Glossary

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

Absorption Line: A dark line at a particular wavelength of a spectrum, formed when a cool, tenuous gas between a hot radiating source and the observer absorbs electromagnetic radiation of that wavelength.
ACE: Advanced Composition Explorer. A spacecraft studying the heliosphere and cosmic rays.
Acoustic Wave: A wave for which pressure is the restoring force. Also known as a sound wave.
Active Region: An area of the Sun where the magnetic fields are very strong. At ultraviolet and X-ray wavelengths they appear bright. In visible light they exhibit sunspots.
Angstrom: Abbreviated ⋭. A unit of length equal to 10^-8 cm (one-hundredth of a millionth of a centimeter). An Angstrom is on the order of the size of an atom.
Arc Degree: A unit of angular measure in which there are 360 arc degrees in a full circle.
Arc Minute: Abbreviated arcmin. A unit of angular measure in which there are 60 arc minutes in 1 arc degree
Arc Second: Abbreviated arcsec. A unit of angular measure in which there are 60 arc seconds in 1 arc minute and therefore 3600 arc seconds in 1 arc degree. One arc second is equal to about 725 km on the Sun.
Astronomical Unit: The average distance between the Earth and Sun, about 150 million kilometers (93 million miles).
Atmosphere: A gaseous envelope surrounding a planet, or the visible layers of a star; also a unit of pressure (abbreviated atm) equal to the pressure of air at sea level on the Earth's surface.
Atmospheric reddening: Preferential scattering of blue light over red by air particles so that an object appears redder than it actually is.
Atom: The smallest particle of an element that exhibits the chemical properties of the element.
Aurora: A display of colored light given off by collisions between charged particles trapped in a planet's magnetic fields and atoms of atmospheric gases near the planet's magnetic poles. Aurora are visible on Earth as the aurora borealis or northern lights and the aurora australis or southern lights.

Big Bang model: A theory of the evolution of the universe that postulates its origin, in an event called the Big Bang, from a hot, dense state that rapidly expanded to cooler, less-dense states.
Blackbody spectrum: The continuous spectrum emitted by a blackbody; the flux at each wavelength is given by Planck's law.
Black Hole: A mass that has collapsed to such a degree that the escape velocity from its surface is greater than the speed of light, so that light is trapped by the intense gravitational field.
Blue shift: A decrease in the wavelength of the radiation emitted by an approaching celestial body as a consequence of the Doppler effect; a shift toward the short-wavelength (blue) end of the spectrum.

