Subtopics

You can jump to any letter in the alphabet: A B C D E F G H I K L M N O P R S T U V W X Y

Absorption Line: A region of low intensity at a particular wavelength in a spectrum, formed when a cool, tenuous gas between a hot radiating source and the observer absorbs electromagnetic radiation of that wavelength. The opposite effect is seen as an emission line.
Active Region: An area of the Sun where the magnetic fields are very strong. At ultraviolet and X-ray wavelengths they appear bright. In visible light they exhibit sunspots.
Angstrom: Abbreviated Å. A unit of length equal to 10^-8 cm (a one-hundred millionth of a centimeter). An Angstrom is about the size of an atom, and 10 Angstroms are equal to one nanometer.
Arc Degree: A unit of angular measure in which there are 360 arc degrees in a full circle.
Arc Minute: Abbreviated arcmin. A unit of angular measure in which there are 60 arc minutes in 1 arc degree The single apostrophe is usually used as the symbol for an arcminute. For example 3' means 3 arcminutes.
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. The double quotation mark is usually used as the symbol for an arcsecond. For example 3" means 3 arcseconds. When looking at the Sun from Earth, one arc second is equal to about 725 km on the Sun's surface.
Astronomical Unit: Often defined as the average distance between the Earth and Sun, about 150 million kilometers. But if you really want to impress (or bore) people, insist on the correct definition which is that the Astronomical Unit is the radius of a circular orbit in which an object of negligible mass, and free of other disturbances, would revolve around the Sun in 2pi/k days, where k is the Gaussian gravitational constant! Now you can see why people cheat and use the first "definition"!
Atom: The basic building block of all matter. All atoms consist of a nucleus containing protons and neutrons surrounded by different numbers of electrons. The different number of protons in the nucleus is what make the atoms appear as different elements (for example, Helium, Oxygen, Carbon).
Aurora: A display of coloured light given off by collisions between charged particles trapped in a planet's magnetic fields and atoms of atmospheric gases near the planet's magnetic poles. Aurora are visible on Earth as the aurora borealis or northern lights and the aurora australis or southern lights.
Black Hole: A region of space that has so much mass concentrated in it, and therefore such a strong gravitational pull, that there is no way for a nearby object, or even light, to escape the gravitational pull.
Blueshift: See Doppler Shift.
CDS: Coronal Diagnostic Spectrometer, or in other words it is a Spectrometer designed to Diagnose the solar Corona - so maybe it should have been called the SDC!
An ultraviolet spectrometer aboard SOHO, designed to study the solar spectrum from 18 to 80 nanometers. Many of the spectra and images illustrated on this CD-ROM were obtained with the CDS.
Celsius: Abbreviated C. A unit of temperature. Zero degrees Celsius is equal to 273 Kelvin. Also known as centigrade. Water freezes at 0° C and boils at 100° C.
CELIAS: Charge, Element, and Isotope Analysis System.
Instrument aboard SOHO which analyzes the constituents of the solar wind.
Centigrade: Abbreviated C. A unit of temperature. Zero degrees Centigrade is equal to 273 Kelvin. Also known as celsius. Water freezes at 0° C and boils at 100° C. In the SunBlock pages the distinction between Kelvin and Centigrade is usually ignored since temperatures cannot be measured accurately enough to make the difference important.
CGS: Centimeter-Gram-Second. 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.
CME: Short for Coronal Mass Ejection.
Conduction: The transfer of energy via collisions of randomly moving atoms and electrons.
Convection: The physical up-welling of hot matter, thus transporting energy from a lower, hotter region to a higher, cooler region. A bubble of gas that is hotter than its surroundings expands and rises. When it has cooled by passing on its extra heat to its surroundings, the bubble sinks again. Convection can occur when there is a substantial decrease in temperature with height, such as in the Sun's convection zone.
Convection Zone: A layer in a star in which convection is the main mechanism by which energy is transported outward. In the Sun a convection zone extends from just below the photosphere to about seventy percent of the solar radius.
