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Laboratory for Atmospheric and Space Physics

Glossary

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absolute accuracy
The extent to which a value or measurement approaches the true value or measurement.
absorption line
In spectroscopy, a characteristic wavelength of emitted radiation that is partially absorbed by the medium between the source and the observer.
active region
A localized, transient volume of the solar atmosphere in which plages, sunspots, facula, flares, etc., may be observed. Active regions are the result of enhanced magnetic fields; they are at least bipolar and may be complex if the region contains two or more bipolar groups.
airglow
A faint luminescence of the night sky originating in photochemical reactions in the upper atmosphere. Also referred to as geocoronal emission.
aperture
The portion of the plane surface perpendicular to the direction of radiation and through which the radiation passes.
aphelion
That point on the path of a Sun-orbiting body most distant from the center of the Sun. Compare perihelion.
apogee
That point on the path of an Earth-orbiting satellite most distant from the center of the Earth. Compare perigee.
arcade

A series of magnetic loops, overlying a solar inversion line.arch filament system (AFS) A system of small, arched linear-absorption features connecting bright, compact plages of opposite polarity. An AFS is a sign of emerging bipolar magnetic flux and possibly rapid or continued growth in an active region.astronomical unit (AU)The mean distance between the Earth and Sun, equal to 214.94 solar radii or 1.496E+11m.atmosphereThe gaseous envelope surrounding a planet or star.attitude controlSpacecraft subsystem capable of pointing the spacecraft toward a selected target.auroraA sporadic, faint visual phenomenon associated with geomagnetic activity that occurs mainly in the high-latitude night sky. Auroras occur within a band of latitudes known as the auroral oval, the location of which is dependent on geomagnetic activity. Auroras are a result of collisions between atmospheric gases and precipitating charged particles (mostly electrons) guided by the geomagnetic field from the magnetotail. Each gas (oxygen and nitrogen molecules and atoms) gives out its own particular color when bombarded, and atmospheric composition varies with altitude. Since the faster precipitating particles penetrate deeper, certain auroral colors originate preferentially from certain heights in the sky. The auroral altitude range is 80 to 1000 km, but typical auroras are 100 to 250 km above the ground; the color of the typical aurora is yellow-green, from a specific transition of atomic oxygen. Auroral light from lower levels in the atmosphere is dominated by blue and red bands from molecular nitrogen and molecular oxygen. Above 250 km, auroral light is characterized by a red spectral line of atomic oxygen. To an observer on the ground, the combined light of these three fluctuating, primary colors produces an extraordinary visual display. Auroras in the Northern Hemisphere are called the aurora borealis or “northern lights.” Auroras in the Southern Hemisphere are called aurora australis. The patterns and forms of the aurora include quiescent arcs, rapidly moving rays and curtains, patches, and veils.auroral ovalAn elliptical band around each geomagnetic pole ranging from about 75 degrees magnetic latitude at local noon to about 67 degrees magnetic latitude at midnight under average conditions. It is the locus of those locations of the maximum occurrence of auroras and widens to both higher and lower latitudes during the expansion phase of a magnetic substorm.

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- B -

B-angle
As viewed from the Earth, the heliographic latitude of the center of the solar disk. The center of the solar disk usually does not coincide with the heliographic equator, due to a tilt of the solar axis with respect to the ecliptic. (See Bo under solar coordinates.)
Bartels’ rotation number
The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartels to begin in January 1833, and the count has continued by 27-day intervals to the present. (For example, rotation 2000 began on 12 November 1979, rotation 2030 on 30 January 1982.) The 27-day period was selected empirically from the observed recurrence of geomagnetic activity attributed to co-rotating features on the Sun. The Sun has an average rotation period (as seen from the Earth) of 27.27 days; therefore, solar longitude slowly drifts with respect to the Bartels rate. Compare Carrington longitude.
bipolar magnetic region
A region of the solar photosphere containing at least two areas of enhanced magnetic fields of opposing polarity.
blackbody temperature
The temperature of an object if it is reradiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature.
bolometer
A bolometer is a device for measuring incident electromagnetic radiation. It was invented in 1878 by the American astronomer Samuel Pierpont Langley.
bow shock
A collisionless shock wave in front of a planetary magnetosphere; the place where the supersonic flow of the solar wind is slowed to subsonic speed by the planetary magnetic field.
burst
A transient enhancement of the solar radio emission, usually associated with an active region or flare.
bus module
That portion of the spacecraft not including the science instruments but supporting their operation. The bus module includes the power system, radio system, computer and data handling, and attitude control system.
butterfly diagram
A plot of observed solar active region latitudes vs. time. This diagram, which resembles a butterfly, shows that the average latitude of active region formation drifts from high to low latitudes during a sunspot cycle.