Celestial equator: An imaginary projection of the earth's pole onto the celestial sphere; a point about which the apparent daily rotation of the stars takes place.
Celestial sphere: An imaginary sphere of very large radius centered on the earth, on which the celestial bodies appear fastened and against which their motions are charted.
Celsius: Abbreviated C. A unit of temperature. Zero degrees Celsius is equal to 273 Kelvin. Also known as centigrade. Water freezes at 0°C and boils at 100°C.
Degrees Fahrenheit = Degrees Celsius *(9/5) +32.
CELIAS: Charge, Element, and Isotope Analysis System.
Instrument aboard SOHO which analyzes the constituents of the solar wind.
CGS: Centimeter-Gram-Second (abbreviated cm-gm-sec or cm-g-s). The system of measurement that uses these units for distance, mass, and time.
CCD: Charge Coupled Device.
A semiconductor light detector which converts light to electrical impulses. Such detectors are used by a number of the instruments on SOHO, including EIT, LASCO, and MDI.
Charge-Exchange: A process by which one atom gives up electrons to a proton or another atom. For example, if a positively-charged proton encounters a neutral atom, the neutral atom may give up one of its electrons to the proton. The proton-electron pair would then be neutral, and the formerly neutral atom would be positively-charged.
Chromosphere: The layer of the solar atmosphere that is located above the photosphere and beneath the transition region and the corona. The chromosphere is hotter than the photosphere but not as hot as the corona.
Chromospheric Network: A large scale cellular pattern visible in hydrogen-alpha and other parts of the spectrum associated with the chromosphere. The network appears at the boundaries of super-granulation cells and contains a magnetic field which has been swept to the edges of the cells by the flow of material in the cell.
CME: Short for Coronal Mass Ejection.
Conduction: The transfer of energy via collisions of randomly moving atoms and electrons.
Continuous spectrum: A spectrum showing emission at all wavelengths, unbroken by either absorption lines or emission lines.
Convection: The physical up-welling of hot matter, thus transporting energy from a lower, hotter region to a higher, cooler region. A bubble of gas that is hotter than its surroundings expands and rises. When it has cooled by passing on its extra heat to its surroundings, the bubble sinks again. Convection can occur when there is a substantial decrease in temperature with height, such as in the Sun's convection zone.
Convection Zone: A layer in a star in which convection currents are the main mechanism by which energy is transported outward. In the Sun, a convection zone extends from just below the photosphere to about seventy percent of the solar radius.
Core: In solar astronomy, the innermost part of the Sun, where energy is generated by nuclear reactions.
Corona: The outermost region of the Sun's atmosphere, consisting of thin, ionized gases at a temperature of about 1,000,000 K. It is visible to the naked eye during a solar eclipse.
Coronagraph: Telescope for observing the corona. Often contains an occulting disk which covers the disk of the Sun so that the corona may be more easily observed.
CDS: Coronal Diagnostic Spectrometer.
An ultraviolet spectrometer aboard SOHO.
Coronal Hole: An area of the corona which appears dark in X-rays and ultraviolet light. They are usually located at the poles of the Sun, but can occur other places as well. The magnetic field lines in a coronal hole extend out into the solar wind rather than coming back down to the Sun's surface as they do in other parts of the Sun.
Coronal interstellar gas: High-temperature interstellar plasma made visible by its X-ray emission.
Coronal Mass Ejection: A huge magnetic bubble of plasma that erupts from the Sun's corona and travels through space at high speed.
Coronal Streamer: Large scale magnetic structures observed in the Sun's corona.
Cosmic blackbody microwave radiation: Radiation with a blackbody spectrum at a temperature of about 2.7 K permeating the universe; believed to be the remains of the primeval fireball in which the Universe was created.
Cosmic Ray: High energy charged particles traveling through interstellar space at nearly the velocity of light.
Cosmic rays: Charged atomic particles moving in space with very high energies (the particles travel close to the speed of light); most originate beyond the solar system, but some of low energy are produced in solar flares.
Cosmology: The study of the nature and evolution of the physical universe.
Crab nebula: A supernova remnant, located in the constellation Taurus, produced by the supernova explosion visible from earth in 1054 CE; a pulsar in the nebula marks the neutron-star corpse of the exploded star.
Critical density: In cosmology, the density that marks the transition from an open to a closed universe; the density that provides enough gravity to just bring the expansion to a stop after infinite time.

D Region: The lowest layer of the Earth's ionosphere. It is between about 50 and 95 kilometers above Earth's surface. This is the layer which reflects radio waves. Also called the D Layer.
Dark cloud: An interstellar cloud of gas and dust that contains enough dust to blot out the light of stars behind it (as seen from the Earth).
Deep Space Network: A NASA radio navigation network used to communicate with spacecraft beyond Earth's orbit.
Degenerate electron gas: An ionized gas in which nuclei and electrons are packed together as much as possible, filling all possible low energy states, so that the perfect gas law relating pressure, temperature, and density no longer applies.
Degenerate neutron gas: Matter made up of neutrons packed together as tightly as possible.
Density: The amount of mass or number of particles per unit volume. In cgs units mass density has units of gm cm^-3. Number density has units cm^-3 (particles per cubic centimeter).
Differential Rotation: The change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).
Diffraction: The spreading of light as it passes a sharp edge of an opaque object.
Diffuse (bright) nebula: A cloud of ionized gas, mostly hydrogen, with an emission-line spectrum.
Disk: The visible surface of the Sun (or any heavenly body) projected against the sky.
Doppler Shift: A change in the wavelength of radiation received from a source because of its motion along the line of sight. A Doppler shift in the spectrum of an astronomical object is commonly known as a redshift when the shift is towards longer wavelengths (the object is moving away) and as a blueshift when the shift is towards shorter wavelengths (the object is approaching).
Dynamo: Something which converts energy of motion into an electric current.

