. (Such electric fields are self-generated by the plasma and act to preserve charge neutrality.)

• Antisunward Convection:

• Atmospheric Explorer (AE): (Sat.)

  The Atmospheric Explorer series were three similar spacecraft which orbited the Earth from 1973 to 1981. AE-C was launched by Delta on Dec. 16, 1973 in a highly elliptical orbit of 160 km to 4300 km altitude, with an inclination of 68 degrees. After about a year its orbit was circularized and maintained within 200km to 400km throughout the rest of its lifetime. AE-C returned to Earth on Dec. 11, 1978. The ill-fated AE-D was usable from Oct. 6, 1975 until Jan. 29, 1976. AE-E lasted from Nov. 19, 1975 to June 10, 1981, with an altitude profile similar to AE-C, but at 19 degrees inclination. The three spacecraft were instrumented primarily for the analysis of the Earth's atmosphere and particles and fields in the magnetosphere.

  AE at NSSDC

• Atmospheric Radiation:

  Infrared radiation emitted by or being propagated through the atmosphere.

• Atmospheric Sounding:

  Measurement of atmospheric phenomena generally with instruments carried aloft by spacecraft, rockets, etc.

• Arecibo:

  Large radiotelescope facility located in Arecibo, Puerto Rico.

• Aureol (3): (Sat.)

• Aurora:

  The display of lights produced when energetic particles are precipitated into the planetary ionospheres. They are geographically confined to magnetic polar and sub-polar regions. This and also their sporadic occurrence is what distinguishes them from air glow. Different auroral colors come from the atmospheric emissions characteristic of the excited constituents. Auroras have been observed on Earth, Jupiter, Saturn, Mercury, and Uranus and the current understanding is that auroras of one from or another should occur with all magnetospheres. A typical aurora on Earth dissipates about 10^14 joules (10^21 ergs) of energy into the atmosphere and this occurs every few hours during a moderate level of solar activity.

• Auroral Blobs:

• Auroral Boundary:

• Auroral Electrojet:

  One of the major features of the high-latitude ionospheric current system. An intense westward flowing current stretching from the early morning sector to just beyond midnight within the auroral belt. The total current in the auroral electrojet is about 10^6 A and is driven by the electrical coupling between the high-latitude ionosphere and the magnetosphere. There's a second weaker, eastward flowing auroral electrojet extending from the evening sector to just before midnight. Eastward electrojet lies equatorward of the westward electrojet in the premidnight sector. The boundary between the two is called the Harang discontinuity.

• Auroral Oval:

  A ring shaped auroral display around the polar regions where hot plasma sheet particles move along the magnetic field lines and precipitate into the atmosphere.

• Auroral Zone:

  Roughly circular bands around either geomagnetic pole above which there is a maximum of auroral activity. The zones lie about 10deg. to 15 deg. of geomagnetic latitude from the geomagnetic poles.

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• Backscattering:

  Deflection of incident particle / radiation through an angle greater than 90 degrees relative to the original direction of motion/propagation.

• Barium Ion Cloud:

• Bartel's Rotation Number:

  The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartel to begin in January 1833.

• Bifurcation:

  The separation or branching into two parts, areas, aspects or connected segments, of anatomical systems or functions.

• Blob:

• Bohm diffusion:

  A rapid loss of plasma across magnetic field lines caused by microinstabilities. Theory formulated by the physicist David Bohm. From Chen's book (see references): "Semiempirical formula for the diffusion coefficient given by Bohm in 1946 (noted by Bohm, Burhop, and Massey, who were developing a magnetic arc for use in uranium isotope separation)." Bohm diffusion was proposed (not derived from first principles) to scale as 1/B rather than the 1/B^2 scaling predicted by classical diffusion. A 1/B scaling results from assuming that particles diffuse across field lines at an optimum rate (effective collision frequency=cyclotron frequency). The 1/B scaling is observed (approximately) in most reactors. (Acknowledgements to Philip Snyder) See also: diffusion, microinstabilities, field lines...

• Boundary Blob:

• Bubbles:

  See plasma bubbles

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• Charge Exchange:

  Phenomenon in which an ion colliding with a molecule (or an atom) neutralizes itself by capturing an electron from the molecule/atom, and transforming the molecule/atom into a positive radical/ion.

• Classical Diffusion:

  In plasma physics, diffusion due solely to scattering (collisions) of charged particles (with unlike charges) via electrical ("Coulomb") interactions. (See also diffusion.)