Core: In solar astronomy, the innermost part of the Sun, where energy is generated by nuclear reactions.
Corona: The outermost layer of the solar atmosphere. The corona consists of a highly rarefied gas with a temperature greater than one million degrees Kelvin. It is visible to the naked eye during a solar eclipse.
Coronagraph: Telescope for observing the corona by producing an artificial eclipse. It contains an occulting disk which covers the disk of the Sun so that the faint corona may be more easily observed. Before coronagraphs were built astronomers had to wait for a natural solar eclipse to blot out the Sun's disk in order to let them study the corona.
Coronal Hole: An area of the corona which appears dark in X-rays and ultraviolet light. They are usually located at the poles of the Sun, but can occur other places as well. The magnetic field lines in a coronal hole extend out into the solar wind rather than coming back down to the Sun's surface as they do in other parts of the Sun.
Coronal Mass Ejection: A huge bubble of plasma that erupts from the Sun's corona and travels through space at high speed.
Coronal Streamer: Large scale magnetic structures observed in the Sun's corona.
Cosmic Ray: High energy charged particles traveling through interstellar space at nearly the velocity of light.
COSTEP: Comprehensive Suprathermal and Energetic Particle Analyzer.
Instrument aboard SOHO which analyzes high energy particles in the solar wind.
Deep Space Network: A NASA network of communication dishes used to communicate with spacecraft beyond Earth's orbit.
Density: The amount of mass or number of particles per unit volume. In cgs units mass density has units of gm cm^-3. Number density has units cm^-3 (particles per cubic centimeter).
Differential Rotation: The change in rotation rate with latitude. Solid bodies like the Earth do not show this effect but the giant planets (eg Jupiter and Saturn) and the Sun do because they are composed of gas. Low latitudes rotate at a faster angular rate. For the Sun the difference is such that low latitudes near the equator rotate at 14 degrees per day but the high latitudes at only 12 degrees per day.
Disk: The visible surface of the Sun (or any heavenly body) projected against the sky.
Doppler Shift: A change in the wavelength of radiation received from a source because of its motion along the line of sight. A Doppler shift in the spectrum of an astronomical object is commonly known as a redshift when the shift is towards longer wavelengths (the object is moving away) and as a blueshift when the shift is towards shorter wavelengths (the object is approaching).
Ecliptic: The plane of Earth's orbit around the Sun.
EIT: Extreme ultraviolet Imaging Telescope.
Telescope aboard SOHO which obtains images of the Sun at ultraviolet wavelengths. Many of the pictures featured on the CD-ROM were taken with the EIT.
Electromagnetic Radiation: Light we see is one kind of electromagnetic radiation - it has a special range of wavelengths that our eyes can respond to. In general, electromagnetic radiation is 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 and transports energy.
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 orbits the nucleus of an atom.
Emission Line: A bright section of a spectrum at a particular wavelength. Usually emitted by a hot gas.
Equator: The great circle on the surface of a body formed by the intersection of the surface of the body and the plane passing through the centre of the body at right angles to the axis of rotation.
ESA: The European Space Agency.
Erg: A cgs unit of energy equal to work done by a force of 1 dyne acting over a distance of 1 cm.
10⁷ (ten million) erg s^-1 (ergs per second) = 1 watt. Also, 1 Calorie = 4.2 × 10¹⁰ (42 billion) ergs.
ERNE: Energetic and Relativistic Nuclei and Electron experiment.
Instrument aboard SOHO which analyzes high energy nuclei and electrons in the solar wind.
Filament: A structure in the corona consisting of cool plasma supported by magnetic fields. Filaments are dark structures when seen against the bright solar disk, but appear bright when seen over the solar limb, Filaments seen over the limb are also known as prominences.
Flare (Solar): Rapid release of energy from a localized region on the Sun in the form of electromagnetic radiation, energetic particles, and mass motions.
Free Electron: An electron that has broken free of its 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.
Gauss: A unit of magnetic field strength. The Earth's magnetic field has a strength of about 0.5 Gauss at the surface.
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).
