Glossary for LEP Education and Outreach

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Absorption Nebula: A nebula characterized by lack of emitted light due to the absorption of light by interstellar dust or dense clouds of matter from within it.
ACE: A spacecraft called Advance Composition Explorer, was launched on September 25, 1997. Its main job is to measure the composition of the solar wind, that is, to measure the various types of its charged gas particles. Some examples of these are positively ionized helium, oxygen, carbon, silicon, and many others, as well as the lightest, hydrogen, which is by far the most abundant element from the Sun.
Achondrite: Stony meteorite without chondrules. Achondrites constitute about 4 percent of known meteorites and are similar in appearance to terrestrial igneous rocks low in silica content, such as basalts, peridotites, and pyroxenites.
Acquisition of Signal (AOS): When a spacecraft is on the other side of an object from the Earth, the signal cannot be heard on Earth. This is known as loss of signal. When the spacecraft returns out from behind the object (the moon or some other body), then its radio signals can once again be heard on Earth, which is called acquisition of signal.
Active Solar Period: The period of time when our Sun has increased solar activity such as sunspots, flares, and associated phenomenon.
Albedo: A ratio of the intensity of received sunlight to that of the reflected sunlight of a planet or other celestial body. Different planets will have a different albedo, due to many factors, and the albedo can change due to the incident angle of measurement. The albedo of Earth ranges between 0 and 1.0.
Alfvén Wave: A wave that occurs in an electrically charged gas in which the vibration of the magnetic field part of the wave is perpendicular to the direction of travel of the wave. These waves were named after the Swedish scientist, and Nobel Prize winner, Hannes Alfvén who used basic physics to explain them.
Altitude: The height above a planet's surface.
Ammeter: An instrument for measuring electric current, based on the galvanometer.
Ampere (Amp): A unit of electrical current equivalent to about 6 x 10¹⁸ electrons per second. Named after the scientist Andre Marie Ampere, who did studies with electricity and magnetism.
Ångstrom (Å): A unit of length used to measure very short distances. For example, it is used for the wavelength of light, especially in and around the visual range and at shorter wavelengths. It is equal to 1/10,000,000,000 meters (m).
Antenna: An electrical conductor (a wire or mesh of wires) used to send or receive radio signals. Large electrical power grids form antennas vulnerable to increased geomagnetic activity.
Antimatter: Matter that is composed of antiprotons, antielectrons (positrons), antineutrons and antineutrinos.
Aperature: The diameter of an opening; usually used to describe the diameter of the primary lens or mirror of a telescope or a camera.
Aphelion: This term means the point of a celestial body's orbit most distant from the Sun.
Apogee: The point in the orbit of an object orbiting Earth that is at the greatest distance from the center of Earth.
Asteroid: A large rocky body in space but small compared to a planet. Typically an asteroid is hundreds of miles across. Their orbits have an average distance from the Sun of about 2.7 AU's. The first three asteroids found were called Ceres, Juno and Vesta. Ceres is about the size of the State of Texas. These bodies lie in a belt of asteroids between Mars and Jupiter. See the table on the planets. NASA has a NEAR mission to an asteroid.
Asteroid Belt: An area between the orbits of Mars and Jupiter where thousands of asteroids orbit the Sun. Ceres, as big as the state of Texas, is the largest asteroid found in the asteroid belt to date.
Astronomer: One who studies the science of celestial bodies and their origins, magnitudes, motions, and compositions.
Astronomy: The study of celestial bodies and their origins, magnitudes, motions, and compositions.
Astronomical Unit (AU): The average distance of the Earth from the Sun. This distance is 150,000,000 km (or about 93 million miles).
Astrophysics: The study, or the physics, of the universe. The word astrophysics consists of the prefix astro meaning star in Greek and physics, the very broad study of matter and energy. Since stars cover all portions of the sky, they came to represent the entire "heavens," the universe. In astrophysics scientists study black holes, stars, galaxies, neutron stars and much more.
Atmosphere: The gas envelope that surrounds a body in space which is held to that body by gravity. The Earth and planets have atmospheres, and even the Sun has one that we call the corona. The Earth's atmosphere is mainly nitrogen and oxygen gas.
Atom: The smallest particle into which an element can be broken. It consists of a very small nucleus of positive electrical charge, at the center, and negatively charged electrons outside of it. See the figure for an example of the simplest atom.
Atomic Mass Unit (AMU): A unit of mass equal to 1/16-th the mass of the atom of oxygen of atomic mass number 16, meaning that there are 16 nucleons in the standard oxygen nucleus. A nucleon is either a proton or a neutron. In terms of energy (using E = mc^2) an AMU = 931 Mev = 1.49 X 10^(-3) ergs.
Attitude Control System (ACS): A system onboard a spacecraft for maintaining or changing the orientation of the spacecraft in relation to the Earth or other celestial body.
Attitude Stabilized: An attitude stabilized spacecraft is one that does not (usually) spin. Its three principal axes remain directionally fixed in space, even as the spacecraft itself moves. Occasionally such a spacecraft may be commanded from the ground to slowly spin (called rolling) for a limited time for purposes of calibrating the onboard instruments. These commands are sometimes programmed in the onboard computer beforehand, or both kind of commanding, from ground or programmed (i.e., automatic), on the same spacecraft are possible.
Aurora: Sometimes called the Northern Lights, which is colorful light coming from high up in the atmosphere in the Northern (and Southern) regions near Earth's poles. These occur during times of active space weather. See the figures.
Auroral Oval: An oval shaped region around each of the Earth's polar regions which sometimes contains colorful lights called the aurora. The oval can change size depending on the amount of space activity.

B Return to Main Index

Bar Magnet: A piece of magnetized material in the shape of a bar. It has magnetic field 'lines of force' that leave one end and go into the other end in a symmetric way. One end where the lines come out is called the north magnetic pole and the other end where the lines go in is called the south pole. See the figure.
Basaltic Achondrite: A type of meteorite consisting of the minerals feldspar and pyroxene; they are similar to basalts from lava flows on Earth.
Beta Rays: Energetic electrons emanating from the nuclei of of certain radioactive elements.
Bipolar: A structure that has two poles. For example, we refer to a bipolar magnetic field region on the Sun's surface when the observed (at a great distance in this case) magnetic field 'lines of force' appear to come out of the Sun's surface (the positive pole) at one place and go into the surface at another place (the negative pole). Similarly, a bar magnet has two poles, so it is bipolar. For contrast see Unipolar Region.
Birkeland Currents: Electric currents flowing along magnetic field lines and linking the Earth's ionosphere with more distant regions in the magnetosphere. These currents are often associated with the polar aurora and with geomagnetic storms and substorms. They were named after Kristian Birkeland, who first proposed such currents around 1900.
Bits per Second (BPS): Data is sent from a spacecraft to a computer on Earth or from computer to computer at specific speeds. These speeds are measured in how many bits of data are sent and received each second.
Boundary Layer: A transition layer between two neighboring regions of plasma in the magnetosphere. Some examples are the plasma sheet boundary layer, which is the transition from the plasma sheet to the tail lobes, and the low latitude boundary layer, just inside the magnetopause, which is the transition between the equatorial magnetosphere and the solar wind in the magnetosheath (which is solar wind modified in various ways by the bow shock).
Bow Shock: This huge shock wave in front of the magnetic field of the Earth results from the very fast solar wind from the Sun ramming into the Earth's magnetic field. It is called a 'bow' shock, because it reminds us of the bow wave that a boat makes in the water due to the motion of the boat in the water. See the figure. Other planets usually have bow shocks too, even if they do not all have magnetic fields. In Venus' case, a bow shock arises from the solar wind ramming into the planet's ionosphere.