E Region: The lowest layer of the Earth's ionosphere. It is between about 95 and 130 kilometers above Earth's surface. Also called the E Layer.
Ecliptic: From the Earth, the apparent yearly path on the celestial sphere of the Sun with respect to the stars; also, the plane of the earth's orbit.
Electromagnetic force: Electromagnetic force is one of the four forces of nature; particles with electromagnetic charge either attract or repel each other depending upon whether the two charges are opposite or identical.
Electromagnetic radiation: A self-propagating electric and magnetic wave, such as light, radio, ultraviolet, or infrared radiation; all types travel at the same speed and can be differentiated by wavelength or frequency.
Electromagnetic Spectrum: The entire range of all the various kinds or wavelengths of electromagnetic radiation, including (from short to long wavelengths) gamma rays, x-rays, ultraviolet, optical (visible), infrared, and radio waves.
Electron: A negatively charged elementary particle that normally resides outside (but is bound to) the nucleus of an atom.
Electron Volt: Abbreviated eV. A unit of energy used to describe the total energy carried by a particle or photon. The energy acquired by an electron when it accelerates through a potential difference of 1 volt in a vacuum. 1 eV = 1.6 × 10^-12 erg.
Element: A substance that is made of atoms with the same chemical properties, and which cannot be decomposed chemically into simpler substances.
Emission Line: A bright line at a particular wavelength of a spectrum emitted directly by a hot gas.
Emission (bright) lines: Light of specific wavelengths or colors emitted by atoms; sharp energy peaks in a spectrum caused by downward electron transitions from a discrete quantum state to another discrete state.
Emission-line spectrum: A spectrum containing only emission lines.
Empirical: Derived from experiment or observation.
Energetic Neutral Atoms (ENAs): Atoms with no charge that move very quickly. These atoms have equal numbers of positively-charged protons and negatively-charged electrons. ENAs form when charged particles from the solar wind travel outward and encounter atoms from the interstellar medium. Because the ENAs are neutral, they do not react to any magnetic fields. Some of these ENAs travel toward the inner solar system and are captured by the IBEX spacecraft.
Erg: A cgs unit of energy equal to work done by a force of 1 dyne acting over a distance of 1 cm. 10⁷ (ten million) erg s^-1 (ergs per second) = 1 watt. Also, 1 Calorie = 4.2 × 10¹⁰ (42 billion) ergs.
Excitation: The process of raising an atom to a higher energy level.
Extinction: The dimming of light when it passes through some medium, such as the earth's atmosphere, or interstellar material.

F Region: The lowest layer of the Earth's ionosphere. It is between about 160 and 400 kilometers above Earth's surface. Also called the F Layer.
F-Mode: A wave mode generated by a surface gravity wave.
Faculae: Bright regions of the photosphere visible in white light near the limb of the Sun. They are brighter than their surroundings because they are higher in temperature and density.
Fahrenheit: Abbreviated F. A unit of temperature. In the Fahrenheit scale, water freezes at 32 °F and boils at 212 °F.
Degrees Celsius = (Degrees Fahrenheit - 32)*5/9.
Filament: A structure in the corona consisting of cool plasma supported by magnetic fields. Filaments are dark structures when seen against the bright solar disk, but appear bright when seen over the solar limb, Filaments seen over the limb are also known as prominences.
Flare (Solar): Rapid release of energy from a localized region on the Sun in the form of electromagnetic radiation, energetic particles, and mass motions.
Flare Star: A member of a class of stars that show occasional, sudden, unpredicted increases in light. The total energy released in a flare on a flare star can be much greater that the energy released in a solar flare.
Flash spectrum: The spectrum that appears immediately before the totality of a solar eclipse as the normal absorption spectrum is replaced briefly by the corona's own emission spectrum.
Flux: The amount of energy flowing through a given area in a given amount of time, usually expressed as counts per area per second.
Frame of reference: A set of axes with respect to which the position or motion of something can be described, or physical laws can be formulated.
Free Electron: An electron that has broken free of it's atomic bond and is therefore not bound to an atom.
Frequency: The number of repetitions per unit time of the oscillations of an electromagnetic wave (or other wave). The higher the frequency, the greater the energy of the radiation and the smaller the wavelength. Frequency is measured in Hertz.
Fusion: A nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy

G-Mode: A wave mode generated by a gravity wave.
Galactic equator: The great circle along the line of the Milky Way, marking the central plane of the Galaxy.
Galactic latitude: The angular distance north of south of the galactic equator.
Galactic longitude: The angular distance along the galactic equator from a zero point in the direction of the galactic center.
Galaxy: A huge assembly of stars (between one hundred and one million), plus gas and dust, which is held together by gravity; the Galaxy, our own galaxy, containing the Sun.
Gamma Ray: The highest energy (shortest wavelength) photons in the electromagnetic spectrum. Gamma rays are often defined to begin at 10 keV, although radiation from around 10 keV to several hundred keV is also referred to as hard x-rays.
Gauss: A unit of magnetic field strength.
Geomagnetic Storm: A worldwide disturbance of the Earth's magnetic field, associated with solar activity.
Geosynchronous Orbit: The orbit of a satellite that travels above the Earth's equator from west to east so that it has a speed matching that of the Earth's rotation and remains stationary in relation to the Earth (also called geostationary). Such an orbit has an altitude of about 35,900 km (22,300 miles).
Giant molecular clouds: Large interstellar clouds, with sizes up to tens of parsecs and containing 100,000 solar masses of material; found in the spiral arms of the Galaxy, giant molecular clouds are the sites of massive star formation.
Granule: A roughly circular region on the Sun whose bright center indicates hot gases rising to the surface, and whose dark edges indicate cooled gases that are descending towards the interior. Individual granules appear and disappear on time scales of about 5 minutes and are typically about 1000 km.
Gravitation: In Newtonian terms, a force between masses that is characterized by their acceleration toward each other; the magnitude of the force depends directly on the product of the masses and inversely on the square of the distance between them; in Einstein's terms, the curvature of space-time.
Gravitational focusing: The directing of the paths of small masses by a larger one so that their paths cross, which enhances their accretion onto the larger mass.
Gravitational lens: The bending effect of a large mass on light rays so that they form an image of the source of light.
Gravitational redshift: The change to longer wavelengths that marks the loss of energy by a photon that moves from a stronger to a weaker gravitational field.
Gravity Wave: In fluid mechanics (and thus helioseismology) this refers to a wave for which buoyancy is the restoring force.