• Cleft:

• Collision Time:

  Typical time which passes between two consecutive collision events for a given particle. Inverse of the collision frequency; equal to the mean free path divided by the particle's velocity. In plasmas, the (Coulomb) collision time decreases with increasing density, and increases with increasing temperature.

• Conductivity:

  Degree to which a substance transmits (conducts) a given physical property, such as heat or electricity. See electrical conductivity, thermal conductivity.

• Convection:

  In general, mass motion within a fluid resulting in transport and mixing of the properties of that fluid. Specifically, in meteorology, atmospheric motions that are predominately vertical.

• Convective Instabilities:

• Corpuscular Radiation:

  Nonelectromagnetic radiation consisting of energetic charged or neutral particles. Used for penetrating particles.

• Cosmic Rays:

  The aggregate of extremely high energy subatomic particles which travel the solar system and bombard the earth from all directions. Cosmic ray primaries seem to be mostly protons, hydrogen nuclei, but also contain heavier nuclei. On colliding with atmospheric particles they produce many different kinds of lower energy secondary cosmic radiation. Used for cosmic radiation.

• Coulomb Collision:

  An interparticle collision where the Coulomb Force (electrical attraction and repulsion) is the governing force. Coulomb collisions have a number of interesting properties, but these are better described in textbooks. The interaction of the charged particles with each other's electric fields results in deflections of the particles away from their initial paths.

• Current Convective Instability:

• Cusp:

• Cyclotron Frequency:

  Number of times per second that a particle orbits a magnetic field line. The frequency is completely determined by the strength of the field and the particle's charge-to-mass ratio. (Often, and incorrectly, called the Larmor frequency. The cyclotron or gyrofrequency is twice the Larmor frequency of precession.)

• Cyclotron Radius:

  Radius of orbit of charged particle about a magnetic field line. Also called gyroradius, Larmor radius.

• Cyclotron Radiation:

  The electromagnetic radiation emitted by charged particles as they orbit in a magnetic field. The radiation arises from the centripetal acceleration of the particle as it moves in a circular orbit.

• Cyclotron Resonance:

  Charged particles in a magnetic field resonate with (and absorb energy from) an electric field (perpendicular to the magnetic field) which oscillates at the particles' cyclotron frequency, or at a harmonic (multiple) of that frequency.

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• DE: (Sat.)

  See Dynamics Explorer.

• Debye Length (radius,sphere):

  A measure of the shielding distance or thickness of the sheath under Debye Shielding.

• Debye Shielding:

  A fundamental characteristic of the behavior of a plasma is its ability to shield out electric potentials that are applied to it. Electric potential in the neighborhood of a positively charged particle in a plasma attracts electrons toward it while ions are repelled and its electrostatic field therefore becomes shielded from the plasma. This is an example of collective behavior of plasma and the net electrostatic field becomes confined to a short range.

• Defense Meteorological Satellite Program: (Sat.)

  The DMSP spacecraft are a series of vehicles built and launched since the 1960's under the direction of the Air Force Space Division for the Air Force Global Weather Central, Offutt AFB, NE as a service to the DoD community. The prime sensor for the spacecraft is the Operational Line Scan (OLS) system which makes high-resolution white-light and infra-red images of the earth's cloud systems. Other instruments which measure the troposhperic and space environment are included as payload capacity and operational needs allow.

  The DMSP spacecraft are launched into an 832 km, 98.7 degree inclination circular orbit. The non-spherical terms of the earth's gravity field cause the orbit plane to precess so that the orbit plane always has the same orientation with respect to the Earth-Sun line; i.e., the orbit is sun-synchronous. The DMSP spacecraft have a planned lifetime on orbit of three years. Two functioning DMSP spacecraft are supposed to be on orbit at all times: one in the 6:00-18:00 meridian plane and one in the 10:30-22:30 meridian plane (often referred to as a noon- midnight orbit.

  (CSS instruments are on board seven of the DMSP satellites, beginning with F8, in the form of the Special Sensor for Ions, Electrons, and Scintillation (SSIES). Each SSIES instrument contains an RPA, a drift meter, a scintillation meter, and a Langmuir probe, along with the associated electronics for these.)

• Diamagnetism:

• Diffuse Aurora:

  The aurora seen near the equatorward edge of the auroral oval and most of the daytime aurora is known as the diffuse aurora. It is caused by the precipitation of particles with a relatively broad spectrum of energies (eg. 100 eV to 100 keV). Since the diffuse aurora mirrors the hot plasma sheet, it is also characterised by the same flow pattern, which is roughly zonally westward before midnight and zonally eastward after midnight. Most of the energy results in strong ionization in the E-region, creating a broad shelf in the electron density profile. But the low energy particles also enhance the F-region density.

