Solar Physics Glossary
- Abbreviated Å. 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.
- 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.
- 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)
- Centimeter-Gram-Second (abbreviated cm-gm-sec or cm-g-s). The system
of measurement that uses these units for distance, mass, and time.
- 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.
- 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
- 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.
- 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.
- 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
- 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".
- 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
- 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.
- 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.
- 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).
- 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
- Intensity Map
- A color-coded map of radiation intensity as a function of position. Different colors or
shades represent different intensities of observed radiation.
- An atom that has lost or gained one or more electrons and has
become electrically charged as a result.
- The process by which ions are produced, typically occurring by
collisions with atoms or electrons ("collisional
ionization"), or by interaction with electromagnetic
- 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
- 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.
- Abbreviated K. A unit of absolute temperature. Zero degrees Celsius is equal to
- One thousand electron volts.
- 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
- 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.
- 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.
- One million electron volts.
- 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.
- The positively charged core of an atom, consisting of protons and neutrons (except for hydrogen), around which electrons
- 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
- A discrete quantity of electromagnetic energy. Short wavelength
(high frequency) photons carry more energy than long wavelength (low frequency)
photons. See Electromagnetic Radiation.
- 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. Plasma is often considered the fourth state
of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the
- 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.
- A positively charged elementary particle. A proton is 1836 times heavier than an electron.
- A neutron star (burnt-out star) that emits radio waves which pulse on and off.
- 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.
- An instrument that spreads light or other electromagnetic
radiation into its component wavelengths
(spectrum), recording the results photographically
- An instrument for measuring the intensity of radiation as a function of wavelength. See Spectrograph.
- 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.
- 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
- 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
- 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
- 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.
- 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.
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.
Solar Flare Theory Homepage
Table of Contents
Web Author: Gordon Holman
Web Author: Sarah Benedict
Responsible NASA Official: Gordon D. Holman,
Heliophysics Science Division
NASA/Goddard Space Flight Center
Solar Physics Laboratory / Code 671
Greenbelt, MD 20771, USA