Halo CME: A CME pointed either towards or away from the Earth so that it looks roughly like a halo or ring around the Sun in images from a coronagraph.
Heliopause: The boundary between the Sun's solar wind and the interstellar medium.
Helioseismology: The study of the interior of the Sun by the analysis of its natural modes of oscillation.
Heliosheath: The place where the solar wind slows down and begins to interact with the interstellar medium. The heliosheath has a few parts: the termination shock (the innermost part of the boundary), the heliopause (the outermost part of the boundary) and the part in between the inner and outer boundary.
Heliosphere: The region around the Sun where the solar wind dominates over the interstellar medium.
Hertz: Abbreviated Hz. A unit of frequency equal to one cycle per second. One kHz = 1000 Hz. One MHz = 10⁶ (one million) Hz. One GHz = 10⁹ Hz.
Hertzsprung-Russell diagram: A diagram which plots temperature (or color) vs. luminosity for a population of stars.
High-velocity clouds: Clouds of gas associated with the Galaxy, moving at speeds of hundreds of kilometers per second.
High-velocity stars: Stars in the Galaxy with velocities greater than 60 km/s relative to the Sun; they have orbits with high eccentricities, often at large angles with respect to the Galactic plane.
Hubble's Law: A description of the expansion of the Universe, such that the more distant a galaxy lies from us, the faster it is moving away; the relation, v = Hd, between the expansion velocity (v),and the distance (d) of a galaxy, where H is the Hubble constant.
Hubble constant: The proportionality constant relating velocity and distance in the Hubble law; the value, now around 75 km/s/Mpc, changes with time as the Universe expands.
Hubble time: Numerically the inverse of the Hubble constant; it represents, in order of magnitude, the age of the universe.
Hydrogen Alpha: Also called H-alpha. Light emitted at a wavelength of 6563 ⋭ from an atomic transition in hydrogen. This wavelength is in the red portion of the visible spectrum and is emitted by plasma at about 10,000 K, mainly in the solar chromosphere.

Infrared telescope: A telescope, optimized for use in the infrared part of the spectrum, fitted with an infrared detector.
Intergalactic medium: The gas and dust found between the galaxies.
Interstellar dust: Small (micrometers in diameter) solid particles in the interstellar medium
Interstellar extinction curve: The amount of extinction from interstellar dust as a function of wavelength.
Interstellar gas: Atoms, molecules, and ions in the interstellar medium.
Interstellar hydrogen: Hydrogen whose origin is from between the stars. This hydrogen is generally neutral, consisting of a single proton-electron pair.
Interstellar medium: All the gas and dust found between stars.
Ion: An atom that has become electrically charged by the gain or loss of one or more electrons.
Ionization: The process by which ions are produced, typically occurring by collisions with atoms or electrons ("collisional ionization"), or by interaction with electromagnetic radiation("photoionization").
Ionized gas: A gas that has been ionized so that it contains free electrons and charged particles; a plasma if it is electrically neutral overall.
Ionosphere: The region of the Earth's upper atmosphere (100 to 700 kilometers above the surface) containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere (oxygen and nitrogen) by solar ultraviolet radiation. The ionosphere significantly influences radiowave propagation of frequencies less than about 30 MHz.
Isotope: One of two or more atoms having the same number of protons in its nucleus, but a different number of neutrons and, therefore, a different mass.
Isotropic: Having no preferred direction in space.
ISTP: International Solar-Terrestrial Physics Science Initiative.
Collaborative effort by US, European, and Japanese space agencies to obtain coordinated, simultaneous investigations of the Sun-Earth space environment over an extended period of time. SOHO is a part of this program.