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c: See Speed of Light.
Chromosphere: Region of the Sun's atmosphere just above the photosphere and below the corona characterized by a rapid rise in temperature with height from the photosphere's temperature of 7,000 degrees Kelvin to about 2 million degrees in the corona. It is approximately 4,000 km thick, so it is really a relatively 'thin layer' on the Sun (whose photosphere's radius is about 700,000 km). It has much fine structure in the form of bright patches called plages and long dark structures called filaments.
Calibration: Tuning a measuring instrument for accuracy before or after measuring. If the calibration is done after the measurement, the calibration information is used to correct the measurement.
Capacitance: The ratio of charge to potential of an electrical device (capacitor). It's the ability of the capacitor to store electrical charge as its potential rises.
Capacitor: A device to measure capacitance and usually consisting of conducting plates or foils separated by thin layers of dielectric (such as mica) with the plates on opposite sides of the dielectric layers. The plates are oppositely charged by a source of voltage and the electrical energy of the charged system stored in the polarized dielectric material.
Carbonaceous Chondrite: A primitive type of meteorite usually with chondrules; they contain water and carbon compounds including organic molecules.
Centrifugal Force: The force that tends to impel any body outward from a center of rotation or the force that an object moving along a circular path exerts on the body constraining the object and that acts outwardly away from the center of rotation. An example is a stone whirled on a string exerts centrifugal force on the string.
Chemistry: The study of the composition, structure, and other properties of substances and the transformations that they experience.
Chondrite: A stony meteorite containing chondrules embedded in a fine grained matrix of pyroxene, olivine, and metallic nickel-iron. Most chondrites are believed to originate in the asteroid belt.
Chondrule: A small rounded body of various materials, chiefly olivine or pyroxene, found embedded in a usually fragmental matrix in certain of the stony meteorites. As chondrites are the only undifferentiated meteorites (that is meteorites that have a solar-like composition that did not experience large scale melting like volcanic rocks) they likely represent the precursor material for planet building.
Coma (of a comet): The layer of gas and dust (100's of thousands of kilometers across) surrounding the comet nucleus.
Comet: A fragile body of ice and dust in orbit around the Sun. They are objects that move in the solar system, usually very far away from the Sun and planets, but which sometimes get close enough to the Sun to be seen as having long trailing comet tails. Comets are sometimes referred to as dirty snowballs, because the part of the comet that has most of the mass, the core, is believed to be mainly made of frozen water with dust and gasses buried inside. See the figure.
Comet Nucleus: A solid body of ice and dust a few kilometers in size.
Comet Tail: The part of a comet most easily recognized. Gas and dust from the comet nucleus is pushed into a long narrow appendage by the solar wind and the Sun's radiation pressure.
Command and Data Handling (C&DH): This is a method of collecting data by instruments on board a spacecraft, sending them to Earth (either in real-time or delayed), and after receiving commands from the Earth, decoding and distributing them to the appropriate spacecraft subsystems. This is usually handled by very complicated software installed on the spacecraft computers.
Conductor (Electrical): A material that conducts electrical current easily. Copper, silver, gold and aluminum are good conductors of electrical current. Other metals are usually good conductors also. The electrical current is the result of the motion of some of the electrons in the metal � the ones that are not too well tied to the metal.
Constellation: A group of stars that make a discernable shape, often named after mythological characters, people, animals and things that resemble the shape. Also, a group of spacecraft working together to accomplish a scientific task.
Convection: Rising warm air and falling cool air (as in the Earth's atmosphere). The transfer of energy by moving currents of fluid or gas containing energy (as in the Sun below the surface).
Core (of Earth or Moon): The central region of a planet or moon frequently made of different materials than the surrounding regions (mantle and crust); Earth and the Moon are thought to have cores of metallic iron and nickel.
Corona: The atmosphere of the Sun that can be observed out to many times the size of the Sun's main surface, which is called the photosphere. The corona is much hotter than the photosphere, but it has a much lower density. The gases in the corona are electrically charged. Warning: The Sun should be observed only with special equipment to protect the eyes.
Coronal Holes: When viewed in x-rays or extreme-ultraviolet light these are dark regions on the Sun's surface that appear like holes, but are really places where the gasses are at a lower temperature than the surrounding regions. In these "holes" magnetic field lines emanate outward or go inward, to or from very far away places in space (and are called open field lines), rather than having both ends attached to the Sun in loop-like structures.
Coronal Mass Ejections (CME's): Bundles or clouds of electrically charged gas ejected from near the Sun's surface on occasion and usually having duration of a few minutes to several hours. These mass ejections are believed to be due to magnetic forces in erupting prominences or solar flares on the Sun's surface.
Cosmic Rays: Very fast moving electrically charged particles believed to come mainly from our galaxy (and maybe even from farther away) and even occasionally from our Sun. Some of the cosmic rays are the fastest moving particles in nature. In fact, the fastest ones move at nearly c, the speed of light.
Coulomb's Law: A mathematical law that states the magnitude of the force between two charges at rest is proportional to the product of the two charges and inversely proportional to the square of the distance between them. The direction of the force is the vector from one charge to the other.
Crater: A hole or depression of which most are roughly circular or oval in outline. On Earth most natural craters visible at this point in geologic time are of volcanic origin. On the Moon most craters are of impact origin.
Crust: The outermost layer of a planet or moon, above the mantle.
Cusps: The regions in the magnetosphere of a planet, including the Earth, where the magnetic field 'lines of force' have a geometrical shape like a funnel, either coming out of the planet or going into it. See the figure.

D Return to Main Index

Data Processing Unit (DPU): This can be a software program or an entire department on Earth to ingest, process, and distribute data and the derived scientific products to domestic and foreign researchers. This is where the data is turned into a useful form for the scientists to study.
Deep Space Network (DSN): This is a network of interplanetary tracking stations in Goldstone, California, Madrid, Spain, and Canberra, Australia, which constantly observes spacecraft as the Earth rotates. This makes the DSN the largest and most sensitive scientific telecommunications system in the world. It is administered by NASA's Jet Propulsion Laboratory.
Density: The measure of the amount of mass per unit of volume of matter. It is sometimes expressed in grams per cubic centimeter (gm/cm³) or kilograms per cubic meter (kg/m³).
Dipole (of magnetic field): A very common structure of magnetic field lines in nature. See the figure. Many planets, including Earth, have almost dipolar field lines, at least in the near region the planet. The "di-" in dipolar refers to the two poles, north and south. The overall structure is symmetric about its axis. In Earth's case the directions of all of these field lines change over very long periods of time and in fact, the dipole structure has polarity (sign) changes so that its north and south poles change places on a time-scale of hundreds of thousands of years.
Disk: The seemingly flat figure of a celestial body (as in the solar disk).
Doppler Effect: The change in the observed frequency (or wavelength) of a wave, caused by the relative motion between the source and the point of observation.
Dynamo: A 'machine' (either made by man or by nature) that can transform mechanical into electrical and magnetic energy or vice versa. The Earth's magnetic field is believed to be the result of a dynamo acting in the center (or core) of the Earth where there are strong electrical currents, and where the driving mechanical energy is associated with the rotation of the Earth. Some other planets are believed to have dynamos which explain their magnetic fields.

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Earth: The third planet from the Sun and home to humans. The Earth contains literally thousands of species of flora and fauna in a constantly changing kaleidoscope of life. Earthly measurements form the basis of all astronomical data. See the table of the planets.
Earth Safe Mode: This is when a spacecraft is placed in a power safe configuration with an emergency mode downlink. The solar panels are pointed directly at the Sun, and the medium gain antenna is pointed towards the Earth. The underlying assumption in Earth Safe Mode is that the Guidance and Control subsystem retains knowledge of the location of the Earth and that the telecommunications subsystem is operational.
Eclipse: The complete or partial blocking of one celestial body by another. An example is when the moon travels between the Sun and the Earth, which is called the solar eclipse.
Ecliptic Plane: The plane in space in which the Earth orbits about the Sun. Most of the other planets in the solar system move in orbits that are very close to the ecliptic plane.
Ejecta (Solar): Material from the Sun which propagates out from the Sun and generates major interplanetary and magnetospheric effects.
Ejecta (Terrestrial): Material thrown out and deposited around an impact crater on Earth.
Electric Current: The net flow of electrical charges.
Electric Field: A region associated with a distribution of electric charge or a varying magnetic field in which electrical forces may result if that charge or field act upon other electric charges.
Electricity: The term used to describe the effects of electric charges, or the study of the effects of electric charges. For example, if the charges are stationary we refer to the study as static electricity, and unbalanced moving charges cause electric currents.
Electromagnet: A core of magnetic material surrounded by a coil of wire through which an electric current is passed to magnetize the core.
Electromagnetic Spectrum: The band of electromagnetic waves that covers those from the highest frequencies (gamma rays) to the lowest frequencies (long wave length radio waves). See the figure, showing the spectrum of electromagnetic waves split into the usual seven parts.
Electromagnetic Wave: Wave energy moving in the form of electric and magnetic 'lines of force' linked to each other. Light, x�rays , and radio waves are examples of such waves. All electromagnetic waves in space move at c, the speed of light, which is 3.0 x 10⁸ meters per second.
Electromotive Force (emf): Something that moves or tends to move electricity: the potential difference derived from an electrical source per unit quantity of electricity passing through the source (as a cell or generator).
Electron: The negatively charged particle in the portion of an atom outside the nucleus, the very dense central part. An electron's mass is about 1/2000 that of a proton's (or that of a neutron's) in the nucleus of an atom.
Electron-Volts (eV): This is the amount of energy needed to move an electron through a one volt electrical potential. So electron-volts (eV) are energy units in the study of electricity and magnetism.
Element: Atoms of one type which cannot be broken down further into a more fundamental type by chemical reactions. Some examples of elements are hydrogen, oxygen, nitrogen, sulfur, uranium and gold. There are now 104 different known elements; most are common in nature but a few (those with a lot of protons in the nucleus of the atoms) were developed by researchers using "atom smashing machines."
Ellipse (elliptical): An ellipse is a closed curve of oval shape. The orbits of the planets, comets, and meteorites often follow elliptical patterns.
Emission Nebula: A cloud of gas and dust which gives off light from its atoms being excited by massive hot stars within the nebula itself.
Energy: The ability to do work. There are many forms of energy: mechanical, electrical, magnetic, gravitational potential, heat, nuclear, chemical, kinetic, etc. Some of those just named are just different ways of saying the same thing. For example, chemical energy is essentially electromagnetic energy. Energy can change forms, but the total amount must stay the same for any closed region and for the universe as a whole, as far as we know. The usual units of energy are electron-volts, ergs, or joules.
Equator: The circle around a planet's surface (including Earth's) that is half�way between the geographic north and south poles. On a planet this circle is said to be at 0� latitude and the poles are at + 90� (north) or � 90� (south) latitudes.
Eros: Eros is a near-Earth asteroid with high concentrations of silicate minerals and metals. Eros has a prominent elongated ridge that extends for as much as 12 miles. Eros is about 21 X 8 X 8 miles in size.
ESA: ESA is the European Space Agency, formed in 1975, and consisting of 14 member states: Austria, Belgium, Denmark, Finland, France, Germany, Ireland, Italy, Norway, the Netherlands, Spain, Sweden, Switzerland and the United Kingdom. Canada sometimes participates in ESA missions under a cooperation agreement.
Excitation (of an atom): A process by which an atom or ion is raised to a higher energy state by moving one of its electrons to a higher energy, i.e., a higher orbit. When an atom (or ion) drops down to the lower energy a photon is emitted.
Extraterrestrial: That which is outside of the Earth or its atmosphere. When used in the name Laboratory for Extraterrestrial Physics, it more or less means Laboratory of Space Physics.

