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 gaseous envelope surrounding a planet, or the visible layers of a star; also a unit of pressure(abbreviated atm) equal to the pressure of air at sea level on the Earth's surface.
A display of colored 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.
A theory of the evolution of the universe that postulates its origin, in an event called the Big Bang, from a hot, dense state that rapidly expanded to cooler, less-dense states.
A mass that has collapsed to such a degree that the escape velocity from its surface is greater than the speed of light, so that light is trapped by the intense gravitational field.
A decrease in the wavelength of the radiation emitted by an approaching celestial body as a consequence of the Doppler effect; a shift toward the short-wavelength (blue) end of the spectrum.
An imaginary sphere of very large radius centered on the earth, on which the celestial bodies appear fastened and against which their motions are charted.
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.
Degrees Fahrenheit =
Degrees Celsius *(9/5) +32.
Charge Coupled Device.
A semiconductor light detector which converts light to electrical
impulses. Such detectors are used by a number of the instruments on
SOHO, including EIT,
LASCO, and MDI.
A process by which one atom gives up electrons to a proton or another atom. For example, if a positively-charged proton encounters a neutral atom, the neutral atom may give up one of its electrons to the proton. The proton-electron pair would then be neutral, and the formerly neutral atom would be positively-charged.
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.
A large scale cellular pattern visible in
hydrogen-alpha and other
parts of the spectrum associated with the
chromosphere. The network
appears at the boundaries of
super-granulation cells
and contains a magnetic field which has been swept to the
edges of the cells by the flow of material in the cell.
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.
A layer in a star in which convection currents are the main mechanism by which energy is transported outward. In the Sun, a convection zone extends from just below the photosphere to about seventy percent of the solar radius.
The outermost region of the Sun's atmosphere, consisting of thin, ionized gases at a temperature of about 1,000,000 K. It is visible to the naked eye during a solar eclipse.
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.
Radiation with a blackbody spectrum at a temperature of about 2.7 K permeating the universe; believed to be the remains of the primeval fireball in which the Universe was created.
Charged atomic particles moving in space with very high energies (the particles travel close to the speed of light); most originate beyond the solar system, but some of low energy are produced in solar flares.
A supernova remnant, located in the constellation Taurus, produced by the supernova explosion visible from earth in 1054 CE; a pulsar in the nebula marks the neutron-star corpse of the exploded star.
In cosmology, the density that marks the transition from an open to a closed universe; the density that provides enough gravity to just bring the expansion to a stop after infinite time.
The lowest layer of the Earth's ionosphere. It is between about 50 and 95 kilometers above Earth's surface. This is the layer which reflects radio waves. Also called the D Layer.
An ionized gas in which nuclei and electrons are packed together as much as possible, filling all possible low energy states, so that the perfect gas law relating pressure, temperature, and density no longer applies.
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).
The change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).
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).
Electromagnetic force is one of the four forces of nature; particles with electromagnetic charge either attract or repel each other depending upon whether the two charges are opposite or identical.
A self-propagating electric and magnetic wave, such as light, radio, ultraviolet, or infrared radiation; all types travel at the same speed and can be differentiated by wavelength or frequency.
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 × 10-12erg.
Light of specific wavelengths or colors emitted by atoms; sharp energy peaks in a spectrum caused by downward electron transitions from a discrete quantum state to another discrete state.
Atoms with no charge that move very quickly. These atoms have equal numbers of positively-charged protons and negatively-charged electrons. ENAs form when charged particles from the solar wind travel outward and encounter atoms from the interstellar medium. Because the ENAs are neutral, they do not react to any magnetic fields. Some of these ENAs travel toward the inner solar system and are captured by the IBEX spacecraft.
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.
Bright regions of the photosphere visible in white light near the limb of the Sun. They are brighter than their surroundings because they are higher in temperature and density.
Abbreviated F. A unit of temperature. In the Fahrenheit scale, water freezes at 32 °F and boils at 212 °F. Degrees Celsius = (Degrees Fahrenheit - 32)*5/9.
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.
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 spectrum that appears immediately before the totality of a solar eclipse as the normal absorption spectrum is replaced briefly by the corona's own emission spectrum.
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.
A huge assembly of stars (between one hundred and one million), plus gas and dust, which is held together by gravity; the Galaxy, our own galaxy, containing the Sun.
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.
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).
Large interstellar clouds, with sizes up to tens of parsecs and containing 100,000 solar masses of material; found in the spiral arms of the Galaxy, giant molecular clouds are the sites of massive star formation.
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.
In Newtonian terms, a force between masses that is characterized by their acceleration toward each other; the magnitude of the force depends directly on the product of the masses and inversely on the square of the distance between them; in Einstein's terms, the curvature of space-time.
The place where the solar wind slows down and begins to interact with the interstellar medium. The heliosheath has a few parts: the termination shock (the innermost part of the boundary), the heliopause (the outermost part of the boundary) and the part in between the inner and outer boundary.
Stars in the Galaxy with velocities greater than 60 km/s relative to the Sun; they have orbits with high eccentricities, often at large angles with respect to the Galactic plane.
A description of the expansion of the Universe, such that the more distant a galaxy lies from us, the faster it is moving away; the relation, v = Hd, between the expansion velocity (v),and the distance (d) of a galaxy, where H is the Hubble constant.
The proportionality constant relating velocity and distance in the Hubble law; the value, now around 75 km/s/Mpc, changes with time as the Universe expands.
Also called H-alpha. Light emitted at a wavelength of 6563 ⋭ from an atomic transition in hydrogen. This wavelength is in the red portion of the visible spectrum and is emitted by plasma at about 10,000 K, mainly in the solar chromosphere.