Kelvin: Abbreviated K. A unit of temperature with its zero point located at "absolute zero," the point at which the random motion of atoms and molecules ceases. Each degree has the same change as in Celsius, and zero degrees Celsius is at 273.16 K
Kilometer: Abbreviated km. 1 km = 1000 meters = 10⁵cm = 0.62 mile.

Lagrange Points: The five gravitational balance points between two orbiting masses. The first Lagrange Point (L₁) is in between the two bodies.
Large Magellanic Cloud (LMC): A small galaxy, irregular in shape, about 50 kpc from the Milky Way.
Latitude: A north-south coordinate measured on the surface of a sphere. It is the angular distance from the equator in the direction of one of the rotational poles.
Lens: A curved piece of glass designed to bringlight rays to a focus.
Light-year: The distance light travels in one earth year, about 30,900,000,000,000 kilometers.
Limb: See Solar Limb.
Local Group: A gravitationally bound group of about 20 galaxies to which our Milky Way Galaxy belongs.
Local Supercluster: The supercluster of galaxies in which the Local Group is located; spread over 10,000,000 pc, it contains the Virgo and Coma clusters.
Longitude: An east-west coordinate measured on the surface of a sphere.
Luminosity: The total rate at which radiative energy is given off by a celestial body, over all wavelengths; the Sun's luminosity is about 4x10²⁶ watts.

Magnetic Field: A field of force that is generated by electric currents. The Sun's average large-scale magnetic field, like that of the Earth, exhibits a north and a south pole linked by lines of magnetic force.
Magnetic Field Lines: Imaginary lines that indicate the strength and direction of a magnetic field. The orientation of the line and an arrow show the direction of the field. The lines are drawn closer together where the field is stronger. Charged particles move freely along magnetic field lines, but are inhibited by the magnetic force from moving across field lines.
Magnetic flux: The number of magnetic field lines passing through an area.
Magnetic reconnection: The process by which two separate magnetic field lines bend toward each other and "fuse" to create a new field line, allowing charged particles that were previously travelling in two different directions to now travel toward the same point.
Magnetogram: A map showing the strength of the magnetic field in different locations.
Magnetometer: A device to measure the strength of a magnetic field.
Magnetosphere: The region around a planet where particles from the solar wind are trapped by the planet's magnetic field.
Magnitude: An astronomical measurement of an object's brightness; larger magnitudes represent fainter objects.
Main Sequence: An area on the Hertzsprung-Russel Diagram containing "middle aged" stars like the Sun.
Mass: A measure of an object's resistance to change in its motion (inertial mass); a measure of the strength of gravitational force an object can produce (gravitational mass).
MDI/SOI: Michelson Doppler Imager/Solar Oscillations Investigation.
Helioseismology instrument aboard SOHO which analyzes the vibrational modes of the Sun. Also measures the Sun's magnetic field in the photosphere.
Megaparsec: One million parsecs (Mpc).
Megaton: An explosive force equal to one million metric tons of TNT. The energy released in the explosion of one megaton of TNT is equal to 4.2 × 10²² ergs.
Mesosphere: Region of the Earth's atmosphere between 50 and 100 km where the temperature falls rapidly.
Metallic hydrogen: A state of hydrogen, reached at high pressures, where it is able to conduct electricity.
Microwave background radiation: A universal bath of low-energy photons having a blackbody spectrum with a temperatureof 2.7 K.
Milky Way: The band of light that encircles the sky, caused by the bending of light from many stars lying near the plane of the Galaxy; also sometimes used to refer to the Galaxy in which the Sun belongs.
Molecular cloud: Large, dense, massive clouds in the plane of a spiral galaxy; they contain dust and a large fraction of gas in molecular form.
Molecule: A combination of two or more atoms bound together electrically; the smallest part of a compound that has the properties of that substances.
Momentum: The product of an object's mass and velocity.

Nanometer: 0.000000001 of a meter; common unit of wavelength measurement for light.
NASA: The National Aeronautics and Space Administration. NASA web site.
Nebula (Latin for "cloud"): A cloud of interstellar gas and dust.
Network: See Chromospheric Network.
Neutrino: An elementary particle with no charge and almost no mass which interacts very weakly with other matter.
Neutron: A subatomic particle with about the mass of a proton and no electric charge; one of the main constituents of an atomic nucleus; the union of a proton and an electron. A neutron is 1839 times heavier than an electron.
North magnetic pole: One of two points on a star or planet from which magnetic lines of force emanate and to which the north pole of a compass points.
Nova (Latin, "new"): A star that has a sudden outburst of energy, temporarily increasing its brightness by hundreds to thousands of times; now believed to be the outburst of a degenerate star in a binary system; also used in the past to refer to some stellar outbursts that modern astronomers now call supernovas.
Nucleosynthesis: The chain of thermonuclear fusion processes by which hydrogen is converted to helium, helium to carbon, and so on through all the elements of the periodic table.