• Diffusion:

  The interpenetration of one substance into another as a result of thermal / random motion of the individual particles. (e.g., the diffusion of a plasma across a magnetic field as a result of collisions which cause particles to move onto new field lines.) See also classical diffusion, neoclassical diffusion, anomalous diffusion, transport.

• Discrete Aurora:

  Poleward of the diffuse aurora is the region of discrete auroral arcs produced by monoenergetic field-aligned particle beams which create a "bubbling" effect in the E-region at the end of the field lines along which they are traveling.

• Dispersion Relation:

  For a given wave, the dispersion relation relates the temporal frequency of a wave (w, or omega) to its wavenumber k and other physical quantities characteristic of the system. Dispersion relations can be quite simple (e.g., w = k * c for light; c being the speed of light), and they can also be quite complex, with interesting mathematical structure. The dispersion relation and its mathematical structure provide important information about the wave, including the phase and group velocities. (See relevant entries.) Note that the meaning of "dispersion relation" is different in plasma physics than in other fields.

• Dissociative Recombination:

  The combination of an electron with a positive molecular ion, followed by dissociation of the molecule in which the resulting atoms/molecules carry off the excess energy released in the recombination.

• Doppler Broadening:

  Frequency spreading which causes broadening of single-frequency radiation (e.g., spectral lines) when the radiating bodies (atoms, molecules, etc.) have different velocities. Radiation from each individual radiating body has a different Doppler shift, and the collection of radiations at different frequencies broadens the peak of the line in an intensity-vs-frequency plot.

• D-Region:

  A daytime layer of the earth's ionosphere approximately 50 to 90 km in altitude.

• Drift Motion:

  Ordinarily particles placed in a magnetic field will simply orbit in circles, but if the magnetic field is not uniform, or curves, or there is an electrical field perpendicular to the magnetic field, or another force is applied perpendicular to the magnetic field, then the "guiding centers" of the particle orbits will drift (generally perpendicular to the magnetic field and to the applied force). There are several sorts of drifts; refer to a plasma physics text for more information (see references). For a good introduction at the undergraduate physics level, see Chen.

• DMSP (F1-F10): (Sat.)

  See Defense Meteorological Satellite Program.

• Drift Velocity:

  Characteristic velocity at which the center of a particle's orbit ("guiding center") drifts when drift motion (see above) occurs.

• Drift Waves:

  Oscillations in a magnetically-confined plasma arising in the presence of density gradients (such as at the plasma's surface). These resemble the waves that propagate at the interface of two fluids with different density in a gravitational field.

• Dynamics Explorer (1-2): (Sat.)

  The Dynamics Explorer spacecraft were two similar satellites launched on 3 August, 1981. They were instrumented primarily for the analysis of particles and fields in the Earth's magnetosphere. DE-A also had an ultraviolet imager, aimed at Earth to study the airglow in the upper atmosphere, which captured the first global images of the aurora. The program was expected to last only three years, but further funding was obtained and DE-A lasted until 17 March 1991, when it was turned off because of a malfunction in the onboard computer.

  DE at NSSDC

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• ExB (Gradient Drift) Instability:

• Eiscat: (Rad.)

  The European Incoherent Scatter Radar system. Used for European Incoherent Scatter Radar.

• Electromagnetic Radiation:

  Energy propagated through space or through material media in the form of an advancing disturbance in electric and magnetic fields existing in space or in media. The term radiation, alone, is used commonly for this type of energy, although it actually has a broader meaning. Used for electromagnetic waves and wave radiation.

• Electron Runaway:

  High acceleration of electrons in a collisional plasma caused by a suddenly applied electric field (which greatly reduces the collision cross section of the electrons).

• E-Region:

  A daytime layer of the earth's ionosphere roughly between the altitudes of 85 and 140 km.

• Esro (4): (Sat.)

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• Fac:

  See Field Aligned Current.

• Field Aligned Current:

  Electric currents aligned along magnetic fields.

• Field Line Integrated Pedersen Conductivity:

• Forbush Decreases:

  The observed decreases in cosmic ray activity in the earth's atmosphere about a day after a solar flare. Used for Forbush effect.