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- C -

calcium K
A narrow wavelength of blue light (393.3 nm) which is emitted and absorbed by ions of the element calcium.
Carrington longitude
A system of fixed solar longitudes rotating at a uniform synodic period of 27.2753 days (a sidereal period of 25.38 days). Carrington selected the meridian that passed through the ascending node of the Sun’s equator at 1200 UTC on 1 January 1854 as the original prime meridian. The daily Carrington longitude of the central point of the apparent solar disk is listed (with other solar coordinates) in the Astronomical Almanac published annually by the U.S. Naval Observatory. Compare Bartels’ rotation number.
centimeter burst
A solar radio burst in the centimeter wavelength range (1 to 10 cm or 0.01 to 0.1 m), or 30 000 to 3000 MHz in the frequency range.
chromosphere
The layer of the solar atmosphere above the photosphere and beneath the transition region and the corona. The chromosphere is the source of the strongest lines in the solar spectrum, including the Balmer alpha line of hydrogen and the H and K lines of calcium, and is the source of the red (chromium) color often seen around the rim of the Moon at total solar eclipses.
continuum
Optical radiation arising from broadband emission from the photosphere.
convection
The bulk transport of plasma (or gas) from one place to another, in response to mechanical forces (for example, viscous interaction with the solar wind) or electromagnetic forces.
corona
The outermost layer of the solar atmosphere, characterized by low densities (<10E+9 per cubic cm or 10E+15 per cubic m and high temperatures (>10E+6 K). It is not visible from the Earth except during a total eclipse of the Sun or by use of a special telescope called a coronagraph.
coronagraph
An optical device that makes it possible to observe the corona at times other than during an eclipse. A simple lens focuses an image of the Sun onto an occulting disk that prevents the light from the solar disk from proceeding farther along the optical path, effectively providing an artificial eclipse.
coronal hole
An extended region of the corona, exceptionally low in density and associated with unipolar photospheric regions having “open” magnetic field topology. Coronal holes are largest and most stable at or near the solar poles, and are a source of high-speed solar wind. Coronal holes are visible in several wavelengths, including solar x-rays and the He 1083 nm emission line.
coronal loops
A typical structure of enhanced corona observed in EUV lines and soft x-rays. They are sometimes related to H alpha loops. Coronal loops represent “closed” magnetic topology.
coronal mass ejection (CME)
A transient outflow of plasma from or through the solar corona. CMEs are often but not always associated with erupting prominences, disappearing solar filaments, and flares.
coronal rain
Material condensing in the corona and appearing to rain down into the chromosphere as observed in H alpha at the solar limb above strong sunspots.
coronal streamer
A large-scale structure in the white-light corona often overlying a principal inversion line in the solar photospheric magnetic fields. (See helmet streamer.)
corrected geomagnetic coordinates
A nonspherical coordinate system based on a magnetic dipole axis that is offset from the Earth’s center by about 450 km toward a location in the Pacific Ocean (15.6 N 150.9 E). This “eccentric dipole” axis intersects the surface at 81N 85 W, and 75 S 120 E.
cosmic ray
Electromagnetic radiation of extremely high frequency and energy; cosmic rays usually interact with the atoms of the atmosphere before reaching the surface of the Earth; some cosmic rays come from outside the solar system while others are emitted from the Sun and pass through holes in the corona.

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differential rotation
The change in solar rotation rate with heliographic latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees/day) than do high latitudes (approx. 12 degrees/day).
disk
The visible surface of the Sun (or any heavenly body) projected against the sky.
disappearing solar filament (DSF)
A solar filament (prominence) that disappears suddenly (on a time scale of minutes to hours). The prominence material is often seen to ascend but is also seen to fall into the Sun or just fade. (Historically, DSFs have been called disparitions brusques because they were first studied by French astronomers.) DSFs are a possible indicator of coronal mass ejections.
Doppler shift
A change in the perceived frequency of a radiated signal caused by motion of the source relative to the observer.
dual tranceivers
Redundant radio system capable of receiving commands and sending data.

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eclipse
The obscuring of one celestial body by another. (1) A Solar Eclipse occurs when the Moon comes between the Earth and the Sun. In a total eclipse, the solar disk is completely obscured; in a partial eclipse the solar disk is only partly obscured. An annular eclipse occurs when the Moon is near its apogee and the apparent diameter of the Moon is less than that of the Sun so that the Sun is never completely obscured. “First and last contacts” are defined as the times of tangency of the solar and lunar disks. A central eclipse (which can be total or annular) has two additional times of tangency: “second contact,” when maximum eclipse begins, and “third contact,” when it ends. The last glimpses of the Sun through the lunar valleys, just before second contact, are known as Baily’s beads. (2) A lunar eclipse occurs when the Moon enters the shadow cast by the Earth. (3) Spacecraft in the Earth’s shadow are said to be in eclipse.
ecliptic
The great circle made by the intersection of the plane of the Earth’s orbit with the celestial sphere. (Less properly, the apparent path of the Sun around the sky during the year.)
electrical substitution radiometer (ESR)
Radiation sensor that maintains a constant temperature by supplying additional electrical heater power if light is removed from the sensor, and conversely removes such power when more light is added to the sensor.
emerging flux region (EFR)
An area on the Sun where new magnetic flux is erupting. An EFR is a bipolar magnetic region that first produces a small bipolar plage visible in the chromosphere, which may develop an arch filament system and the initial spots of a sunspot group. An EFR may be isolated from other solar activity or may occur within an active region.
emission line
In spectroscopy, a particular wavelength of emitted radiation, more intense than the background continuum.
Evershed effect
Horizontal motion of the solar atmosphere near a sunspot, having velocities of a few kilometers per second. In the photosphere, matter streams away from the umbra. In the chromosphere, the direction of flow is toward the umbra.
extreme ultraviolet (EUV)
A portion of the electromagnetic spectrum from approximately 10 to 100 nm.