Nucleus: The massive and positively charged core of an atom, consisting of protons and neutrons(except for hydrogen), around which electrons orbit.

Observable universe: The parts of the Universe that can be detected by the light they emit.
Opacity: The property of a substance that hinders (by absorption or scattering) light passing through it; opposite of transparency.
Optical Radiation: Electromagnetic radiation (light) that is visible to the human eye.
Orbit: The curved path, usually elliptical, described by a planet, satellite, spaceship, etc., around a celestial body, such as the Sun.
Orbital Period: The amount of time it takes a spacecraft or other object to travel once around it's orbit.
Orbiting Solar Observatories: A series of eight Sun observing satellites launched from 1962 to 1975.
Orion Nebula: A hot cloud of ionized gas that is a nearby region of recent star formation, located in the sword of the constellation of Orion; also catalogued as Messier 42 (M42).

P-Mode: A wave mode generated by an acoustic wave (or "sound wave").
Parallax: The change in an object's apparent position when viewed from two different locations; specifically, half the angular shift of a star's apparent position as seen from opposite ends of the Earth's orbit.
Parsec (pc): The distance an object would haveto be from the earth so that its heliocentric parallax would be 1 second of arc; equal to 3.26 light years; a kiloparsec is 1000 parsecs.
Period: The time interval for some regular event to take place; for example, the time required for one complete revolution of a body around another.
Photodissociation: The breakup of a molecule by the absorption of light with enough energy to break the molecular bonds.
Photometer: A light-sensitive detector placed at the focus of a telescope; it is used to make accurate measurements of small photon fluxes.
Photon: A discrete quantity of electromagnetic energy. Short wavelength (high frequency) photons carry more energy than long wavelength (low frequency) photons. See Electromagnetic Radiation.
Photosphere: The visible surface of the Sun. It consists of a zone in which the gaseous layers change from being completely opaque to radiation to being transparent. It is the layer from which the light we actually see (with the human eye) is emitted.
Pickup ion: An atom from the interstellar medium which has been ionized by the Sun's radiation, and is then swept along with the solar wind.
Plage: Bright areas seen near sunspots in the chromosphere. Seen in H-alpha light. From the French for beach.
Planck curve: The continuous spectrum of a blackbody radiator.
Planck's constant: The number that relates the energy and frequency of light; it has a value of 6.63 x 10^-34joule seconds.
Planetary nebula: A thick shell of gas ejected from and moving out from an extremely hot star; thought to be the outer layers of a red giant star thrown out into space, the core of which eventually becomes a white dwarf.
Plasma: Plasma consists of a gas heated to sufficiently high temperatures that the atoms ionize. The properties of the gas are controlled by electromagnetic forces among constituent ions and electrons, which results in a different type of behavior. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the plasma state.
Polar Plume: Bright structure of out-flowing gas which occur along magnetic field lines in coronal holes. These field lines extend into the solar system. Although plumes usually occur at the poles, they can appear anywhere there is a coronal hole.
Prominence: A structure in the corona consisting of cool plasma supported by magnetic fields. Prominences are bright structures when seen over the solar limb, but appear dark when seen against the bright solar disk. Prominences seen on the disk are also known as filaments.
Proton: A massive, positively charged elementary particle; one of the main constituents of the nucleus of an atom. A proton is 1836 times heavier than an electron.
Pulsar: A neutron star (burnt-out star) that emits radio waves which pulse on and off.

Radiation: Usually refers to electromagnetic waves, such as light, radio, infrared, X-rays, ultraviolet; also sometimes used to refer to atomic particles of high energy, such as electrons (beta-radiation), helium nuclei (alpha-radiation), and so on.
Radiation Belt: A ring-shaped region around a planet in which electrically charged particles (usually electrons and protons) are trapped. The particles follow spiral trajectories around the direction of the magnetic field of the planet. The radiation belts surrounding Earth are known as the Van Allen belts.
Radiative Zone: An interior layer of the Sun, lying between the core and the convection zone, where energy travels outward by radiation.
Red Giant: A large, bright, cool star. Red giants are formed when a star runs out of nuclear fuel in its core. The star starts to contract, which in turn leads to heating and nuclear reactions in layers outside the core and the expansion of the star's outer layers. These outer layers become cooler and redder as they expand.
Reddening: The preferential scattering or absorption of blue light by small particles, allowing more red light to pass directly through.
Red shift: An increase in the wavelength of the radiation received from a receding celestial body as a consequence of the Doppler effect; a shift towards the long-wavelength (red) end of the spectrum.
Reflection nebula: A bright cloud of gas and dust that is visible because of the reflection of starlight by the dust.
Relativistic Doppler shift: Wavelength shift from the radial velocity of a source as calculated in special relativity, so that very large red shifts do not imply that the source moves faster than light.
Relativity: Two theories proposed by A. Einstein; the special theory describes the motion of nonaccelerated objects, and general relativity is a theory of gravitation.