• F-Region: (F1,F2)

  The upper layer of the ionosphere, approximately 120 to 1500 km in altitude. The F region is subdivided into the F1 and F2 regions. The F2 region is the most dense and peaks at altitudes between 200 and 600 km. The F1 region is a smaller peak in electron density, which forms at lower altitudes in the daytime.

• Freezing:

• foF2:

  The maximum ORDINARY MODE radiowave frequency capable of reflection from the F2 REGION of the ionosphere.

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• Geomagnetic Latitude:

  Angular distances from the geomagnetic equator, measured northward of southward through 90 degrees and labeled N or S to indicate the direction of measurement.

• Geomagnetism:

  The magnetic phenomena, collectively considered, exhibited by the earth and its atmosphere and by extension the magnetic phenomena in interplanetary space. The study of the magnetic field of the earth. Used for geomagnetic field and terrestrial magnetism.

• Global Positioning System:

  A satellite navigation system which will display many (up to 24) satellites in three sets of orbits by means of a precise time standard and three-dimensional information on position and velocity.

• GPS:

  See Global Positioning System.

• Gradient Drift Instability:

• Gradient Scale Length (L):

• Group Velocity:

  The velocity of a wave disturbance as a whole, i.e., of an entire group of component simple harmonic waves.

• Growth Rate (of Irregularities):

• Gyrofrequency:

  The natural period of revolution of a free electron in the Earth's magnetic field.

• Gyroradius:

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• Hall Effect:

  The electrical polarization of a horizontal conducting sheet of limited extent, when that sheet moves laterally through a magnetic field having a component vertical to the sheet. The Hall effect is important in determining the behavior of the electrical currents generated by winds in the lower atmosphere. Used for Hall coefficient and Hall currents

• Harang Discontinuity Region:

  The boundary between the eastward and westward auroral electrojets. (See. Auroral Electrojet)

• Heliosphere:

  The region around the sun whose plasma processes are dominated by solar wind.

• Hilat: (Sat.)

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

• IDM:

  See Ion Drift Meter

• IMF:

  See Interplanetary Magnetic Field.

• Incoherent Scattering:

  The phenomena of generating waves with random variations in phase, amplitude, polarization, and direction of propagation when an incident wave encounters matter.

• Incoherent Scatter Radar (ISR): (Sondrestrom, Chatanika)

  Radar used in the study of the ionosphere, thermosphere, etc.

• In Situ Measurement:

• Interchange Instability: (MFTI)

• Interplanetary Magnetic Field (IMF):

  Coronal magnetic field which is trapped in or ``Frozen-in'' the ever expanding solar wind. At Earth the IMF is about 5x10^(-5) gauss. This is about 10^(-4) times the Earth's surface magnetic field.

• Invariant Latitude:

• Ion Density (CONCENTRATION):

  In atmospheric electricity, the number of ions per unit volume of a given sample of air; more particularly, the number of ions of a given type (positive small ion, negative small ion, positive large ion, or negative large ion) per unit volume of air.

• Ion Drift Meter (IDM):

  IDM at NSSDC

• Ionization:

  The process by which electrons are lost from or transferred to neutral molecules or atoms to form positively or negatively charged particles. Used for electron ionization.

• Ionization Potential:

  The energy required to ionize an atom or molecule. The energy is usually given in terms of electron volts.

• Ionogram:

• Ionosonde:

• Ionosphere:

  The region of the earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoioniza- tion of the constituents of the atmosphere by solar ultraviolet radiation at very short wavelengths (l.t.1000 angstroms). The ionosphere significantly influences radiowave propagation of fre- quencies less than about 30 MHz.

• Irregularity:

• ISR: See Incoherent Scatter Radar.

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• Kelvin-Helmholtz Instability:

  A type of instability that arises when the different layers of a stratified heterogeneous fluid are in relative horizontal motion.The instability of the plane interface between the two fluids, when it occurs in this instance is called the Kelvin-Helmholtz Instability.

• Kinetic Theory:

  The derivation of the bulk properties of fluids from the properties of fluids from the properties of their constituent molecules, their motions and interactions.

• Kp index:

  A 3-hourly planetary geomagnetic index of activity generated in Gottingen, Germany, based on the K index from 12 or 13 stations distributed around the world.

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• Landau Damping:

  The damping of a space charge wave by electrons which move at the phase velocity of the wave and gain energy transferred from the wave.

• Langmuir Probe:

• Larmor Radius: (Larmour)

  For a charged particle moving transversely in a uniform magnetic field, the radius of curvature of the projection of its path on a plane perpendicular to the field.