F Return to Main Index

Fast Auroral Snapshot (FAST): The Fast Auroral Snapshot (FAST) was launched on August 21, 1996 to study the detailed plasma physics of the Earth's auroral regions.
Field Lines (Electric or Magnetic): For an electric force these are lines along which the electric force operates on a charged particle. For a magnetic force these lines are perpendicular to the magnetic force operating on a moving charge (or current). Field lines can also be applied to the gravitational force, and in fact, field lines are a very general term which can be used in connection with other forces in physics.
Filament: Elongated, dark features observed against the disk of the Sun around imagined lines where the magnetic field appears to change sign; such lines are called inversion or neutral lines. Filaments correspond to bright prominences when seen on the limb of the Sun, and these are either quiet or active types. The material responsible for filaments is suspended in the corona and is cooler and denser than the very hot surrounding gasses making it appear dark.
Flare: See Solar Flare.
Fluoresce: The process of emitting visible light after a gas has absorbed ultraviolet light (or other short wavelength light).
Force: A push or pull which, if unbalanced, will cause a change of momentum of an object. Examples: electromagnetism, gravity, the strong nuclear force, and the weak force.
Frequencies: The number of cycles of a vibrating object (including wave cycles) per unit of time, usually the second. A common unit for measuring cycles per second is called the Hertz (Hz), after Heinrich Hertz (1857-1894), the scientist who is famous for his studies of electromagnetic waves.

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Galaxy: The large collection of stars in fairly tight gravitational connection with each other. Our galaxy, the Milky Way, contains about 200 hundred billion stars (200,000,000,000). Our galaxy is so large that it takes light 90,000 years to go from one edge to the opposite edge. Some galaxies have beautiful spiral patterns (see the figure) and others are shaped like footballs or are quite irregular. Some have a shape made up of a combination of a spiral and a central bar.
Galvanometer: An instrument for measuring a small electric current by movements of a magnetic needle or of a coil in a magnetic field.
Gamma Rays: These are very high frequency electromagnetic waves with very small wavelengths, i.e., near 1 cm/10,000,000,000,000 (1 cm/10 trillion). They usually come from the nucleus of an atom or from solar flares of extremely high energies.
Generator: A machine by which mechanical energy is changed into electrical energy.
Geocentric Solar Ecliptic Coordinates (GSE): A 3�axis (x,y,z) coordinate system (see graphic), with the Earth at its center, and where the x�axis points to the Sun, the z�axis is perpendicular to the ecliptic plane, and the y�axis is perpendicular to both the x and z axes with a positive direction opposite to the Earth's motion around the Sun.
Geocentric Magnetospheric Coordinates (GSM): This is similar to the Geocentric Solar Ecliptic Coordinate (GSE) system, except the y and z axes rock around the x�axis every day, because they depend on the direction of the Earth's magnetic dipole, which is always moving on the surface of an imaginary cone as the Earth rotates. This coordinate system is useful for scientists who study the Earth's magnetosphere.
Geochemist: A scientist who studies the chemical composition of, and chemical changes in, the Earth or other planetary bodies.
Geocorona: The outermost low density layer of the Earth's neutral atmosphere appearing as a huge cloud of hydrogen surrounding our planet. Although its density diminishes with distance, the geocorona has been observed to distances as far as 5 or 6 Earth radii.
Geographic Indices: Parameters used for providing a global picture of the degree of geomagnetic disturbance level. These are based on magnetic field data at the Earth's surface.
Geographic Poles: These are points on the Earth's surface in the North and South that mark where the rotation axis of the planet is located.
Geologist: A scientist who studies Earth, its materials, the physical and chemical changes that occur on the surface and in the interior, and the history of the planet and its life forms. Planetary geologists extend their studies to the Moon, planets, and other solid bodies in the solar system.
Geomagnetic Activity: Whenever there are changes in the Earth's magnetosphere brought on by electric and magnetic forces caused by the solar wind. Usually this means that a magnetic storm or magnetic substorm is occurring.
Geomagnetic Field: The Earth's magnetic field measured close to its surface. This is mainly the result of a dynamo that operates in the core of the Earth. This field is approximately dipolar in structure. The solar wind and its magnetic field ramming into the geomagnetic field can cause small changes in its strength and direction.
Geophysics: The science of the composition, structure, and processes of the Earth's interior. Extensive experimental and modeling studies of the Earth and other planetary bodies with respect to their structure and development.
Geospace: The space surrounding Earth.
Geosynchronous Orbit: A satellite in this orbit will always stay above some specific spot on the Earth's equator. To satisfy the laws of physics it must stay at a distance of about 6.6 Re (or about 42,000 km) from Earth's center and move at about 3 kilometers in one second.
GEOTAIL: A spacecraft built by the Japanese Space Agency, called ISAS, launched by the U.S on July 24, 1992, to study the Earth's magnetotail and other parts of space in the ISTP program (International Solar Terrestrial Physics Program).
Goddard Space Flight Center (GSFC): One of several National Aeronautics and Space Administration centers around the world. Goddard is located in Greenbelt, Maryland, and encompasses directorates in Earth Science, Space Science (including the Laboratory for Extraterrestrial Physics), Applied Engineering, Flight Missions, and Suborbital Flights.
Granulation: Numerous little bright areas surrounded by darker regions on the Sun.
Gravity: A fundamental physical force of attraction between particles, between aggregations of matter (as stars and planets), and between particles (such as electrons, protons, etc.) and aggregations of matter. The strength of the gravity depends on the mass of the two objects and their distance from one another.
Guiding center: A charged particle in a magnetic field is constrained to move in circular motion around a local ("guiding") magnetic field line, while the "center" of this circular motion slides up or down along the field line, slowly drifting from one guiding field line to its neighbor. The center of that circle is known as the particle's guiding center, and this kind of motion is called guiding center motion.
Gyration: A term used in physics for the circular motion of a charged particle around its guiding center.