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- F -

F corona
Of the white-light corona (the corona seen by the eye at a total solar eclipse), that portion which is caused by sunlight scattered or reflected by solid particles (dust) in interplanetary space. The same phenomenon produces zodiacal light.
F region
The upper region of the ionosphere, above approximately 160 km altitude. F region electron densities are highly variable, depending on the local time, solar activity, season, and geomagnetic activity.
facula
(White-light plage). A bright region of the photosphere seen in white light, seldom visible except near the solar limb. Corresponds with concentrated magnetic fields that may presage sunspot formation.
facular brightening
Brighter faculae seen against the quiet Sun – especially obvious when faculae are near the limb (edge) of the Sun.
Fery Prism
A wedge of glass that disperses light into its component colors, but with the additional feature that one face is slightly concave and the other convex so that it focuses the light as well.
fibril
A linear feature in the H alpha chromosphere of the Sun, occurring near strong sunspots and plages or in filament channels. Fibrils parallel strong magnetic fields, as if mapping the field direction.
filament
A mass of gas suspended over the chromosphere by magnetic fields and seen as dark ribbons threaded over the solar disk. A filament on the limb of the Sun seen in emission against the dark sky is called a prominence. Filaments occur directly over magnetic-polarity inversion lines, unless they are active.
filament channel
A broad pattern of fibrils in the chromosphere, marking a portion of a magnetic polarity inversion line where a filament may soon form or where a filament recently disappeared. Filament channels interconnect separate filaments and active regions on a common inversion line.
flare
A sudden eruption of energy in the solar atmosphere lasting minutes to hours, from which radiation and particles are emitted.
fluence
Time-integrated flux.
flux
The rate of flow of a physical quantity through a reference surface.
follower spot
In a magnetically bipolar or multipolar sunspot group, the main spot in that portion of the group east of the principal inversion line is called the follower or f-spot. Leader and follower describe the positions of spots with respect to apparent motion due to solar rotation. (Compare leader spot.)
Fraunhofer spectrum
The system of dark lines superposed on the continuous solar spectrum formed by the absorption of photons by atoms and molecules in the solar and terrestrial atmospheres.

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- G -

geocorona
The outer region of the Earth’s atmosphere lying above the thermosphere and composed mostly of hydrogen.
geomagnetic activity
Natural, short-term variations and disturbances in the geomagnetic field, due to interactions of the geomagnetic field and the magnetosphere with the solar wind and other energetic products of solar activity.
geomagnetic field
The magnetic field in and around the Earth. The intensity of the magnetic field at the Earth’s surface is approximately 32,000 nT at the equator and 62,000 nT at the north pole (the place where a compass needle points vertically downward). The geomagnetic field is dynamic and undergoes continual slow secular changes as well as short-term disturbances. (See geomagnetic activity.) The geomagnetic field can be approximated by a centered dipole field, with the axis of the dipole inclined to the Earth’s rotational axis by about 11.5 degrees. Geomagnetic dipole north is near geographic coordinate 78.3 N 69 W (Thule, Greenland), and dipole south is near 79 S 110 E (near Vostok, Antarctica). The observed or dip poles, where the magnetic field is vertical to the Earth’s surface, are near 76 N 101 W, and 66 S 141 E. The adopted origin of geomagnetic longitude is the meridian passing through the geomagnetic poles (dipole model) and the geographic south pole.
geomagnetic storm
A worldwide disturbance of the Earth’s magnetic field, distinct from regular diurnal variations.
geosynchronous
Term applied to any equatorial satellite with an orbital velocity equal to the rotational velocity of the Earth. The geosynchronous altitude is near 6.6 Earth radii (approximately 36000 km above the Earth’s surface). To be geostationary as well, the satellite must satisfy the additional restriction that its orbital inclination be exactly zero degrees. The net effect is that a geostationary satellite is virtually motionless with respect to an observer on the ground.
global average albedo
The average fraction of incident radiation (as light) that is reflected by the Earth’s surface and atmosphere.
granulation
Cellular structure of the photosphere visible at high spatial resolution. Individual granules, which represent the tops of small convection cells, are 200 to 2000 km in diameter and have lifetimes of 8 to 10 minutes.
green line
A coronal emission line at 530.3 nm from Fe XIV (an iron atom from which 13 electrons have been stripped). The green line is one of the strongest (and first-recognized) visible coronal lines. It identifies moderate-temperature regions of the corona; it is enhanced in coronal streamers above inversion lines, and diminished in coronal holes.
greenhouse effect
Increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere (carbon dioxide is the major factor).
ground station
A radio receiving and transmitting station capable of communicating with a satellite as it passes overhead.

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- H -

H alpha
The first atomic transition in the hydrogen Balmer series; wavelength = 656.3 nm. This absorption line of neutral hydrogen falls in the red part of the visible spectrum and is convenient for solar observations. The H alpha line is universally used for patrol observations of solar flares, filaments, prominences, and the fine structure of active regions.
Hale boundary
A large-scale magnetic inversion line of a particular magnetic orientation in the solar photosphere or across a sector boundary in the solar wind. If the polarity of the western (leading) side of the boundary is the same as that of the nearer solar pole at the start of a sunspot cycle, the boundary is said to be “Hale.” If the polarity is opposite, the boundary is “anti-Hale.” At the beginning of Cycle 22 (1987), the northern solar pole was negative; therefore in the northern hemisphere a Hale boundary separates a leading negative polarity region from a following positive one. The boundary between the leader spot and follower spot of a typical sunspot group in either hemisphere is a Hale boundary.
heliographic
Referring to coordinates on the solar surface referenced to the solar rotational axis. See solar coordinates.
heliopause
The boundary surface between the solar wind and the external galactic medium.
heliosphere
The magnetic cavity surrounding the Sun, carved out of the galaxy by the solar wind.
helmet streamer
A feature of the white light corona (seen in eclipse or with a coronagraph) that looks like a ray extending away from the Sun out to about 1 solar radius, having an arch-like base containing a cavity usually occupied by a prominence.
Hertzsprung-Russell diagram
Plot of stars’ absolute magnitude plotted against their surface temperature or color, used to study stellar evolution.
high-speed stream
A feature of the solar wind having velocities exceeding approximately 600 km/s (about double average solar wind values). High-speed streams that originate in coronal holes are less dense than those originating in the average solar wind.
homologous flares
Solar flares that occur repetitively in an active region, with essentially the same position and with a common pattern of development.
Hyder flare
A filament-associated two-ribbon flare, often occurring in spotless regions. The flare is generally slow (30-60 minutes rise time in H alpha and x-ray) and follows the disappearance of a quiescent filament. The flare presumably results from the impact on the chromosphere of infalling filament material. The Hyder flare is named for Dr. C. Hyder, who published studies of such flares in 1967.
Hydrogen Lymann-alpha emission
Spectral line with energy corresponding to the transition of the hydrogen atom from its ground energy state to its lowest excited state.

