Solar Physics Glossary



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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 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.
Aurora
A colorful, rapidly varying glow in the sky caused by the collision of charged particles in the magnetosphere with atoms in the Earth's upper atmosphere. Auroras are most often observed at high latitudes and are enhanced during geomagnetic storms.
Black Hole
A region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull.
Bremsstrahlung
Radiation that is emitted when a free electron is deflected by an ion, but the free electron is not captured by the ion. Generally, it is a type of radiation emitted when high energy electrons are accelerated. (German for braking radiation)
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.
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.
Contour Map
A map showing the intensity of radiation as a function of position. Each contour line corresponds to a specific intensity of radiation, with inner contours corresponding to higher intensities than outer contours. Therefore, a closed contour encircles a region where the intensity of the emitted radiation is greater than or equal to the intensity on the contour line. The contours outline the shape of the emitting source.
Convection
The physical upwelling 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.
Corona
The outermost layer of the solar atmosphere. The corona consists of a highly rarefied gas with a low density and a temperature greater than one million degrees Kelvin. It is visible to the naked eye during a solar eclipse.
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).
Electromagnetic Radiation
Radiation that travels through vacuous space at the speed of light and propagates by the interplay of oscillating electric and magnetic fields. This radiation has a wavelength and a 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 Flux
The rate of flow of electrons through a reference surface. In cgs units, measured in electrons s-1, or simply s-1.
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 x 10-12 erg.
Energy Flux
The rate of flow of energy through a reference surface. In cgs units, measured in erg s-1. Also measured in watts, where 1 watt = 1 x 107 erg s-1. Flux density, the flux measured per unit area, is also often referred to as "flux".
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 x 1010 (42 billion) ergs.
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.
Footpoint
The intersection of magnetic loops with the photosphere.
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.
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.
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).
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.
Hydromagnetic Wave
A wave in which both the plasma and magnetic field oscillate.
Intensity Map
A color-coded map of radiation intensity as a function of position. Different colors or shades represent different intensities of observed radiation.
Ion
An atom that has lost or gained one or more electrons and has become electrically charged as a result.
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").
Ionosphere
The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere 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.
Kelvin
Abbreviated K. A unit of absolute temperature. Zero degrees Celsius is equal to 273.16 Kelvin.
keV
One thousand electron volts.
Kilometer
Abbreviated km. 1 km = 1000 meters = 105 cm = 0.62 mile.
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.
Magnetosphere
The region around a planet such as the Earth within which the motion of charged particles is influenced by the planet's magnetic field. The Earth's magnetosphere consists of a dipole field, similar to that of a bar magnet, and a long tail on the night side produced by the interaction of the solar wind with the Earth's magnetic field.
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 x 1022 ergs.
MeV
One million electron volts.
Neutron
An electrically neutral elementary particle. A neutron is 1839 times heavier than an electron.
Nonthermal Particle
A particle that is not part of a thermal gas. These particles cannot be described by a conventional temperature.
Nonthermal Radiation
Radiation emitted by nonthermal electrons.
Nucleus
The positively charged core of an atom, consisting of protons and neutrons (except for hydrogen), around which electrons orbit.
Optical Radiation
Electromagnetic radiation (light) that is visible to the human eye.
Orbital Period
The amount of time it takes a spacecraft or other object to travel once around it's orbit.
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.
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.
Poloidal Radius
The radius of the actual loop structure. For a doughnut, it is measured from the center to the edge of the pastry (not from the center of the hole). See also Toroidal Radius.
Proton
A positively charged elementary particle. 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.
Quasar
A faint blue, star-like object commonly considered to be extremely distant, probably an unusual nucleus of a galaxy. It has a tendency to flare.
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.
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 Limb
The apparent edge of the Sun as it is seen in the sky.
Solar Wind
A stream of charged particles flowing outward from the Sun's corona. The speed of the solar wind at the Earth is typically 450 kilometers per second, but varies from about 200 kilometers per second to 900 kilometers per second.
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 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 its 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.
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.
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.
Surface Plot
A three-dimensional plot mapping the intensity of radiation from a region as a distorted surface. More intense radiation is represented by higher points on the surface. Therefore, regions of intense radiation resemble mountains on the earth.
Thermal Gas
A collection of particles that collide with each other and exchange energy frequently, giving a distribution of particle energies that can be characterized by a single temperature.
Thermal Particle
A particle that is part of a thermal gas.
Thermal Radiation
Electromagnetic radiation emitted by electrons in a thermal gas.
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.
Toroidal Radius
In a solar loop structure, it is the distance from the axis of the loop to the center of the "semi-circle" that the loop forms. Half of the distance from one loop footpoint to the other loop footpoint. For a doughnut, it is the distance from the center of the doughnut hole to the center (circular axis) of the pastry. See also Poloidal Radius.
Ultraviolet Radiation
Electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays.
Universal Time
Abbreviated UT. The same as Greenwich Mean Time (GMT) in England. Eastern Standard Time (EST) is five hours earlier than Universal Time.
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 or radiation belts.
Wavelength
The distance from crest to crest or trough to trough of an electromagnetic wave (see electromagnetic radiation) or other wave.
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.



www icon Some of these definitions are derived from the On-Line Glossary of Solar-Terrestrial Terms and Kenneth R. Lang's book: Sun, Earth, and Sky.


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Web Author: Gordon Holman
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Responsible NASA Official: Gordon D. Holman,
Heliophysics Science Division
NASA/Goddard Space Flight Center
Solar Physics Laboratory / Code 671
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Gordon.D.Holman@nasa.gov