H Return to Main Index

Heliocentric: Referred to or measured from the Sun's center or appearing as if seen from the Sun's center.
Heliographic Coordinates: A graphical coordinate system in which a feature on the Sun is given in terms of the two angles, latitude (measured up and down from the Sun's equator) and longitude (measured around in the equator plane).
Heliopause: The boundary of the heliosphere.
Heliosphere: The huge region around the Sun (possibly reaching to over 100 AU) that encloses the solar wind and its magnetic field. Its boundary is where the solar wind meets the interstellar gas. This boundary is called the heliopause. The heliosphere is believed to have an elongated shape, because of its motion through the interstellar medium. See the figure.
Heliospheric Current Sheet: A relatively thin electrical current layer that separates the Sun's extended magnetic field lines according to their polarity, away from the Sun on one side and toward the Sun on the other side. It is one of the largest structures in the heliosphere. See the figure.
Heliospheric Plasma Sheet: An enhanced density at and around the heliospheric current sheet compared to the usual density of the solar wind. See the figure (by Karen Lepping) that shows a rendition of this structure relatively close to the Sun, which is the light yellow ball at the center. The red represents denser material and the yellow is for the lower densities of the regular wind further from the current sheet. This particular version shown is for the case when there are four magnetic field sectors.
Heterogeneous: The characteristic of having or containing many different parts within a single entity; being composed of several different types of material.
High Gain Antenna (HGA): A spacecraft's high gain antenna is used for direct communications with Earth. It has a narrow beam width and is therefore very directional. On Earth high gain antennas are best used in flat areas like the mid-west where there are no mountains to interfere with the signal. If a spacecraft is not correctly aligned with Earth for transmission and reception of radio waves, the low gain antenna (which has a much broader but weaker beam) can be used to send commands to reorient the spacecraft.
High Speed Stream: A plasma stream within the solar wind believed to come from a coronal hole on the Sun and having a speed up to about 800 km/s, and sometimes even higher.
Homogeneous: Composed of one type of material, of uniform structure or composition throughout.

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Igneous: Rocks or processes involving the formation and solidification of hot, molten magma on the surface of a planetary body or below the surface.
IKI: As the leading organization of the Russian Academy of Sciences in the field of investigations of Outer Space, Solar System planets and other objects of the Universe, Space Research Institute (IKI) is primary in charge of long-range planning and elaboration of space research programs of which a considerable part is performed within the framework of international space research cooperation.
IMAGE: The Imager for Magnetopause to Aurora Global Exploration (IMAGE) is scheduled for launch February 15, 2000. It will use neutral atom, ultraviolet, and radio imaging techniques to identify the dominant mechanisms for injecting plasma into the magnetosphere on substorm and magnetic storm time scales; determine the directly driven response of the magnetosphere to solar wind changes; and, discover how and where magnetospheric plasmas are energized, transported, and subsequently lost during substorms and magnetic storms.
IMP 8: The eighth of a series of NASA IMP spacecraft launched in October 1973, in order to study the space around the Earth out to about the distance of the Moon, which includes the Earth's magnetosphere, and the near�by solar wind. The spacecraft stays in an oval orbit at a distance of about 35 times the radius of the Earth on average. IMP means Interplanetary Monitoring Platform. IMP 8 is also referred to as Explorer 50.
Inclination: The angular deviation of a line from a plane, measured in a plane perpendicular to the first plane.
Infra�red radiation: Electromagnetic radiation (or "light waves") with wave lengths just a little longer than visual light. The wavelength range for infrared radiation is 7/10,000 millimeters to 1/10 centimeters (cm). Infra�red radiation is sometimes referred to as heat radiation.
Interplanetary Dust Particles: Microscopic bits of asteroids or comets, these particles can be collected from high in the atmosphere.
Insulator: A material of very low electrical conductivity, which means that electrical current does not travel through it, or not very easily. Glass and rubber are good examples of electrical insulators. Copper is a very bad example. In fact, copper is a good conductor of electricity.
Interferometry: The location and examination of sources of electromagnetic waves through the simultaneous use of two or more separated telescopes. Similarly for sound waves, using receivers.
International Astronomical Union (IAU): The IAU was founded in 1919 to promote and safeguard the science of astronomy in all its aspects through international cooperation.
International Geophysical Year(IGY): A year covering parts of years 1957 and 1958 was designated the IGY. It was emphasized above other years by scientific leaders, because of the active condition of the Sun at the time. It really marked the beginning of major cooperation in the Space Age among space and Earth scientists.
International Solar Terrestrial Physics (ISTP) Program: Part of a big worldwide program of scientific space studies. This program involves the major space institutes from the United States (NASA), Europe, Japan, and Russia, as well as the participation of many other countries, such as Canada. It incorporates spacecraft making measurements in the solar wind and in the Earth's magnetosphere, a spacecraft observing the Sun, observatories on Earth and scientists who do theory.
Interplanetary Magnetic Field (IMF): The magnetic field that exists in the solar wind which carries the field away from the Sun. It is commonly restricted to mean the solar wind that lies outside of the bow shocks of the planets.
Interplanetary space: The space between the planets. It usually means the solar wind.
Interstellar Gas: Gas in the interstellar region, that is, in the region that is outside of the influence of the Sun's wind. It still is influenced by the Sun's gravitational pull if it is close enough. Interstellar means between the stars.
Ion: An atom (or molecule) that has lost or gained one or more electrons and therefore is electrically charged. In space physics it is most common that an electron is lost, not gained.
Ionization: The process that forms an ion out of an atom or molecule. In space physics this means that one or more electrons are pulled away from an atom by high frequency light or from some particle colliding with the atom.
Ionosphere: This is the part of a planet's atmosphere where its gas is at least partially electrically charged. We refer to this charged gas as being "ionized." In the Earth's case the ionosphere occurs at an altitude of about 90 km (50 miles). This electrical charging of an otherwise "neutral" atmospheric gas is the result of the Sun's ultraviolet light bombarding the atmospheric gas, which is mainly oxygen and nitrogen particles (called molecules) in the Earth's case.
ISAS: ISAS - The Japanese Institute of Space and Astronautical Science, established on April 14, 1981. It was the outgrowth of the Institute of Space and Aeronautical Science of the University of Tokyo.
Isotope: An element having an identical number of protons in its nucleus as another but differing in the number of neutrons.

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Jet Propulsion Laboratory (JPL): One of several National Aeronautics and Space Administration centers around the world. The Jet Propulsion Laboratory is located in Pasadena, California. It is responsible for many deep space missions, such as Mariner 10, Voyagers 1 & 2 etc.
Joule: A unit of work or energy equal to the work done by a force of one Newton acting through a distance of one meter. Named after James Prescott Joule (1818-1889), an English physicist who formulated a law describing the rate at which heat is produced by an electric current.
Jupiter: The largest planet in the solar system. It is one of the four so�called giant outer planets and is about 5 times as far from the Sun as Earth is. It has a diameter which is about 11 times Earth's and, traveling in a circle, it has a year that is about 12 Earth�years. It is a giant gas ball without a solid surface, and is distinguished by a big red spot just below its equator. See the table of the planets.

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Kelvin (K): A unit of measurement named after Lord Kelvin (1824-1907), a Scottish physicist and mathematician who calculated that molecular motion stops at -273 deg C. He called this temperature absolute zero, the lowest possible temperature.
Kilogram (kg): The mass of a thousand grams, since the prefix "kilo" means a thousand. A kilogram is equal to 2.205 pounds (on the Earth's surface), or a better way of saying it: A kilogram is the mass that weighs 2.205 pounds (lbs).
Kilometer: A thousand meters, and a meter is about 39.4 inches.
Kinetic Energy: The energy of a body or mass in motion. The kinetic energy increases with greater speed and mass.
Kuiper Belt: A disk shaped region in which comets have been stored since their origin. Scientists believe this region lies in the plane of the Solar system with its inner edge near 30 A.U. and its outer edge extending to 1000 A.U. It is thought that comets with the shorter periods come from this region of space.