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- I -

IDL .sav file
A file created in a binary format by IDL. IDL .sav files can only be read with an IDL procedure (.pro file).

index of refractionRatio of the speed of light in a vacuum to the speed of light in another medium – an indication of how much a ray of light is bent as it passes from one medium into vacuum.infraredThe portion of the electromagnetic spectrum (light) between visible light and microwaves, invisible to the human eye.instrument module (IM)Portion of the SORCE spacecraft that includes the four scientific instruments and their support electronics and thermal control systems. The IM serves as an optical bench aligning all instruments to a common axis.integral particle fluxThe integral directional particle flux J(E,w) is literally the mathematical integral, with respect to the energy E, of the differential particle flux j(E,w). It denotes the number of particles of energy equal to or greater than E, per unit area, per unit solid angle, per unit time, passing through an area perpendicular to the viewing direction.interplanetary magnetic fieldThe magnetic field carried with the solar wind.inversion lineThe locus of points on the solar surface where the radial magnetic field vanishes. Inversion lines separate regions of opposing polarity and are often superposed by thin, dark filaments, which can be used as tracers. Inside active regions, the areas close to and along inversion lines are preferred places of flare occurrence. Filament channels, plage corridors, arch filament systems, and fibril patterns surrounding active regions can be used to infer the positions of inversion lines.ionAn atom or molecular fragment that has a positive electrical charge due to the loss of one or more electrons; the simplest ion is the hydrogen nucleus, a single proton.ionosphereThe region of the Earth’s upper atmosphere containing free electrons and ions produced by ionization of the constituents of the atmosphere by solar ultraviolet radiation at very short wavelengths (<100 nm) and energetic precipitating particles.ionospheric stormA disturbance in the F region of the ionosphere, which occurs in connection with geomagnetic activity. In general, there are two phases of an ionospheric storm, an initial increase in electron density (the positive phase) lasting a few hours, followed by a decrease lasting a few days. At low latitudes only the positive phase is usually seen. Individual storms can vary, and their behavior depends on geomagnetic latitude, season, and local time. The phases of an ionospheric storm are not related to the initial and main phases of a geomagnetic storm.

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- K -

K corona
Of the white-light corona (that is, the corona seen by the eye at a total solar eclipse), that portion which is caused by sunlight scattered by electrons in the hot outer atmosphere of the Sun. This is the “true” corona. Coronagraphs are specifically constructed to separate the K corona from the F corona.

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- L -

leader spot
In a magnetically bipolar or multipolar sunspot group, the main spot in that portion of the group west of the principal inversion line; also called the preceding or p-spot. Leader and follower describe the positions of spots with respect to apparent motion due to solar rotation. (Compare follower spot.)
light bridge
Observed in white light, a bright tongue or streaks penetrating or crossing sunspot umbrae. Light bridges typically develop slowly and have lifetimes of several days. The appearance of a light bridge is frequently a sign of impending active region division or dissolution. The more brilliant forms occur with overlying bright plage and often occur during the most active phase of the sunspot group.
limb
The outer edge of the apparent disk of a celestial body; the edge of the solar disk, corresponding to the level at which the solar atmosphere becomes transparent to visible light.
limb darkening
For certain solar spectral lines, a lessening of the intensity of the line from the center of the solar disk to the limb, caused by the existence of a temperature gradient in the Sun and the line-of-sight through the solar atmosphere.
loop prominence system
A system of prominences in the form of loops associated with major flares, bridging the magnetic inversion line. The lifetime of an LPS is a few hours. Loop prominences observed in H alpha are distinctly brighter than other prominences, and material typically flows downward along both legs from condensation “knots” near the top of the loop. LPSs show a high correlation with proton flares.
low Earth orbit (LEO)
An orbital altitude typically around 350 – 1400 km above the Earth’s surface.

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- M -

magnetograph
A special telescope which analyzes the color and polarization of sunlight in order to measure the magnetic field of the Sun.
magnetometer
A device for measuring the magnitude and direction of the Earth’s magnetic field.
magnetopause
The boundary surface between the solar wind and the magnetosphere, where the pressure of the magnetic field of the object effectively equals the dynamic pressure of the solar wind.
magnetosheath
The region between the bow shock and the magnetopause, characterized by very turbulent plasma. For the Earth, along the Sun-Earth axis, the magnetosheath is about 2 Earth radii thick.
magnetosphere
The magnetic cavity surrounding a magnetized body, carved out of the passing solar wind by virtue of the magnetic field, which prevents, or at least impedes, the direct entry of the solar wind plasma into the cavity.
magnetotail
The extension of the magnetosphere in the antisunward direction as a result of interaction with the solar wind. In the inner magnetotail, the field lines maintain a roughly dipolar configuration. But at greater distances in the antisunward direction, the field lines are stretched into northern and southern lobes, separated by a plasmasheet. There is observational evidence for traces of the Earth’s magnetotail as far as 1000 Earth radii downstream.
Maunder minimum
An approximately 70-year period, centered near 1670, during which practically no sunspots were observed.
mesosphere
The region of the Earth’s atmosphere between the upper limit of the stratosphere (approximately 30 km altitude) and the lower limit of the thermosphere (approximately 80 km altitude). In the mesosphere, temperature decreases with height.
Mount Wilson magnetic classification
Classification of the magnetic character of sunspots according to rules set forth by the Mount Wilson Observatory in California.

