Solar Physics Glossary (7/29/01)
Angstrom
A unit of length equal to 10^-8 cm (one-hundredth of a millionth of a centimeter). A(n) _____ is on the order of the size of an atom.
Arc Degree
A unit of angular measure in which there are 360 _____ in a full circle.
(two words)
Arc Second
A unit of angular measure in which there are 60 _____s in 1 arc minute and therefore 3600 _____s in 1 arc degree. One _____ is equal to about 725 km (450 miles) on the Sun.
(two words)
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.
(two words)
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 _____ 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.
(two words)
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. _____ can occur when there is a substantial decrease in temperature with height, such as in the Sun's _____ zone.
Convective Zone
A layer in a star in which convection currents are the main mechanism by which energy is transported outward. In the Sun, a(n) _____ extends from just below the photosphere.
(two words)
Corona
The outermost layer of the solar atmosphere. The _____ 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 _____ has units of gm cm^-3. Number _____ 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.
(two words)
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.
(two words)
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.
(two words)
Electron Volt
A unit of energy used to describe the total energy carried by a particle or quantum. 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.
(two words)
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 10^7 erg s^-1. Flux density, the flux measured per unit area, is also often referred to as "flux".
(two words)
Erg
A cgs unit of energy equal to work done by a force of 1 dyne acting over a distance of 1 cm. 10^7 (ten million) _____ s^-1 (_____s per second) = 1 watt. Also, 1 Calorie = 4.2 x 10^10 (42 billion) _____s.
Flare
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(n) _____ can be much greater that the energy released in a solar flare.
(two words)
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.
(two words)
Frequency
The number of repetitions per unit time of the oscillations of an electromagnetic wave (or other wave). The higher the _____, the greater the energy of the radiation and the smaller the wavelength. _____ is measured in Hertz.
Gamma Ray
The highest energy (shortest wavelength) quanta in the electromagnetic spectrum. _____s 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.
(two words)
Geomagnetic Storm
A worldwide disturbance of the Earth's magnetic field, associated with solar activity.
(two words)
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).
(two words)
Hertz
A unit of frequency equal to one cycle per second. One kHz = 10^00 Hz. One MHz = 10^6 (one million) Hz. One GHz = 10^9 Hz.
Hydromagnetic Wave
A wave in which both the plasma and magnetic field oscillate.
(two words)
Intensity Map
A color-coded map of radiation intensity as a function of position. Different colors or shades represent different intensities of observed radiation.
(two words)
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 _____"), or by interaction with electromagnetic radiation ("photo_____ ").
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 _____ 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
A unit of absolute temperature. Zero degrees Celsius is equal to 273.16 K.
keV
One thousand electron volts.
Kilometer
1 km = 1000 meters = 10^5 cm = 0.62 mile.
Magnetic Field
A field of force that is generated by electric currents. The Sun's average large-scale _____ , like that of the Earth, exhibits a north and a south pole linked by lines of magnetic force.
(two words)
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 _____ , but are inhibited by the magnetic force from moving across field lines.
(three words, plural)
Megaton
An explosive force equal to one million metric tons of TNT. The energy released in the explosion of one _____ of TNT is equal to 4.2 x 10^22 ergs.
MeV
One million electron volts.
Neutron
An electrically neutral elementary particle. A(n) _____ 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.
(two words)
Nonthermal Radiation
Radiation emitted by nonthermal electrons.
(two words)
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.
(two words)
Orbital Period
The amount of time it takes a spacecraft or other object to travel once around it's orbit.
(two words)
Photon
A discrete quantity of visible electromagnetic energy. Short wavelength (high frequency violet) _____s carry more energy than long wavelength (low frequency red) _____s. 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
_____ 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. _____ is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the _____ 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.
(two words)
Proton
A positively charged elementary particle. A(n) _____ 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 _____s surrounding Earth are known as the Van Allen belts.
(two words)
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 _____ include the photosphere, chromosphere, and the corona.
(two words)
Solar Limb
The apparent edge of the Sun as it is seen in the sky.
(two words)
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.
(three words)
Spectral Line
A line in a spectrum due to the emission or absorption of electromagnetic radiation at a discrete wavelength. _____s result from discrete changes in the energy of an atom or molecule. Different atoms or molecules can be identified by the unique sequence of _____s associated with them.
(two words)
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.
Spectrum
Electromagnetic radiation arranged in order of wavelength. A rainbow is a natural _____ of visible light from the Sun. _____(plural) 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. _____s 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.
(two words)
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.
(two words)
Thermal Particle
A particle that is part of a thermal gas.
(two words)
Thermal Radiation
Electromagnetic radiation emitted by electrons in a thermal gas.
(two words)
Thermonuclear Fusion
The combination of atomic nuclei at high temperatures to form more massive nuclei with the simultaneous release of energy. _____ is the power source at the core of the Sun. Controlled _____ reactors, when successfully implemented, could become an attractive source of power on the Earth.
(two words)
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.
(two words)
Universal Time
The same as Greenwich Mean Time (GMT) in England. Eastern Standard Time (EST) is five hours earlier than _____ .
(two words)
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.
(two words, plural)
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.
(two words)
X-ray
The part of the electromagnetic spectrum whose radiation has somewhat greater frequencies and smaller wavelengths than those of ultraviolet radiation. Because _____s are absorbed by the Earth's atmosphere, _____ astronomy is performed in space.