GOLF: Global Oscillations at Low Frequencies.
Helioseismology instrument aboard SOHO which analyzes the vibrational modes of the Sun.
Granule: A roughly circular region on the Sun whose bright center indicates hot gases rising to the surface, and whose dark edges indicate cooled gases that are descending towards the interior. Individual granules appear and disappear on time scales of about 5 minutes and are typically about 1000 km.
Helioseismology: The study of the interior of the Sun by the analysis of its natural modes of oscillation.
Heliosphere: The region around the Sun where the solar wind dominates over the interstellar medium.
Hertz: Abbreviated Hz. A unit of frequency equal to one cycle per second. One kHz = 1000 Hz. One MHz = 10⁶ (one million) Hz. One GHz = 10⁹ Hz.
Hydrogen Alpha: Light emitted at a wavelength of 656.3 nanometers from an atomic transition in hydrogen. This wavelength is in the red portion of the visible spectrum and is emitted in the solar chromosphere.
Infrared: The part of the electromagnetic spectrum whose radiation has longer wavelengths than optical radiation, but shorter wavelengths than radio waves . Because much infrared light is absorbed by the Earth's atmosphere, infrared astronomy is often performed in space.
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.
ISTP: International Solar-Terrestrial Physics Science Initiative.
Collaborative effort by US, European, and Japanese space agencies to obtain coordinated, simultaneous investigations of the Sun-Earth space environment over an extended period of time. SOHO is a part of this program.
Kelvin: Abbreviated K. A unit of absolute temperature. Zero degrees Celsius is equal to 273 Kelvin. Zero Kelvin is "absolute zero" - the coldest temperature possible.
Kilometer: Abbreviated km. 1 km = 1000 meters = 10⁵ cm = 0.62 mile.
Lagrange Points: The five "gravitational balance" points between two orbiting masses. The first Lagrange Point (L₁) is in between the two bodies. The SOHO spacecraft orbits the Sun at a place near the (L₁) point. It is often wrongly stated that the (L₁) point is where the gravitational pull of the Sun and the Earth balance. From your knowledge of the mass of the Sun and Earth and how the strength of gravity changes with distance can you show why that statement is wrong?
LASCO: Large Angle and Spectrometric Coronagraph.
Coronagraph aboard SOHO.
Latitude: A north-south coordinate measured on the surface of a sphere. It is the angular distance from the equator in the direction of one of the rotational poles.
Light Year: The distance light travels in 1 year (approximately 9.5 million million kilometers).
Limb: See Solar Limb.
Longitude: An east-west coordinate measured on the surface of a sphere.
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.
Magnetogram: A map showing the strength of the magnetic field in different locations.
Magnetosphere: Area around a planet in which the planet's magnetic field is stronger than the magnetic field carried by the solar wind.
MDI/SOI: Michelson Doppler Imager/Solar Oscillations Investigation.
Helioseismology instrument aboard SOHO which analyzes the vibrational modes of the Sun. Also measures the Sun's magnetic field in the photosphere.
Naked eye: Using only the human eye for observations. That is with no artificial help, eg binoculars or a telescope.
NASA: The National Aeronautics and Space Administration
Neutrino: An elementary particle with no charge and almost no mass. which interacts very weakly with other matter.
Neutron: An electrically neutral elementary particle. A neutron is 1839 times heavier than an electron.
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.
Orbiting Solar Observatories: A series of eight Sun observing satellites launched from 1962 to 1975.
Orbit: The path of an object revolving around another object or point.
Orbital Period: The amount of time it takes a spacecraft or other object to travel once around its orbit.
Parsec: A unit of distance equal to 3.16 light years. The name is derived from PARallax and SECond since one parsec is the distance at which the Earth's orbit subtends an angle of one arcsecond.
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 behaviour. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the plasma state.
Polar Plume: Bright structure of out-flowing gas which occur along magnetic field lines in coronal holes. These field lines extend into the solar system. Although plumes usually occur at the poles, they can appear anywhere there is a coronal hole.
Pole: The intersections of the surface of a body and its axis of rotation.