  • alpha. A unipolar sunspot group.
  • beta. A sunspot group having both positive and negative magnetic polarities (bipolar), with a simple and distinct division between the polarities.
  • gamma. A complex active region in which the positive and negative polarities are so irregularly distributed as to prevent classification as a bipolar group.
  • beta-gamma. A sunspot group that is bipolar but which is sufficiently complex that no single, continuous line can be drawn between spots of opposite polarities.
  • delta. A qualifier to magnetic class (see below) indicating that umbrae separated by less than 2 degrees within one penumbra have opposite polarity.
  • beta-delta. A sunspot group of general beta magnetic classification but containing one (or more) delta spot(s).
  • beta-gamma-delta. A sunspot group of beta-gamma magnetic classification but containing one (or more) delta spot(s).
  • gamma-delta. A sunspot group of gamma magnetic classification but containing one (or more) delta spot(s).

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network
Chromospheric: a large-scale brightness pattern in chromospheric (see chromosphere) and transition region spectral lines, which is located at the borders of the photospheric (see photosphere) supergranulation and coincides with regions of local magnetic enhancement. These cellular patterns are typically 30000 km across. (2) Photospheric: a bright pattern that appears in spectroheliograms in certain Fraunhofer spectrum lines. It coincides in gross outline with the chromospheric network.
neutrino
A fundamental particle with neutral charge and near-zero mass, supposedly produced in massive numbers by the nuclear reactions in stars; they are very hard to detect since the vast majority of them pass completely through the Earth without interacting.
nickel phosphorous (NiP)
Metallic black absorbing layer, stable under ultraviolet exposure.
nuclear fusion
A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones; the difference in mass is converted to energy by Einstein’s famous equivalence E=mc^2; this is the source of the Sun’s energy and therefore ultimately of (almost) all energy on Earth.

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- P -

Pegasus
Orbital Sciences Corporation rocket vehicle that is carried aloft by an airplane (L-1011) and is then released. After a short fall the motor ignites and propels the payload to orbit.
penumbra
The sunspot area that may surround the darker umbra or umbrae. In its mature form it consists of linear bright and dark elements radial from the sunspot umbra.
perigee
That point on the orbit of an Earth-orbiting satellite nearest to the Earth. Compare apogee.
perihelion
That point on the orbit of a Sun-orbiting body nearest to the Sun. Compare aphelion.
photodiode
a photoelectric semiconductor device for detecting and often measuring radiant energy (as light)
photometer
An instrument for measuring the luminous intensity of a light source.
photosphere
The lowest visible layer of the solar atmosphere; corresponds to the solar surface viewed in white light. Sunspots and faculae are observed in the photosphere.
plage
On the Sun, an extended emission feature of an active region that is seen from the time of emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background. Magnetic fields are more intense in plage, and temperatures are higher than in surrounding, quiescent regions.
plage corridor
A low-intensity division in chromospheric (see chromosphere) plage coinciding with a polarity inversion line and marked by narrow filament segments and/or fibrils spanning the corridor.
plasma
A gas that is sufficiently ionized so as to affect its dynamical behavior.
plasmapause
The outer surface of the plasmasphere.
plasmasheet
In the magnetosphere, the core of the magnetotail in which the plasma is hotter and denser than in the tail lobes north and south of it. The plasmasheet is thought to be separated from the tail lobes by the sheet of the “last closed field lines” and it typically lies beyond geosynchronous orbit.
plasmasphere
In the magnetosphere, a region of relatively cool (low energy) and dense plasma that may be considered an outer extension of the ionosphere with which it is coupled. Like the ionosphere, the plasmasphere tends to co-rotate with the Earth.
polar crown
A nearly continuous ring of filaments occasionally encircling either polar region of the Sun (latitudes higher than 50 degrees).
polar plumes
Fine, ray-like structures of the solar corona, best observed in the solar polar regions during solar minimum.
polarimeter
An instrument for measuring the angle of polarization of a polarized light source, or the proportion of polarized light in a partially polarized source.
pore
A feature in the photosphere, 1 to 3 arc seconds in extent, usually not much darker than the dark spaces between photospheric granules. It is distinguished from a sunspot by its short lifetime, 10 to 100 minutes.
precision
The quality of being exactly defined. Indicated by the minimum number of significant digits required for an adequat representation of a measurement.
prominence
A term identifying cloud-like features in the solar atmosphere. The features appear as bright structures in the corona above the solar limb and as dark filaments when seen projected against the solar disk. Prominences are further classified by their shape (for example, mound prominence, coronal rain) and activity. They are most clearly and most often observed in H alpha.
proton flare
Any flare producing significant counts of protons with energies exceeding 10 MeV in the vicinity of the Earth.

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- Q -

quiescent prominence
A long, sheet-like prominence nearly vertical to the solar surface. Except in an occasional activated phase, shows little large-scale motion, develops very slowly, and has a lifetime of several solar rotations. Quiescent prominences form within the remnants of decayed active regions, in quiet areas of the Sun between active regions, or at high solar latitudes where active regions seldom or never form. (See filament.)