• Loss Cone:

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• Magnetic Flux Tube:

• Magnetic L-Shells:

• Magnetic Mirror:

• Magnetic Storms:

  Worldwide disturbances of the earth's magnetic field. Used for geomagnetic storms and magnetic substorms.

• Magnetohydrodynamics: (MHD)

  The study of the interaction that exists between a magnetic field and an electrically conducting fluid. Used for geometrical hydromagnetics, hydromagnetics, hydromagnetism, and magnetogasdynamics.

• Magnetohydrodynamic Waves:

  Low frequency waves in an electrically highly conducting fluid (such as a plasma) permeated by static magnetic fields. The restoring forces of the waves are, in general, the combination of a magnetic tensile stress along the magnetic field lines and the comprehensive stress between the field lines and the fluid pressure. Used for Alfven waves, hydromagnetic waves, and plasma sound waves.

• Magnetosphere:

  The magnetic cavity surrounding the earth, carved out of the passing solar wind by virtue of the geomagnetic field, which prevents, or at least impedes, the direct entry of the solar wind plasma into the cavity.

• Mid-Latitude Plasma Trough:

  Nightside plasma depletion that can exceed and order of magnitude. Occurs on flux tubes which flow west at the same speed as the earth rotates. Plasma loss due to recombination is responsible for the observed depletion.

• Mobility:

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• Neutral Wind:

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• Particle Precipitation:

• Patch: (Ionization Patch)

  Solar-produced plasma that has been detached from the dayside subauroral ionosphere by a changing convection pattern and transported.

• Pedersen Conductivity:

• Phase Velocity:

  Of a traveling plane wave at a single frequency, the velocity of an equiphase surface along the wave normal.

• Photoionization:

  The ionization of an atom or molecule by the collision of a high energy photon with the particle.

• Pitch Angle:

• Plasma:

  Electrically conductive gases comprised of neutral particles, ionized particles, and free electrons but which, taken as a whole, are electrically neutral. Plasmas are further characterized by relatively large intermolecular distances, large amounts of energy stored in the internal energy levels of the particles, and the presence of plasma sheaths at all boundaries of the plasma. Plasmas are sometimes referred to as a fourth state of matter. Used for electrostatic plasma, ionized plasma, magnetoionic plasma, magnetoplasmas, and plasmoids.

• Plasma bubbles:

  Pockets of very low electron density in the equatorial F region of the ionosphere in which the plasma density is lower than the ambient density.

• Plasma Frequency:

• Plasma Sheaths:

  The boundary layers of charged particles between plasmas and their surrounding walls, electrodes, or other plasmas. Envelopes of ionized gases that surround bodies moving through an atmosphere at hypersonic velocities.

• Plasma Sheet:

• Plasma Structures:

  Multi-scale variations in plasma density due to electric fields, heating, mixing, precipitation, recombination and various instability mechanisms.

• Polar Cap:

  An area in the high latitude ionosphere where the magnetic field lines are connected to the interplanetary magnetic field.

• Polar Hole:

  High latitude winter phenomena which occurs when the plasma remains for a long time on flux tubes with no sunlight and no particle precipitation. Even a slow decay rate is sufficient to deplete the plasma density.

• Polar Showers:

• Pulsation Event:

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• Quasi-Neutral Plasma:

• Quiet Auroral Arcs:

  East-west aligned auroral arcs which are much smaller in latitudinal extent than in longitude.

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• Radar Backscatter:

• Radiation Belts:

  Envelopes of charged particles trapped in the magnetic field of a spatial body. Used for geomagnetically trapped particles and Van Allen radiation belts.

• Radiosondes:

  Instruments, usually balloonborne, for the simultaneous measurement and transmission of meteorological data while moving vertically through the atmosphere.

• Radio Waves:

  Waves produced by oscillation of an electric charge at a frequency useful for radio communication. Used for radio frequency radiation.

• Rayleigh-Taylor Instability:

  An instability that derives from the character of the equilibrium of an incompressible heavy fluid of variable density (i.e of a heterogeneous fluid). Of two fluids of different densities superposed one over the other; the instability of the plane interface between the two fluids, when it occurs is called the Rayleigh-Taylor Instability.

• Recombination Coefficients:

  A measure of the specific rate at which oppositely charged ions join to form neutral particles (a measure of ion recombination).

• Region 1-2 Currents:

• Retarding Potential Analyzer RPA):

  RPA at NSSDC

• RPA:

  See Retarding Potential Analyzer

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• San Marco D/L: (Sat.)