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Lagrangian Point: The Lagrangian point between the Sun and the Earth is a point where the pull of the Sun's gravity is equal to the pull of the Earth's gravity. A spacecraft can have an orbit around this point, as if the point were a planet, provided that some spacecraft "fuel" (gas shot out of the spacecraft) occasionally is used to keep it from slowly drifting away from the orbit. The Lagrangian point is about 235 times the Earth's radius away from the Earth, toward the Sun; this is 1,500,000 km away or almost a million miles away.
Latitude: The angular distance North or South from the Earth's equator measured in degrees with the equator being 0^o and the poles 90^o N and 90^o S.
Lava: Fluid magma that flows onto the surface of a planet or moon, erupted from a volcano or fissure. The rock formed by solidification of magma.
LIght Detection and Ranging (LIDAR): The optical analogue of RADAR (RAdio Detection and Ranging). That is, lidar is the same as radar, except the energy source, a laser, is in the optical part of the electromagnetic spectrum. LIDARs are active remote sensors since they include the light source on which the measurement depends.
Light-Year: The distance that light travels in one year, which is 9.5 X 10¹⁵ meters or 9,500,000,000,000,000 meters.
Limb: The outer edge of the apparent disk of a celestial body, such as the limb of the Sun.
Lines of Magnetic Field: These lines, sometimes called lines of magnetic force, really represent a way of thinking about magnetic force. However, they are not really force; they are part of the force. A magnetic force requires two things: the magnetic field (represented by the lines in space) and a moving electrical charge or electrical current. The force is perpendicular to both the field lines and the direction of the moving electrical charges.
Lithosphere: The rocky crust of a planet.
Loss of Signal (LOS): When a spacecraft is on the other side of an object from the Earth, the signal cannot be heard on Earth. This is known as loss of signal. When the spacecraft returns out from behind the object (the moon or some other body), then its radio signals can once again be heard on Earth, which is called acquisition of signal.
Longitude: The angular distance East or West between the meridian of a particular place on Earth and that of Greenwich, England, expressed in degrees or time.
Low Gain Antenna (LGA): A low gain antenna has a broad radiation beam and is good for use in mountainous areas here on Earth, and as a backup system on spacecraft. Because the beam area is so large, obstacles in the way are surrounded much like a stream will divide around a rock in its path. Low gain antennas are often used in spacecraft as backups since the high gain antennas are so dependent on a narrow, uninterrupted path to the receiver.

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Magma: The term applied to molten rock in the interior of a planet or moon. When it reaches the surface, magma is called lava.
Magnetic Cloud: A large structure (about 1/4 AU across) observed in the solar wind characterized by a relatively strong magnetic field, a slow, but large change of the magnetic field's direction, and a low temperature of protons inside. Magnetic clouds are seen at Earth's orbit and even much further out in the solar wind. See the figure.
Magnetic Field Sectors: Regions in the solar wind in which the magnetic field either leaves the or enters the Sun in a spiral pattern on average. The boundaries separating these regions coincide with the heliospheric current sheet. Typically there are two, four, or six sectors for any single rotation of the Sun.
Magnetic Field Investigation (MFI): Magnetic Field Investigation (MFI), which is the research program associated with the magnetic field data obtained from the magnetometers on-board the WIND spacecraft. This includes the development of the magnetometer hardware, its testing, the processing and distribution of data of various types, and finally the production of science products, along with science collaborations.
Magnetic Fields: Imagined "lines of force" that arise from the motion of electrically charged particles, either as familiar electrical currents or, on a microscopic scale, as tiny currents working together in magnetized materials. Magnetic fields are very common in space.
Magnetic Reconnection: When magnetic field lines merge with each other and give off energy from the lost field, we say the field "reconnected," and this is the process of magnetic reconnection. The energy that results goes into heating up of the surrounding gas and into the streaming of some particles of that gas. This process is known to occur sometimes at the magnetopause of Earth, especially the front part, because of the solar wind ramming into the Earth's magnetosphere.
Magnetic Storm: Similar to a Magnetic Substorm but is much longer lasting and has major changes in the intensity of particles in the Van Allen radiation belts. The changes in the Earth's field strength at the surface of planet are stronger than for a substorm. Also the beautiful aurora that we see during a magnetic storm is usually seen at lower latitudes on the Earth than for a substorm, that is, further from the Earth's poles.
Magnetic Substorm: A series of magnetic disturbances on a planet causing auroras (in Earth's case) and rapid changes in the planet's field. These are caused by changes in the solar wind's speed and its magnetic field, especially the field's direction. This solar wind effect occurs through a process called magnetic reconnection at the front of the Earth's magnetosphere. As well as Earth, Mercury is believed to have substorms based on observations from the Mariner 10 spacecraft made in 1973 and 1974.
Magnetograph: A graph of curves of the components of the vector magnetic field vs. time. These are generated at various stations around the Earth and from data from spacecraft, in order to help to understand the variations in the magnetic field.
Magnetohydrodynamics(MHD): The study of the average motion, density, temperature, etc. of charged particles (plasma) that work together like a fluid which remains electrically neutral as a whole. For example, an MHD wave is a hybrid wave acting partly like a sound wave but also according to the influence of electric and magnetic forces.
Magnetometer: An instrument that measures the strength of the magnetic field. Most magnetometers can measure both the strength and direction of the magnetic field.
Magnetopause: The boundary of the magnetosphere.
Magnetosphere: A region around a magnetic planet formed by the motion of the solar wind ramming into that magnetic field. It can be thought of as a cavity in the solar wind caused by the presence of the planet's field. At the boundary of the magnetosphere, called the magnetopause, the inside pressure of the magnetosphere itself and the outside pressure of the solar wind are equal when the boundary is steady. The magnetosphere has a very elongated magnetotail. See the figure.
Magnetotail: The elongated region of a planet's magnetosphere extending away from the Sun, caused by the solar wind flow stretching the magnetosphere. The magnetotail's axis is approximately aligned with the solar wind flow direction. The Earth's magnetotail may be 1,000 or more times as long as the radius of the Earth under normal conditions. See the figure.
Magnitude (for a star): The intensity of the visual light produced by a star. Ancient astronomers described the 20 brightest stars as of the first magnitude. Hipparchus in Greece extended the scale by dividing the stars into five magnitudes. Magnitudes were estimates without quantitative definition, but today there is a logarithmic scale which classifies magnitude precisely.
Mantle: A mostly solid layer of Earth lying beneath the crust and above the core, consisting mostly of iron and silicate materials.
Mars: The fourth planet from the Sun, Mars is sometimes called the red planet, because of its reddish appearance as seen in a telescope. Its diameter is about one half Earth's, and its year is almost 2 Earth�years. Mars has two moons. See the table of the planets.
Mass: The property of a body that is a measure of its inertia and that is commonly taken as a measure of the amount of material it contains and causes it to have weight in a gravitational field.
Maunder Minimum: The period between the years 1645 and 1715 when there was a marked decrease of sunspots compared to what would be expected based on earlier and later periods.
Mercury: The planet closest to the Sun. It is a small planet, its diameter is approximately 1/3 Earth's, and its year is only 88 Earth�days. A planet closer to the Sun like Mercury must travel faster than one farther away, in order to stay in orbit. The planet's orbit has an average radius that is about 1/3 of an AU, which is the average distance from the Sun to the Earth. See the table of the planets.
Medium Gain Antenna (MGA): Antennas can not add power to a signal (amplify the signal) they can only direct the output and increase the effective radiated power (ERP). This is why they are called "gain" antennas. A high gain antenna has a very directional narrow beam area of signal, whereas a low gain antenna has a very large "balloon" shaped area of signal. A medium gain antenna can also serve the purposes of the low gain but is more directional.
Metal: Any of various opaque, fusible, ductile, and typically lustrous substances that are good conductors of electricity and heat, especially one that is a chemical element as distinguished from an alloy.
Metamorphic: Rocks that have recrystallized in a solid state as a result of changes in temperature, pressure, and chemical environment.
Meteor: A relatively small body of matter traveling through interplanetary space. Usually these are directly observable only by their incandescence from frictional heating on entry into the atmosphere.
Meteorite: A metallic or stony body that has fallen on the Earth, Moon or other planetary body from outer space.
Meteor Shower: A group of meteors or comet dust, traveling together, which enter the atmosphere within a few hours.
Meteorite Shower: A large number of similar meteorites falling together, caused by the breakup of a large meteorite in the atmosphere.
Micrometeorite: Meteorites smaller than 1 mm in diameter.
Milky Way: A gravitationally bound collection of roughly a hundred billion stars. Our Sun is one of these stars and is located roughly 27 thousand light years (or 8.2 kiloparsecs) from the center of the Milky Way, or two-thirds of the way out. The bulk of these stars are contained in a rim disk.
Mineral: Naturally occurring inorganic solid with a definite chemical composition and crystal structure.
Mineralogy: The study of minerals: formation, occurrence, use, properties, composition, and classification. The mix of minerals which make up a rock.
Missions Operations Center (MOC): The place from where the satellite or spacecraft receives coded instructions and to which it delivers data. The coordination place (usually on Earth) for any space mission.
Mission Elapsed Time (MET): It is easier to plan functions of a mission if they are planned in terms of time after liftoff instead of a specific time of day. Thus at the moment of liftoff the clock is set at zero and counts forward in normal days, hours, minutes, and seconds. This becomes especially helpful if a flight is delayed or rescheduled for any reason. A particular experiment could be scheduled for 2 days, 31 minutes after liftoff and it would not matter at what time the spacecraft actually lifted off, the experiment would take place 2 days and 31 minutes into the flight.
Moons: The natural satellites orbiting around the planets. See the table of the planets. (A spacecraft orbiting about anything is a man�made satellite.) Now there are 64 known moons in the solar system. Half of these were discovered by cameras on the Voyager spacecraft. Before the Voyager mission there were only 32 known moons, some quite big and easily seen from the Earth by using a telescope.