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- R -

R-number
See sunspot number.
radiation belts
Regions of the magnetosphere roughly 1.2 to 6 Earth radii above the equator in which charged particles are stably trapped by closed geomagnetic field lines. There are two belts. The inner belt is part of the plasmasphere and corotates with the Earth; its maximum proton density lies near 5000 km. Inner belt protons are mostly high energy (MeV range) and originate from the decay of secondary neutrons created during collisions between cosmic rays and upper atmospheric particles. The outer belt extends on to the magnetopause on the sunward side (10 Earth radii under normal quiet conditions) and to about 6 Earth radii on the nightside. The altitude of maximum proton density is near 16000-20000 km. Outer belt protons are lower energy (about 200 eV to 1 MeV) and come from the solar wind. The outer belt is also characterized by highly variable fluxes of energetic electrons. The radiation belts are often called the “Van Allen radiation belts” because they were predicted in 1958 by a research group at the University of Iowa led by Professor J. A. Van Allen, and later detected by the satellite Explorer I.
radio emission
Emission of the Sun in radio wavelengths from centimeters to dekameters, under both quiet and disturbed conditions. Some patterns, known variously as noise storms, bursts, and sweeps, are identified as described below. These types of emission are subjectively rated on an importance scale of 1 to 3, 3 representing the most intense.

  • Type I. A noise storm composed of many short, narrow-band bursts in the meter wavelength range (300-50 MHz), of extremely variable intensity. The storm may last from several hours to several days.
  • Type II. Narrow-band emission (sweep) that begins in the meter range (300 MHz) and sweeps slowly (tens of minutes) toward dekameter wavelengths (10 MHz). Type II emissions occur in loose association with major flares and are indicative of a shock wave moving through the solar atmosphere.
  • Type III. Narrow-band bursts that sweep rapidly (seconds) from decimeter to dekameter wavelengths (500-0.5 MHz). They often occur in groups and are an occasional feature of complex solar active regions.
  • Type IV. A smooth continuum of broad-band bursts primarily in the meter range (300-30 MHz). These bursts occur with some major flare events; they begin 10 to 20 minutes after the flare maximum and can last for hours.
  • Type V. Short-duration (a few minutes) continuum noise in the dekameter range usually associated with Type III bursts.
reaction wheel
A massive wheel on a motor shaft that can be spun one direction or the other to impart angular momentum to the spacecraft, turning the spacecraft about the wheel’s axis of rotation.
reconnection
A process by which differently directed field lines link up, allowing topological changes of the magnetic field to occur, determining patterns of plasma flow, and resulting in conversion of magnetic energy to kinetic and thermal energy of the plasma. Reconnection is invoked to explain the energization and acceleration of the plasmas that are observed in solar flares, magnetic substorms, and elsewhere in the solar system.
red line
An intense coronal emission line at 637.4 nm from Fe X (an iron atom from which nine electrons have been stripped). It identifies relatively cooler regions of the corona.
redundant
A second (or third) system that can substitute for the primary system if the primary fails.
relative accuracy
An estimate of how well two measurements compare to each other. Systematic errors in the two measurements will cancel, and the relative accuracy will therefore be better than the absolute accuracy.
RI
The international standard relative sunspot number.

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- S -

S-band
radio frequencies from 1.55 GHz to 3.9 GHz.
S component
The slowly varying (weeks or longer) fluctuation observed in solar radio emission at microwave frequencies (wavelengths from 3 to 100 cm).
sector boundary
In the solar wind, the area of demarcation between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward the Sun (a negative sector), or away from the Sun (a positive sector). The sector boundary separating fields of opposite polarity is normally narrow, passing the Earth within minutes to hours as opposed to the week or so needed for passage of a typical sector. The solar wind velocities in the boundary region are typically among the lowest observed.
sidereal
Referring to a coordinate system fixed with respect to the distant stars.
simultaneous flares
Unrelated solar flares that occur at nearly the same time. Compare sympathetic flares.
smoothed sunspot number
An average of 13 monthly RI numbers, centered on the month of concern. The 1st and 13th months are given a weight of 0.5.
soft X-ray (XUV)
Photon radiation with wavelength of 1 to 200 nm.
solar activity
Transient perturbations of the solar atmosphere as measured by enhanced x-ray emission, typically associated with flares.
solar array
Area covered by photovoltaic cells that collects solar energy and converts it to spacecraft electrical power.
solar constant
The total radiant energy received vertically from the Sun, per unit area per unit of time, at a position just outside the Earth’s atmosphere when the Earth is at its average distance from the Sun. Radiation at all wavelengths from all parts of the solar disk is included. Its value is approximately 2.00 cal/sq cm/min = 1.37 kW/sq m and it varies slightly (by approximately 0.l%) from day to day in response to overall solar features.
solar coordinates
Specifications for a location on the solar surface. The location of a specific feature on the Sun (for example, a sunspot) is complicated by the fact that there is a tilt of 7.25 degrees between the ecliptic plane and the solar equatorial plane as well as a true wobble of the solar rotational axis. (Only twice a year are the solar north pole and the celestial north pole aligned.) Consequently, to specify a location on the solar surface, three coordinates (P, B, L) are necessary to define a grid. Daily values for the coordinates in Coordinated Universal Time (UTC) are listed in the Astronomical Almanac published annually by the U.S. Naval Observatory. The terms used to refer to the coordinates are defined as follows:

  • P-angle (or P). The position angle between the geocentric north pole and the solar rotational north pole measured eastward from geocentric north. The range in P is +/- 26.3l degrees.
  • Bo. Heliographic latitude of the central point of the solar disk; also called the B-angle. The range of Bo is +/- 7.23 degrees, correcting for the tilt of the ecliptic with respect to the solar equatorial plane. Example: If (P,Bo) = (-26.21 degrees, -6.54 degrees), the heliographic latitude of the central point on the solar disk is -6.54 degrees (the north rotational pole is not visible), and the angle between the projection onto the disk of the geocentric north pole and the solar north rotational pole is 26.21 degrees to the west.
  • Lo. Heliographic longitude of the central point of the solar disk. The longitude value is determined with reference to a system of fixed longitudes rotating on the Sun at a rate of 13.2 degrees /day (the mean rate of rotation observed from central meridian transits of sunspots). The standard meridian on the Sun is defined to be the meridian that passed through the ascending node of the Sun’s equator on 1 January 1854 at 1200 UTC and is calculated for the present day by assuming a uniform sidereal period of rotation of 25.38 days.
  • Once P, Bo, and Lo are known, the latitude, central meridian distance, and longitude of a specific solar feature can be determined as follows:
    • Latitude. The angular distance from the solar equator, measured north or south along the meridian.
    • Central meridian distance (CMD). The angular distance in solar longitude measured from the central meridian. This position is relative to the view from Earth and will change as the Sun rotates; therefore, this coordinate should not be confused with heliographic positions that are fixed with respect to the solar surface.
    • Longitude. The angular distance from a standard meridian (0 degrees heliographic longitude), measured from east to west (0 degrees to 360 degrees) along the Sun’s equator. It is computed by combining CMD with the longitude of the central meridian at the time of the observation, interpolating between ephemeris values (for 0000 UT) by using the synodic rate of solar rotation (27.2753 days, 13.2 degrees per day).
solar cycle
See sunspot cycle.
solar maximum
The month(s) during the sunspot cycle when the smoothed sunspot number reaches a maximum. A recent solar maximum occurred in December 1979.
solar minimum
The month(s) during the sunspot cycle when the smoothed sunspot number reaches a minimum. A recent solar minimum occurred in September 1986.
solar rotation rate
(1) synodic: l3.39 degrees -2.7 degrees sin^2 (solar latitude)/day. (2) sidereal: 14.38 degrees -2.7 sin^2 (solar latitude)/day. The difference between sidereal and synodic rates is the Earth orbital motion of 0.985 degrees/day.
solar sector boundary (SSB)
The boundary between large-scale unipolar magnetic regions on the Sun’s surface, as determined from inversion lines mapped using filaments and filament channels, or large-scale magnetograms. The supposed solar signature of an interplanetary sector boundary.
solar wind
The outward flow of solar particles and magnetic fields from the Sun. Typically at 1 AU, solar wind velocities are near 375 km/s and proton and electron densities are near 5 per cubic centimeter. The total intensity of the interplanetary magnetic field is nominally 5 nT.
South Atlantic anomaly (SAA)
A region of the Earth centered near 25 degrees S 50 degrees W (geographic coordinates, near the Atlantic coast of Brazil) of low geomagnetic field intensity owing to the fact that the geomagnetic field axis is offset from the center of the Earth (see corrected geomagnetic coordinates.) One consequence of the SAA is that trapped particles in the plasmasphere drift closer to the Earth’s surface and can more easily be lost into the atmosphere. The result is that the F region (see ionosphere) is highly variable in this region, and satellites in low Earth orbits suffer greater radiation doses when they pass through the SAA. There is a corresponding location of maximum geomagnetic field intensity in Southeast Asia.
spectral solar irradiance (SSI)
Solar energy per unit time over a unit area perpendicular to the Sun’s rays at the top of Earth’s atmosphere at a particular wavelength.
spectral type (OBAFGKM)
Star classification according to its surface temperature (color). The Sun is spectral type G.
spectrograph
A spectroscope provided with a photographic camera or other device for recording the spectrum.
spectroheliogram
A photograph of the Sun in one spectral band.
spectroheliograph
An instrument used to photograph the Sun in one spectral band.
spectrometer
A spectroscope equipped with a photoelectric photometer to measure radiant intensities at various wavelengths.
spectroscope
An optical instrument consisting of a slit, collimator lens, grating (or prism), and a telescope or objective lens which produces a spectrum for visual observation.
spectroscopy
The branch of physics concerned with the production, measurement, and interpretation of electromagnetic spectra arising from either emission or absorption of radiant energy by various substances.
spicules
Rapidly changing, predominantly vertical, spike-like structures in the solar chromosphere observed above the limb. Spicules appear to be ejected from the low chromosphere at velocities of 20 to 30 km/s reaching a height of about 9000 km and then falling back or fading. The total lifetime is 5 to 10 minutes.
spray
Luminous material ejected from a solar flare with sufficient velocity to escape the Sun (675 km/s). Sprays are usually seen in H alpha with complex and rapidly changing form. There is little evidence that sprays are focused by magnetic fields. Compare surge.
star tracker
An optical device that when pointed to a star or arbitrary star field determines the direction of its line-of-sight.
stratosphere
That region of the Earth’s atmosphere between the troposphere and the mesosphere. It begins at an altitude of temperature minimum at approximately 13 km and defines a layer of increasing temperature up to about 30 km.
substorm
A geomagnetic perturbation lasting 1 to 2 hours, which tends to occur during local post-midnight nighttime. The magnitude of the substorm is largest in the auroral zone, potentially reaching several thousand nanoteslas. A substorm corresponds to an injection of charged particles from the magnetotail into the auroral oval.
sun sensor
An optical device that when pointed at the Sun determines the direction of its line-of-sight toward the Sun’s center.
sunspot
An area seen as a dark spot, in contrast with its surroundings, on the photosphere of the Sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere. Larger and darker sunspots sometimes are surrounded (completely or partially) by penumbrae. The dark centers are umbrae. The smallest, immature spots are sometimes called pores.
sunspot cycle
The approximately 11-year quasi-periodic variation in the sunspot number. The polarity pattern of the magnetic field reverses with each cycle. Other solar phenomena, such as the 10.7-cm solar radio emission, exhibit similar cyclical behavior.
sunspot number
A daily index of sunspot activity (R), defined as R = k(10g +s) where s = number of individual spots, g = number of sunspot groups, and k is an observatory factor (equal to 1 for the Zurich Observatory and adjusted for all other observatories to obtain approximately the same R number). The standard number, RI, once derived at Zurich (see Wolf number), is now being derived at Brussels and is denoted by RI. Often, the term “sunspot number” is used in reference to the widely distributed smoothed sunspot number.
supergranulation
A system of large-scale velocity cells that does not vary significantly over the quiet solar surface or with phase of the solar cycle. The cells are presumably convective in origin with weak upward motions in the center, downward motions at the borders, and horizontal motions of typically 0.3 to 0.4 km/s. Magnetic flux is more intense along the borders of the cells.
Suprasil 300
Synthetic quartz glass free from bubbles and inclusions, characterized by a very high optical transmission in the UVand the visible spectral ranges. Suprasil 300 has the widest transmission range of all known grades of quartz.
surge
A jet of material from an active region that reaches coronal heights and then either fades or returns into the chromosphere along the trajectory of ascent. Surges typically last 10 to 20 minutes and tend to recur at a rate of approximately 1 per hour. Surges are linear and collimated in form, as if highly directed by magnetic fields. Compare spray.
sympathetic flares
Solar flares in different active regions that apparently occur as the common result of activation of a coronal connection between the regions. Compare simultaneous flares.
synodic
Referring to a coordinate system fixed on the Earth.
synoptic chart
A map of the whole Sun in absolute heliographic coordinates, displaying an integrated view of solar features observed during a Carrington rotation. (See Carrington longitude.)