Scattering (of light): The change in the paths of photons without absorption or change in wavelength.
Scientific model: A mental image ofhow the natural world works, based on physical, mathematical, and aestheticideas.
Second of arc: 1/3600 of a degree, or 1/60 of a minute of arc.
Skylab: A space station which orbited Earth in the 1970s. Performed many important observations of the Sun.
SOHO: The Solar and Heliospheric Observatory.
Solar Atmosphere: The atmosphere of the Sun. An atmosphere is generally the outermost gaseous layers of a planet, natural satellite, or star. Only bodies with a strong gravitational pull can retain an atmosphere. Atmosphere is used to describe the outer layer of the Sun because it is relatively transparent at visible wavelengths. Parts of the solar atmosphere include the photosphere, chromosphere, and the corona.
Solar-B: Solar-B. Japanese spacecraft to observe the Sun
Solar core: Region of the Sun's interior where temperatures and densities are high enough for fusion reactions to take place.
Solar cosmic rays: Low energy cosmic rays generated in solar flares.
Solar Cycle: The approximately 11-year quasi-periodic variation in frequency or number of sunspots, coronal mass ejections, solar flares, and other solar activity.
Solar flare: Sudden burst of electromagnetic energy and particles from a magnetic loop in an active region.
Solar Limb: The apparent edge of the Sun as it is seen in the sky.
Solar Maximum: The month(s) during the solar cycle when the number of sunspots reaches a maximum. The most recent solar maximum occurred in 2014.
Solar Maximum Mission: A satellite dedicated to observing the Sun, especially solar flares. It was in orbit throughout the 1980s.
Solar Minimum: The month(s) during the solar cycle when the number sunspots is lowest. The most recent minimum occurred in 2009.
Solar wind: A stream of charged particles, mostly protons and electrons, that escapes into the Sun's outer atmosphere at high speeds and streams out into the solar system.
South Atlantic Anomaly: The region over the South Atlantic Ocean where the lower Van Allen belt of energetic, electrically charged particles is particularly close to the Earth's surface. The excess energy in the particles presents a problem for satellites in orbit around the Earth.
Spectral Continuum: A broad, continuous band of light in a spectrum.
Space-time: A 4-dimensional universe with space and time unified; a continuous system of one time coordinate and three space coordinates by which events can be located and described.
Spectral Line: A line in a spectrum due to the emission or absorption of electromagnetic radiation at a discrete wavelength. Spectral lines result from discrete changes in the energy of an atom or molecule. Different atoms or molecules can be identified by the unique sequence of spectral lines associated with them.
Spectrograph: An instrument that spreads light or other electromagnetic radiation into it's component wavelengths (spectrum), recording the results photographically or electronically.
Spectrometer: An instrument for measuring the intensity of radiation as a function of wavelength. See Spectrograph.
Spectroscope: An instrument for examining spectra; also a spectrometer or spectrograph if the spectrum is recorded and measured.
Spectroscopy: The analysis of light by separating it by wavelengths (colors, in visible light).
Spectrum: Electromagnetic radiation arranged in order of wavelength. A rainbow is a natural spectrum of visible light from the Sun. Spectra are often punctuated with emission or absorption lines, which can be examined to reveal the composition and motion of the radiating source.
Spicule: A predominantly vertical structure extending from the solar chromosphere into the corona, observed in Hydrogen-Alpha. Spicules are concentrated on the boundaries of super-granulation cells, have lifetimes of 5 to 10 minutes, and are about 1000 km across and 10,000 km long.
Spiral galaxy: A galaxy with spiral arms; the presumed shape of our Milky Way galaxy.
Stellar nucleosynthesis: A process in which nuclear fusion builds up heavier nuclei while supplying the energy by which stars shine.
Streamer: See Coronal Streamer
SUMER: Solar Ultraviolet Measurements of Emitted Radiation.
An ultraviolet spectrometer aboard SOHO.
Sunspot: A temporary disturbed area in the solar photosphere that appears dark because it is cooler than the surrounding areas. Sunspots consist of concentrations of strong magnetic flux. They usually occur in pairs or groups of opposite polarity that move in unison across the face of the Sun as it rotates.
Super-Granulation Cells: Convective cells about 30,000 km in diameter in the solar photosphere. The boundaries between super-granule cells can be seen by emission in the chromosphere known as the chromospheric network and contain concentrations of magnetic flux tubes.
Supernova: A stupendous explosion of a massive star, which increases its brightness hundreds of millions of times in a few days.
Supernova remnant: Expanding gas cloud from the outer layers of a star blown off in a supernova explosion; detectable at radio wavelengths; moves through the interstellar medium at high speeds.