Prominence: A structure in the corona consisting of cool plasma supported by magnetic fields. Prominences are bright structures when seen over the solar limb, but appear dark when seen against the bright solar disk. Prominences seen on the disk are also known as filaments.
Proton: A positively charged elementary particle. A proton is 1836 times heavier than an electron.
Radiation: See Electromagnetic Radiation
Radiation Belt: A ring-shaped region around a planet in which electrically charged particles (usually electrons and protons) are trapped. The particles follow spiral trajectories around the direction of the magnetic field of the planet. The radiation belts surrounding Earth are known as the Van Allen belts.
Radiative Zone: An interior layer of the Sun, lying between the core and the convection zone, where energy travels outward by radiation.
Radio waves: The lowest energy (longest wavelength) photons in the electromagnetic spectrum.
Redshift: See Doppler Shift.
Skylab: A space station which orbited Earth in the 1970s. Performed many important observations of the Sun.
SOHO: The SOlar and Heliospheric Observatory. The largest and most comprehensive solar observatory ever put into space. It was launched in December 1995.
Solar Atmosphere: 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 Cycle: The approximately 11-year variation in frequency or number of sunspots, coronal mass ejections, solar flares, and other solar activity.
Solar Limb: The apparent edge of the Sun as it is seen in the sky.
Solar Maximum: The month(s) during the solar cycle when the number of sunspots reaches a maximum. The next solar maximum will occur in about the year 2000.
Solar Maximum Mission: A satellite dedicated to observing the Sun, especially solar flares, which occur more frequently during solar maximum. It was in orbit throughout the 1980s.
Solar Minimum: The month(s) during the solar cycle when the number sunspots is lowest. The most recent minimum occurred in 1996.
Solar Wind: A stream of particles, primarily electrons and protons, flowing outward from the Sun at speeds as high as 900 km/s. The solar wind is essentially the hot solar corona expanding into interplanetary and interstellar space.
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.
Spectrometer: An instrument that spreads light or other electromagnetic radiation into its component wavelengths (a spectrum), recording the results photographically or electronically. From that it is possible to measure the intensity of radiation as a function of wavelength.
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.
Streamer: See Coronal Streamer
SUMER: Solar Ultraviolet Measurements of Emitted Radiation.
An ultraviolet spectrometer aboard SOHO. A companion instrument to CDS, but which studied the solar spectrum at longer wavelengths (up to 160 nanometers).
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.
SWAN: Solar Wind Anisotropies.
Instrument aboard SOHO which analyzes large scale variations in the solar wind by observing radiation given off by neutral hydrogen.
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.
TRACE: The Transition Region and Coronal Explorer. Launched in April 1998, this satellite is providing some of the best images of the Sun's outer atmosphere ever seen.
Transition Region: The layer in the Solar Atmosphere located above the chromosphere and below the corona. In this region the temperature appears to rise dramatically from ten thousand Kelvin to over one million kelvin as you enter the lower corona. Recent observations from SOHO have found this region to be very dynamic in nature.
Ultraviolet: The part of the electromagnetic spectrum whose radiation has shorter wavelengths than optical radiation, but longer wavelengths than X-rays . Because ultraviolet light is absorbed by the Earth's atmosphere, ultraviolet astronomy is performed in space.
Universal Time: Abbreviated UT. A time system used internationally for recording the timing of events, particularly in astronomy. The same as Greenwich Mean Time (GMT).
UVCS: Ultraviolet Coronagraph Spectrometer.
An ultraviolet spectrometer aboard SOHO.
Van Allen Belts: Two ring-shaped regions that girdle the Earth's equator in which electrically charged particles are trapped by the Earth's magnetic field. See South Atlantic Anomaly and radiation belts.
VIRGO: Variability of Solar Irradiance and Gravity Oscillations.
Helioseismology instrument aboard SOHO which analyzes variations in electromagnetic radiation from the Sun.
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 colours 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.
Yohkoh: A satellite which observes X-rays from the Sun. Launched in 1991.