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NASA: NASA is the National Aeronautics and Space Administration, which was established in 1958, through the National Aeronautics and Space Act, as an outgrowth of the National Advisory Committee for Aeronautics.
NEAR Mission: A NASA mission to the asteroids Mathilde (visited in June 27, 1997) and Eros (planned for January 10, 1999). The NEAR (Near Earth Asteroid Rendezvous) spacecraft was launched in February 1996.
Near-Earth Asteroid: Most known asteroids are found between the orbits of Mars and Jupiter, within the so-called asteroid belt. Some asteroids, however, travel an orbit that brings them closer to Earth. The asteroids that come within 1.3 AU of the Sun are called Near-Earth asteroids.
Near-Real Time (NRT): The time during which something takes place is called real-time. When data is received from a satellite or spacecraft within a short time (usually a few hours) from the time it was collected, the data is said to be near real-time data. Sometimes data is collected and stored on the spacecraft or satellite by a tape recorder and then transmitted to Earth at a later date. By marking data real-time, near real-time or no marking, the scientists who study the data know when it was collected.
Nebula/Nebulae: A generic term given to cloudy-looking patches in space. Better equipment has shown us that some of these are galaxies, some are clusters of stars, some are planetary nebulae and some are supernova remnants.
Neptune: The last giant outer planet, measured by distance from the Sun, of the four giant outer planets. Its diameter is almost 4 times Earth's diameter and its year is 165 Earth�years. See the table of the planets.
Neutral Point: In the cross�sectional view of the Earth's magnetotail (that view, or plane, containing the tail�axis and the north�south axis) a point along the tail axis that has zero field magnitude. The total field magnitude at this point may not be zero, however, because there may be an existing component of the field perpendicular to this plane. See the figure of the magnetosphere.
Neutral Sheet: The sheet of electrical current that separates the northern and southern "lobes" of the Earth's magnetotail. It is called 'neutral,' because the magnetic fields in the two lobes are oppositely directed, and at this central 'boundary' between them the field must become very weak. That is, the field is almost 'neutralized.' It is interesting that, as weak as it gets there, it is usually not of exactly zero magnitude.
Neutrinos: Elementary particles of neutral charge having either no mass or a mass so low that it is as yet not measurable. They are generated in the core of the Sun (and elsewhere) and escape, and those coming in our direction usually pass right through the Earth and our bodies without stopping.
Neutrons: Along with protons, the neutron is the other principal component particle of atomic nuclei. Neutrons are of approximately the same mass as protons (very slightly heavier) but are electrically neutral. As reactions occur in the Sun's core, neutrons escape and sometimes can be detected at the Earth, just as they break up into more fundamental particles: a proton, a neutrino, and an electron. Now you can see why the neutron must be slightly heavier than the proton.
NOAA: The National Oceanic and Atmospheric Administration (NOAA) warns of dangerous weather, charts our seas and skies, guides our use and protection of ocean and coastal resources, and conducts research to improve our understanding and stewardship of the environment which sustains us all. An organization of the Department of Commerce, NOAA is composed of the National Ocean Service, National Weather Service, National Marine Fisheries Service, National Environmental Satellite Data and Information Service, and Office of Oceanic and Atmospheric Research. (We thank NOAA for the information here which comes directly from their web site.)
Nuclear Fusion: A nuclear reaction in which two atomic nuclei combine to form another element with the release of energy.
Nucleus: The very high density central part of an atom consisting of protons and neutrons.

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Obliquity: The angle between a planet's orbital plane and its equator plane, or more precisely, the angle between a planet's rotational axis and the normal to its orbital plane. In Earth's case the orbital plane is the Ecliptic plane.
Ohm (Ohm's Law): Georg Simon Ohm (1787-1854), German physicist, discovered the relationship, V=IR, where I is the strength of electrical current, V is the electromotive force, and R is the resistance of an electrical element of interest. An ohm is a volt/ampere.
Oort Cloud: A spherical region in which a trillion or so comets are stored. The cloud is thought to surround the Sun and to extend about 150,000 A.U., about half way to our nearest star. It is believed that comets with the longest periods come from the Oort cloud region.
Open Magnetosphere: A magnetosphere that has magnetic field lines of force passing from the solar wind into and out of a magnetosphere through its magnetopause boundary, making the boundary "open" also. Because of this property of "openness" solar wind particles can more easily enter into the magnetosphere, and likewise particles in the magnetosphere can exit to the solar wind.
Orbit: The path or trajectory of a body in space. For example, the Moon travels in an orbit around the Earth. They both have orbits around the Sun. We can refer to the orbit of a natural body in space or to that of a spacecraft, made by people.
Ordinary Chondrites: The most common class of meteorites, consisting of variable amounts of metal and chondrules in a matrix of mostly silicate minerals.
Oscillating Fields: Fields (either electric or magnetic � or even gravitational) that vary with some period.
Outer Planets (Giant Planets): Jupiter, Saturn, Uranus, and Neptune. See the table of the planets.

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Penumbra: The region of partial shade between the totally dark part (umbra) of a shadow and the fully illuminated region outside.
Periapsis: In a satellite orbit, the point that is closest to the gravitational center of the system consisting of the primary body and the satellite. Note: In an orbit about the Earth, periapsis is called perigee. In an orbit about the Moon, periapsis is called perilune, and in an orbit about the Sun, it is called perihelion.
Perigee: The point in the orbit of an object orbiting Earth that is at the smallest distance from the center of Earth.
Perihelion: The point of a celestial body's orbit closest to the Sun.
Perturbation (orbital): A disturbance of the regular and usually elliptical course of motion of a celestial body that is produced by some force additional to that which causes its regular motion.
Phase Angle: The angle between the Sun, an object, and an observer. 0 degrees phase means the Sun is behind the observer. [Our thanks to the Galileo glossary - http://galileo.ivv.nasa.gov/glossary.html]
Photometry: A branch of science that deals with measurement of the intensity of light.
Photon: A bundle of light energy thought of as a particle but not having real mass, so-called rest mass, like a proton or electron does. In modern theory, Gamma rays, x-rays, light waves, radio waves, etc. (all electromagnetic wave) can be viewed as either waves or particles.
Photosphere: The visual surface of the Sun. It is the part of the Sun seen "by the eye." Warning: The Sun should be observed only with special equipment to protect the eyes. The average temperature in the photosphere is about 6,000 deg Kelvin (or 11,000 deg F).
Physics: The study of matter and energy and the processes that link them. Physics is usually broken down into these categories: mechanics, thermodynamics, sound, light, electricity and magnetism, atomic, nuclear, elementary particles, relativity theory, and quantum mechanics. The last five are sometimes referred to as modern physics.
Plage: A bright, clustered region which spatially coincides with active regions on the Sun's surface and seen especially in special spectral emission lines.
Planet: The word planet comes from the Greek word meaning "wanderer." The planets in our solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Other planets have been observed outside of the solar system orbiting about distant stars. See table of planets.
Plasma: Sometimes called the 4th state of matter, a plasma is an electrically charged gas that responds to electric and magnetic fields, and has approximately as many positive charged particles as negative ones and neutral particles (having no charge) may be present too. Plasmas are very common throughout the universe. The Sun and most stars are made of plasmas. For a plasma to exist the gas must be so hot that its heat energy overcomes the natural tendency for the oppositely charged particles within it to attract and come together.
Plasmasheet: A 'sheet' of plasma around the neutral sheet in the Earth's magnetotail where the magnetic field is relatively weak. See the figure of the magnetosphere.
Plasmasphere: See magnetosphere.
Pluto: Usually the most distant planet, but sometimes the planet Neptune is farther from the Sun than Pluto. It is a small planet, having a diameter only about 1/4 the diameter of Earth. It is different from all the other known planets in the solar system whose orbits are nearly circular and which are close to the Earth's orbital plane, called the ecliptic plane. Pluto's orbit is very tipped and quite oval in shape. See the table of the planets.
POLAR: This spacecraft is named Polar, because its orbit takes it above the Earth's north and south poles. It orbits around the Earth and stays close enough so that it can take beautiful pictures of the aurora when it occurs. It was built by NASA, but has many scientist and engineers from around the world involved. It was launched in 1995. It is part of a big worldwide program of scientific space studies called the International Solar Terrestrial Physics Program.
Polar Cusp: See Cusps.
Prominences: Masses of relatively cool (compared to the very hot surrounding gasses of a few million degrees) and dense material suspended in the Sun's corona by magnetic fields. They can be thought of in two categories: active ones and quiet ones. The active ones sometimes send material out into the solar wind, and later it is sometimes measured by spacecraft measuring the solar wind, along with the magnetic fields inside it.
Proton: A small, positively charged particle having a very small mass of 1.67/1,000,000,000,000,000,000,000,000,000 kg. A proton is one of the main particles of the nucleus of an atom, but it can exist outside of a nucleus.