The process by which ions are produced, typically occurring by collisions with atoms or electrons ("collisional ionization"), or by interaction with electromagnetic radiation ("photoionization").
The region of the Earth's upper atmosphere (100 to 700 kilometers above the surface) containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere (oxygen and nitrogen) by solar ultraviolet radiation. The ionosphere significantly influences radiowave propagation of frequencies less than about 30 MHz.
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.
Abbreviated K. A unit of temperature with its zero point located at "absolute zero," the point at which the random motion of atoms and molecules ceases. Each degree has the same change as in Celcius, and zero degrees Celcius is at 273.16 K
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.
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.
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.
The process by which two separate magnetic field lines bend toward each other and "fuse" to create a new field line, allowing charged particles that were previously travelling in two different directions to now travel toward the same point.
A measure of an object's resistance to change in its motion (inertial mass); a measure of the strength of gravitational force an object can produce (gravitational mass).
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.
The band of light that encircles the sky, caused by the bending of light from many stars lying near the plane of the Galaxy; also sometimes used to refer to the Galaxy in which the Sun belongs.
A subatomic particle with about the mass of a proton and no electric charge; one of the main constituents of an atomic nucleus; the union of a proton and an electron. A neutron is 1839 times heavier than an electron.
A star that has a sudden outburst of energy, temporarily increasing its brightness by hundreds to thousands of times; now believed to be the outburst of a degenerate star in a binary system; also used in the past to refer to some stellar outbursts that modern astronomers now call supernovas.
The chain of thermonuclear fusion processes by which hydrogen is converted to helium, helium to carbon, and so on through all the elements of the periodic table.
A hot cloud of ionized gas that is a nearby region of recent star formation, located in the sword of the constellation of Orion; also catalogued as Messier 42 (M42).
The change in an object's apparent position when viewed from two different locations; specifically, half the angular shift of a star's apparent position as seen from opposite ends of the Earth's orbit.
The distance an object would haveto be from the earth so that its heliocentric parallax would be 1 second of arc; equal to 3.26 light years; a kiloparsec is 1000 parsecs.
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.
A thick shell of gas ejected from and moving out from an extremely hot star; thought to be the outer layers of a red giant star thrown out into space, the core of which eventually becomes a white dwarf.
Plasma consists of a gas heated to sufficiently high temperatures that the atoms ionize. The properties of the gas are controlled by electromagnetic forces among constituent ions and electrons, which results in a different type of behavior. Plasma is often considered the fourth state of matter (besides solid, liquid, and gas). Most of the matter in the Universe is in the plasma state.
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.
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.
A massive, positively charged elementary particle; one of the main constituents of the nucleus of an atom. A proton is 1836 times heavier than an electron.
Usually refers to electromagnetic waves, such as light, radio, infrared, X-rays, ultraviolet; also sometimes used to refer to atomic particles of high energy, such as electrons (beta-radiation), helium nuclei (alpha-radiation), and so on.
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.
A large, bright, cool star. Red giants are formed when a star runs out of nuclear fuel in its core. The star starts to contract, which in turn leads to heating and nuclear reactions in layers outside the core and the expansion of the star's outer layers. These outer layers become cooler and redder as they expand.
An increase in the wavelength of the radiation received from a receding celestial body as a consequence of the Doppler effect; a shift towards the long-wavelength (red) end of the spectrum.
Wavelength shift from the radial velocity of a source as calculated in special relativity, so that very large red shifts do not imply that the source moves faster than light.
Two theories proposed by A. Einstein; the special theory describes the motion of nonaccelerated objects, and general relativity is a theory of gravitation.
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.
A stream of charged particles, mostly protons and electrons, that escapes into the Sun's outer atmosphere at high speeds and streams out into the solar system.
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.
A 4-dimensional universe with space and time unified; a continuous system of one time coordinate and three space coordinates by which events can be located and described.
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.
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 predominantly vertical structure extending from the solar
chromosphere into the
corona, observed in
Hydrogen-Alpha. Spicules are concentrated on the boundaries of super-granulation cells, have lifetimes of 5 to 10 minutes, and are about 1000 km across and 10,000 km long.
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.
Convective cells about 30,000 km in diameter in the solar photosphere. The boundaries between super-granule cells can be seen by emission in the chromosphere known as the chromospheric network and contain concentrations of magnetic flux tubes.
Expanding gas cloud from the outer layers of a star blown off in a supernova explosion; detectable at radio wavelengths; moves through the interstellar medium at high speeds.
The boundary marking one of the outer limits of the Sun's influence. At the termination shock, solar wind particles slow down as they begin to press into the particles forming the interstellar medium. The solar wind particles then continue to travel outward at a slower rate of speed. This is similar to cars speeding along a highway which then slow down as they encounter many more cars involved in a traffic jam. The jammed cars continue to move outward, although much more slowly.
An idea in science which is supported by numerous pieces of evidence, and which has thus far withstood the rigors of testing by other scientists. It is different from a hypothesis, which is an idea based on observations but which has not yet been tested, or does not have further evidence supporting it. Theory in the scientific sense is very different from the colloquial definition, where a theory is essentially a guess. Examples include the Big Bang theory, the theory of gravitation, and the theory of evolution.
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.
The part of the electromagnetic spectrum whose radiation has somewhat smaller 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.
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.
A splitting of spectral lines in response to a magnetic field. It is used to measure magnetic field strengths on the Sun and on other astronomical objects.
A pale glow sometimes visible in the night sky in the
path of the Earth's orbit around the Sun. (The constellations
in this path make up the zodiac). It is caused by the
scattering of sunlight off of dust in the plane of Earth's orbit.