  The San Marco D/L satellite was the fourth in a series of international cooperative projects between Italy's Centro Ricerche Aerospaziale and NASA. It was launched on 25 March, 1988 into an equatorial orbit of about 300 by 600 km. The spacecraft made studies of the lower magnetosphere until 7 December, 1988, when it succumbed to aerodynamic drag and fell back to Earth.

• Scattering:

  The process by which small particles suspended in a medium of a different index of diffraction diffuse a portion of the incident radiation in all directions. In scattering, no energy transformation results, only a change in the spatial distribution of the radiation. Used for scatterers.

• Scintillation:

  Generic term for rapid variations in apparent position, brightness, or color of a distant luminous object viewed through the atmosphere. A flash of light produced in a phosphor by an ionizing event. On a radar display, a rapid apparent displacement of the target from its mean position.

• Shock Waves:

  Surfaces or sheets of discontinuity (i.e., abrupt changes in conditions) set up in a supersonic fields of flow, through which the fluids undergo a finite decrease in velocity accompanied by a marked increase in pressure, density, temperature, and entropy, as occurs, e.g., in supersonic flows about bodies. Used for bow shock waves.

• Slip Velocity:

  Plasma drift relative to the neutral gas.

• Soft-Particle Precipitation:

• Solar cycle:

  The approximately 11-year quasi-periodic variation in frequency or number of solar active events.

• Solar Maximum:

  The month(s) during the solar cycle when the 12-month mean of monthly average sunspot numbers reaches a maximum. The most recent solar maximum occurred in July 1989.

• Solar Minimum:

  The month(s) during the solar cycle when the 12-month mean of monthly average sunspot numbers reaches a minimum. the most recent minimum occurred in September 1986.

• Solar Terminator:

• Solar Wind:

  A proton electron gas that streams past the Earth with a mean velocity of 400-500 km/sec and a mean proton and electron density of about 5 cm^(-3)

• Sounding:

  Any penetration of the natural environment for scientific observation usually by sounding rockets or balloons. Used for sounders.

• Space Plasmas:

  Concentrations of free electrons and protons in the ionosphere, plasmasphere, and beyond.

• Spectral Index:

• Sporadic E:

  A phenomenon occurring in the e region of the ionosphere, which significantly affects HF radiowave propagation. Sporadic E can occur during daytime or nighttime and it varies markedly with latitude.

• Stirring:

• Striation(s):

• Subauroral Blob:

• Substorm:

• Sudden Ionospheric Disturbances: (SID)

  Complex combinations of sudden changes in the conditions of the ionosphere and the effects of these changes. Used for geomagnetic crotchets and SID (ionospheric disturbances).

• Sun-Aligned Arcs:

  Increased auroral activity in the polar cap region. A very long sun-aligned arc can form in the polar cap when an ordered multi cell flow pattern forms under IMF northward conditions. The circular auroral oval is linked from day to night by a sun-aligned auroral emission. The entire feature is referred to as theta aurora. The particle energies associated with these auroras are in the ``soft'' category (<100ev) and both the plasma production and auroral airglow emissions therefore occur at lower F-region or upper E-region altitudes. Other experiments also show that sun-aligned arcs seem to occur in a region of shear in the plasma. Data suggests that electron precipitation occurs in a region where the flow vorticity has a positive sign.

• Sunward Convection:

• Suprathermal Electron Bursts:

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• TEC:

  See Total Electron Content.

• Theta Aurora:

• Tongue of Ionization:

• Total Electron Content (TEC):

• Triad: (Sat.)

• Trough:

• Turbulence:

  A state of fluid flow in which the instantaneous velocities exhibit irregular and apparently random fluctuations so that in practice only statistical properties can be recognized and subjected to analysis.

• Turbulent Flow:

  Fluid motion in which random motions of parts of the fluid are superimposed upon a simple pattern of flow. All or nearly all fluid flow displays some degree of turbulence. The opposite is laminar flow.

• Two-Cell Convection Pattern:

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• Upper Atmosphere:

  The general term applied to the atmosphere above the troposphere. Used for upper air.

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• Velocity Shear:

• Very High Frequency (VHF):

  30 TO 300 MHZ

• Viscous Flow:

  The flow of a fluid through a duct under conditions such that the mean free path is very small in comparison with the smallest dimensions of a transverse section of the duct. This flow may be either laminar or turbulent.

• Viscous Interaction:

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• Whistlers:

  Radiofrequency electromagnetic signals generated by some lightning discharges.

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