Temperature: A measure of the average random speeds of the microscopic particles in a substance.
Termination Shock: The boundary marking one of the outer limits of the Sun's influence. At the termination shock, solar wind particles slow down as they begin to press into the particles forming the interstellar medium. The solar wind particles then continue to travel outward at a slower rate of speed. This is similar to cars speeding along a highway which then slow down as they encounter many more cars involved in a traffic jam. The jammed cars continue to move outward, although much more slowly.
Tesla: In the SI system, a unit of measure of magnetic flux.
Theory: An idea in science which is supported by numerous pieces of evidence, and which has thus far withstood the rigors of testing by other scientists. It is different from a hypothesis, which is an idea based on observations but which has not yet been tested, or does not have further evidence supporting it. Theory in the scientific sense is very different from the colloquial definition, where a theory is essentially a guess. Examples include the Big Bang theory, the theory of gravitation, and the theory of evolution.
Thermal radiation: Electromagnetic radiation due to the fact that a body is hot; often characterized by a blackbody spectrum.
Thermonuclear Fusion: The combination of atomic nuclei at high temperatures to form more massive nuclei with the simultaneous release of energy. Thermonuclear fusion is the power source at the core of the Sun. Controlled thermonuclear fusion reactors, when successfully implemented, could become an attractive source of power on the Earth.
Thermosphere: A layer in the earth's atmosphere, above the mesosphere, heated by X-rays and ultraviolet radiation from the Sun.
Time: A measure of the flow of events.
TRACE: Transition Region and Coronal Explorer
A NASA satellite launched in 1998. TRACE observes the Sun's atmosphere in ultraviolet wavelengths.
Transition Region: Area of the Sun's atmosphere with temperatures between those of the chromosphere and the corona, 20,000 - 1,000,000°C.

Ultraviolet: The part of the electromagnetic spectrum whose radiation has somewhat smaller wavelengths than optical radiation, but longer wavelengths than X-rays . Because ultraviolet light is absorbed by the Earth's atmosphere, ultraviolet astronomy is performed in space.
Universal Time: Abbreviated UT. The same as Greenwich Mean Time (GMT) in England. US Eastern Standard Time (EST) is five hours earlier than Universal Time.
Universe: The totality of all space and time; all that is, has been, and will be.

Van Allen Belts: Two ring-shaped regions that girdle the Earth's equator in which electrically charged particles are trapped by the Earth's magnetic field. See South Atlantic Anomaly and radiation belts.

Wavelength: The distance from crest to crest or trough to trough of an electromagnetic wave (see electromagnetic radiation) or other wave.
White Dwarf: A star which has used up its nuclear fuel and collapsed to a very small size.
White Light: Visible light that includes all colors and, therefore, all visible wavelengths.

X-ray: The part of the electromagnetic spectrum whose radiation has somewhat greater frequencies and smaller wavelengths than those of ultraviolet radiation. Because x-rays are absorbed by the Earth's atmosphere, x-ray astronomy is performed in space.
X-rays High-energy: electromagnetic radiation with a wavelength of about 10^-10 meter.

Yohkoh: A satellite which observes X-rays from the Sun. Launched in 1991.

Zeeman Effect: A splitting of spectral lines in response to a magnetic field. It is used to measure magnetic field strengths on the Sun and on other astronomical objects.
Zodiacal Light: A pale glow sometimes visible in the night sky in the path of the Earth's orbit around the Sun. (The constellations in this path make up the zodiac). It is caused by the scattering of sunlight off of dust in the plane of Earth's orbit.

References

Introductory Astronomy & Astrophysics, Third Edition
On-Line Glossary of Solar-Terrestrial Terms and Kenneth R. Lang's book: Sun, Earth, and Sky.
Solar Flare Theory web site by Gordon Holman and Sarah Benedict.