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Quantum Number: According to quantum mechanics, an atom can reside only in certain definite energy states. It is impossible for the atom to have an energy between these values and an atom can change its energy only by moving from one discrete energy state to another. In doing so, the atom absorbs or emits an amount of energy equal to the difference between the energies of the two states. The principal quantum number, an integer, denotes the atom's discrete energy level.

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Radiation Belts: Donut shaped regions of charged particles in a magnetosphere. The belts contain ions and electrons. In the Earth's case, they are called the Van Allen Radiation Belts.
Radiation Pressure: The force per unit area exerted by electromagnetic radiation (photons) on matter. (For example, the Sun exerts radiation pressure on solar sails for propulsion. Click here for a Web site about solar sails.)
Radio Emissions: Electromagnetic emissions in the wavelength range of radio waves.
Radio Waves: Electromagnetic waves like light waves but at much longer wavelengths, starting at a wavelength of about 30,000,000,000 cycles per second to about 10,000 cycles per second; a cycle per second is called a Hertz (Hz). This goes through a spectrum of types of radio waves called super high, ultra high, very high, high, medium, low, and very low radio waves, the latter being that near the 10,000 Hz.
Real-Time (RT): The actual time during which something takes place. When data is received on Earth from a satellite or spacecraft immediately as the satellite or spacecraft obtains it, the data is said to be real-time data.
Reconnection (of magnetic field lines): The process in which oppositely directed magnetic "lines of force" merge together and tend to cancel each other out. This causes magnetic energy (from the old field lines) to convert to heat and particle flows away from the region where the reconnection process is centered.
Reflection Nebula: When dust in a cloud of gas reflects incident light.
Regolith: Loose, unconsolidated rock, mineral, and glass fragments on the Moon and some other planetary bodies. This debris is produced by impacts and blankets the surface.
Relativistic: Refers to something, usually a particle, moving at a speed near the speed of light, c.
Resolution (Visual): Ability to distinguish visual detail, usually expressed in terms of the size of the smallest features that can be distinguished.
Resonance (Orbital): A relationship in which the orbital period of one body is related to that of another by a simple integer fractions, such as 1/2, 2/3, 3/5.
Retrograde: The orbit of a planet or an object that appears to move in the opposite direction of Earth's orbit as seen from Earth. The retrograde motion is caused by planets and objects moving at different speeds around the Sun.

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Satellite: A man-made satellite is a spacecraft that orbits another body, such as a planet or the Sun. A natural satellite is another term for a moon. Our Moon is a natural satellite of Earth.
Saturn: The sixth planet from the Sun that is distinguished by its beautiful rings. It is one of the four giant outer planets, the others being Jupiter, Uranus, and Neptune. Its diameter is almost 10 times Earth's diameter and, traveling in a circle, its year is about 30 Earth�years. It is a giant gas ball without a solid surface. See the table of the planets.
Science Data Center (SDC): Just as the name implies, this is a place or organization that collects, processes, stores and distributes data sent to it from other organizations or directly from a spacecraft.
SEC: The Space Environment Center (SEC) provides space weather alerts and warnings to the nation and the world for disturbances that can affect people and equipment working in space and on Earth.
Shock Wave: A wave appearing as an abrupt change in pressure and temperature in a fluid (or gas) in which the wave's speed is faster than some signal speed in the fluid. In everyday experience the signal has the sound wave speed. A shock wave in front of a jet plane moving faster than the speed of sound is an example of this. In space, where usually there are magnetic fields, the signal speed depends on the magnetic field and the speed of sound.
Sidereal: A measurement dealing with respect to the stars, rather than the Sun. An example is sidereal time.
Silicate: Minerals that contain the elements silicone and oxygen plus one or more of the metals such as magnesium, iron, calcium, sodium, aluminum, etc.
SOHO: This means Solar and Heliospheric Observatory, which is a spacecraft built by the European Space Community and was launched in December 1995. It is part of a big worldwide program of scientific space studies called the International Solar Terrestrial Physics Program. It orbits around the Lagrangian Point.
Solar Cycle: A cyclical change in solar activity as seen by the number of Sun Spots on the Sun's surface. The cycle is 11 years. There is a related 22 magnetic solar cycle also.
Solar flare: A very rapid release of energy from the Sun's surface. This energy is in the form of light waves of various wavelengths, shock waves, and hot and fast charged particles. This can happen anywhere from a few minutes to about a day. The effects of flares are often measured by spacecraft and seen at Earth.
Solar Power: Energy derived from the Sun or sunlight for use as a source of electricity.
Solar System: The Solar System consists of: one star, the Sun, nine planets, 65 moons (number known as of June 1998), asteroids (thousands known), comets (less than 1,000 recorded, possibly more than a trillion trapped beyond Pluto) dust. The Sun is the largest body in the Solar System, with more than 1,000 times the mass of everything else added together. It's the Sun's huge gravity that keeps all of the nine planets, their moons, the asteroids, the comets, and the dust between the planets all orbiting the Sun.
Solar Wind: An electrically charged gas that usually blows almost directly outward from the Sun typically at 400 km/s (which is about one million miles and hour). It has a density so low that it is nearly a vacuum compared to our atmosphere at the Earth's surface. The Solar Wind is the extension of the Sun's corona, its atmosphere. Its density at Earth's distance from the Sun is typically about 6 particles per cubic centimeter. It is composed of the same matter as the Sun itself being principally protons and electrons.
Spacecraft: A vehicle that is launched into space to observe something in nature (or made by man) for scientific or technical purposes. A spacecraft is sometimes correctly referred to as a satellite, if it orbits a body, such as a satellite of Earth. But sometimes the word satellite is used incorrectly. For example, the two Voyager spacecraft are not satellites, as there are sometimes called, because they are not orbiting around anything. They will eventually leave the solar system to go into the stars.
Space physics: The study of everything in the solar system above Earth's lower atmosphere. Traditionally the study of the solid bodies of the solar system is excluded from space physics; this is included in astrophysics or geology.
Space weather: The study of the effects of the Sun and the solar wind on the magnetic environment of the Earth. Space weather involves many important areas of study including concern for astronauts when exposed to solar hazards. Some other concerns that arise because of such space activity are possible communication disruptions, overloads of electrical power grids, increased drag on satellites which could cause them to fall to Earth earlier than planned, and confusion about the Earth's main (almost steady) magnetic field when changing conditions are occurring.
Spectroscopy: The excitation of the spectrum, its visual or photographic observation, and the precise determination of wavelengths.
Spectrum: A continuum of color (wavelength) formed when a beam of white light is dispersed (as by passage through a prism) so that its component wavelengths are arranged in order. Also, an ordered arrangement by a particular characteristic (as frequency or energy). It also applies to sound waves and energetic particles.
Speed of Light (called c): c = 186,284 miles/s or 299,796 km/s. Physics principles tell us that nothing can move faster than c.
Speed of Sound: At Earth's surface the speed of a sound wave, which is typically 730 miles/hr (or 330 m/s), depends on the local density and pressure of the atmosphere. As the pressure increases the speed increases, and as the density increases the speed decreases.
Spicules: Flames of gas that shoot upward from the surface of the Sun approximately 10,000 km in altitude with lifetimes of 2-5 minutes.
Spin Stabilized: A spin stabilized spacecraft is one that rotates at some constant spin period about one of its principal axes. For example, the WIND spacecraft completes one rotation in 3 seconds. By spinning, the spacecraft maintains a fixed, i.e., stable, alignment of its spin axis, even as the spacecraft itself moves in space. This fixed alignment (meaning that the axis always points to the same "fixed" star) is why the spacecraft is called spin stabilized.
Star: Stars are observed as twinkling points of light on an otherwise clear night sky. They are located at very great distances from us. The closest one is about 3 light�years away and some are far as the edge of the universe. Most stars are very hot gas balls composed mostly of the element hydrogen. They get their heat energy from nuclear reactions at their centers.
Star Tracker (ST): A navigational device on board a spacecraft used to observe or plot the moving path of a star so that the spacecraft can make small course adjustments and stay on a prescribed course in relation to that star. It is also used to help measure the amount of roll of a spacecraft whenever rolling is necessary, i.e., for calibration of instruments.
Static Electricity: The study of electric charges at rest or electricity that consists of isolated motionless charges. For example, those produced by scuffing your feet across a carpet during the winter.
Stony-Iron: A class of meteorites composed mostly of an intimate mixture of silicates and iron metal.
Stony Meteorite: A class of meteorites composed mostly of silicate minerals.
Storm: See Magnetic Storm
Stratosphere: The part of the Earth's atmosphere which extends from about 7 miles (11 kilometers) above the surface to 31 miles (50 kilometers) and in which temperature increases gradually to about 32� Fahrenheit (0� C) and clouds rarely form.
Sublimate: The process of changing directly from the solid phase into a gas without going though a liquid stage.
Substorm: See Magnetic Substorm
Sudden Ionospheric Disturbance: A disturbance in the form of increased ionization of atoms or ions in the Earth's ionosphere. This is caused by either x�ray bursts or ultraviolet emission from solar flares.
Sun: The Earth's nearest star. Similar to most typical stars, it is a large ball of hot electrically charged gas which obtains its energy from nuclear reactions in its core, much like energy expected from a countless number of hydrogen bombs exploding. It is about 100 million miles from Earth and has a diameter of about 1 million miles. (For more detail and a figure).
Sunspots: Dark regions on the Sun's surface, the photosphere, where it is not as hot as the surrounding areas. The presence of strong magnetic fields cause these regions to be cooler. These spots appear in bipolar Regions.
Synchronous: The instantaneous alignment of two or more events in time. Events may occur at irregular intervals, though at the same instant and still be synchronous.