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- T -

thermosphere
That region of the Earth’s atmosphere above the mesosphere where the neutral temperature increases with height. It begins at the mesopause at about 80-85 km and extends to the exosphere where the temperature regime becomes isothermal.
torque rod
Magnetic rod fixed to the spacecraft that when turned on couples to the Earth’s magnetic field and imparts a torque to turn the spacecraft. These can be used to control the attitude of a spacecraft.
total solar irradiance (TSI)
Solar energy per unit time over a unit area perpendicular to the Sun’s rays at the top of Earth’s atmosphere.
transition region
That region of the solar atmosphere lying between the chromosphere and the corona where the temperature rises from 10000 K to 1000000 K. The transition region is only a few thousand kilometers thick.
transverse
Component of magnetic field vector perpendicular to direction of view, e.g., of the solar magnetic field and parallel to the solar surface at disk center.
troposphere
The lowest layer of the Earth’s atmosphere, extending from the ground to the stratosphere at approximately 13 km of altitude. In the troposphere, temperature decreases with height.

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- U -

ultraviolet (UV)
Electromagnetic radiation at wavelengths shorter than the violet end of visible light (5 to 300 nm); the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.
umbra
The dark core or cores (umbrae) in a sunspot with penumbra, or a sunspot lacking penumbra.
unipolar magnetic region
A large-scale photospheric region where the magnetic elements are predominantly of one polarity (for example, the solar polar regions).

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- V -

 Van Allen radiation belts
See radiation belts.
visible radiation
Radiation detectable by the human eye, from 400 to 700 nanometers.

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- W -

white light
The sum of all visible wavelengths of light (400-700 nm) so that all colors are blended to appear white to the eye. No pronounced contribution from any one spectral line (or light-emitting element)is implied.
white-light flare
A major flare in which small parts become visible in white light. This rare continuum emission is caused by energetic particle beams bombarding the lower solar atmosphere. Such flares are usually strong x-ray, radio, and particle emitters.
wing
Portion of a spectroscopic absorption (or emission) line between the core of the line and the continuum adjacent to the line.
Wolf number
An historic term for sunspot number. In 1849, R. Wolf of Zurich originated the general procedure for computing the sunspot number. The record of sunspot numbers that he began has continued to this day.

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- X -

 x-ray
Electromagnetic radiation of very short wavelength (less than 1 nm) and very high energy; x-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays. “Soft” x-rays are those of energies less than 20 keV, or wavelengths longer than 0.05 nm.
x-ray background
A daily average background x-ray flux in the 0.1 to 0.8 nm range. It is a midday minimum given in terms of x-ray flare class.

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- Y -

yellow line
A coronal emission line at 569.4 nm from Ca XV (a calcium atom from which 14 electrons have been stripped). It identifies the hottest regions of the corona.

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- Z -

Zeeman effect
The splitting of spectral emission lines due to the presence of a strong magnetic field. Briefly, the lines split into three or more components of characteristic polarization; the components are circular if the local magnetic field is parallel to the line of sight, and linear if the field is perpendicular to the line of sight. The amount of splitting is proportional to the strength of the field.
zodiacal light
A diffuse band of luminosity occasionally visible on the ecliptic, it is sunlight diffracted and reflected by dust particles in the solar system within and beyond the Earth’s orbit.
Zurich sunspot number
See sunspot number.

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