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Tectonic: Pertaining to rock forms due to deformation in the crust of Earth and other planetary bodies.
Telemetry: Radio signals from a spacecraft used to encode and transmit data to a ground station.
Terrain: Area of the surface with a distinctively physical or geological character.
Terrestrial: Anything having to do with the Earth.
Terrestrial Planets: Mercury, Venus, Earth, and Mars. See the table of the planets.
Torque: The product of force, acting about some axis, and some distance representing a lever. For example, on a spacecraft, from commands usually sent from the ground, pairs of jets set far from the axis of the spacecraft spurt out gas (supplying a force) to impart a torque to the spacecraft in order to make it spin or to slow down its spin, depending on the direction of the spurt. See the figure.
Trajectory Correction Maneuver (TCM): This operation is to make small corrections in the trajectory of a spacecraft to change its speed, direction, orientation, etc., to better serve the mission. Some missions will have multiple trajectory correction maneuvers while others require fewer.
Transition Region (Sun's): The region of the Sun's atmosphere between the chromosphere and the corona (the outermost part) where a large rise of temperature, from about 20,000 to 2,000,000 degrees K, takes place. It is still not completely understood why the corona is so hot compared to its visible surface, the photosphere.
Troposphere: The lowest, densest part of the Earth's atmosphere in which most weather changes occur and temperature generally decreases rapidly with altitude and which extends from the surface to the bottom of the stratosphere.

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Ultra High Frequency (UHF): A radio wave that is between 30 and 300 megahertz in frequency is referred to as an ultrahigh frequency (UHF) radio wave.
Ultraviolet (UV) Light: Electromagnetic radiation (or "light waves") with wavelengths that are just a little shorter than visual light. The wavelength range for this light is from 400 angstroms (Å) to 4000 Å. It is mainly UV wavelength light that causes severe sunburn if the skin is exposed to sunlight too long. The eye cannot see in this wavelength.
Umbra: The central region of a shadow that is totally dark because no light reaches it. (See also penumbra)
Uniform: Always having the same form, manner, or degree.
Unipolar Region: A relatively large region on the Sun's surface showing magnetic field of only one polarity, i.e., either positive polarity (coming out of the surface and into the solar wind) or negative polarity (going into the surface from the solar wind. For contrast see Bipolar (Region).
Uranus: The seventh planet from the Sun that is distinguished by the fact that its axis of rotation almost lies in the plane of its orbit. (This plane is close to Earth's ecliptic plane.) It is one of the four giant outer planets, the others being Jupiter, Saturn, and Neptune. Its diameter is a little more than four times Earth's diameter and, traveling in a circle, its year is about 84 Earth�years. It is a giant gas ball without a solid surface. See the table of the planets.

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Vacuum: The name of a cleaning machine that your mother (or father!) uses. It is also space where there is very little or no matter. The term is usually used for regions in outer space, far away from planets and stars , where there is almost no matter. A perfect vacuum is impossible to attain, especially if a very big volume of space is considered.
Van Allen Radiation Belts: Belt�shaped regions in the Earth's magnetic field where mostly energetic protons and electrons are trapped by the field. They were named after James A. Van Allen whose team was the first to identify the trapped particles from measurements from the early Explorer satellites. These moving charged particles are sometimes referred to as radiation. See the figure.
Van de Graaff Generator: Essentially a device that "pumps" electrical charges onto a hollow conducting metal sphere to very high voltages by mechanical means. See the figure, which shows how a motor driven rubber (or silk) belt is used to transfer electrons from a glass cylinder (or a battery) to the sphere, which is insulated from its surroundings, so the charge will not leak off as it accumulates. After awhile the charge will get very high and the electric potential between the sphere and the ground goes to very high voltages. Such an electrostatic generator can be used to accelerate particles to high speeds.
Vaporize: To change something from a liquid or a solid to a gaseous state as in rock that is completely altered to gas during large impacts.
Vector: Thought of as an arrow, it is anything having both magnitude and direction. The magnitude could be strength or intensity or length, and many other possibilities. Some examples of vectors in space physics are the magnetic field observed at any point in space, the velocity of a particle, solar wind particle flux, its acceleration, any force, and the electric and gravitational fields, and even the displacement of a spacecraft from one point to another.
Velocity: Rate of motion in a specific direction.
Venus: The second planet from the Sun, Venus comes closest to the Earth of any planet as it orbits the Sun, sometimes getting as close as 26 million miles from Earth. It is one of the four terrestrial (or Earth�like) planets. Its heavy cloud structure makes it one of the most beautiful planets, but it also makes the planet very hot by trapping the Sun's heat. Its diameter is almost as large as Earth's diameter and, traveling in a nearly perfect circle, its year is about 225 Earth�days. See the table of the planets.
Very High Frequency (VHF): A radio wave that is between 300 and 3000 megahertz in frequency is referred to as a very high frequency (VHF) radio wave.
Volcanism: The physical processes of a volcano. Volcanism also refers to all natural processes resulting in volcanoes and other igneous surface events.
Volcano: A vent in the crust of the Earth or other planet from which usually molten or hot rock and steam issue. A hill or mountain formed from the eruption of igneous matter through a vent.
Volt: A unit of electromotive force that will cause a current of one ampere (Amp) to flow through a resistance of one ohm. Named in honor of the Italian physicist Alessandro Volta.
Voltage: Electric potential or potential difference measured in volts.
Volume: The amount of space occupied by a three-dimensional object as measured in cubic units (as quarts or liters).
Voyager Spacecraft: Launched in August and September of 1977 the Voyager 1 and 2 spacecraft were each originally planned for visits to the planets Jupiter (1979) and Saturn (in 1980 and 1981), but after those successful encounters Voyager 2 was sent to Uranus and Neptune in the late 1980's. The main interest of these missions was to study the surfaces, atmospheres, and the magnetospheres of the Outer Planets, as well as any planetary rings and moons. See the table of planets.

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Watt: The unit of power equal to kg m²/s³ (in the meter-kilogram-second system). It is the work done at the rate of one joule per second or the power produced by a current of one ampere across a potential difference of one volt: 1/746 horsepower.
Wave: An oscillatory motion of something in nature. This may be the variation of gas molecules to give a sound wave, or electric and magnetic fields to give a light wave or regular motion of a solid surface to give a mechanical wave. Maybe the most easily recognized waves are the water waves in the ocean. The distance between any maximum position and the next maximum position in the wave (of the water wave, for example, as it bobs up and down) is called the wavelength.
WIND: This spacecraft is named after the solar wind which it studies and where it spends most of its time. It was built by NASA, but has many scientists and engineers from around the world involved. It was launched in early November 1994. It is part of a big worldwide program of scientific space studies called the International Solar Terrestrial Physics Program.

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X�ray: Light waves with wavelengths shorter than those of ultraviolet light but longer than those of gamma rays . These are often emitted from the Sun, but cannot be seen on Earth's because of their absorption by the Earth's atmosphere. However, these solar x�rays can be measured by spacecraft that are above the atmosphere, such as by the Japanese spacecraft Yohkoh.