Glossary of Space Science Terms

Aphelion: The point in the orbit of a planet, comet, etc. which is furthest from the sun.
See also: Perihelion.
Astrophysics: The branch of astronomy and physics that deals with the physics of astronomical objects.
Atmosphere: The layer of gas surrounding the earth or other planets. The upper atmosphere is the region of Earth's atmosphere above the troposphere (which extends to about 20 km). Regions of the upper atmosphere are the stratosphere, mesosphere and thermosphere.
Aurora (or Southern/Northern Lights): The bright emission of atoms and molecules in the polar upper atmosphere that appears as permanent, ring shaped belts called the auroral oval around the north and south geomagnetic poles. It is associated with a global electrical discharge process caused by energetic particles impinging on the upper atmosphere of Earth.
See also: Auroras: Billboards for Electric Space.
Auroral Oval: The pattern of auroral light around the north and south magnetic poles. The auroral oval expands and contracts over a period of hours and days, depending on geomagnetic activity.
Comet: A small body of ice and dust which orbits the Sun.
Corona: The very hot outer layer of the Sun's atmosphere, composed of highly diffused, superheated, ionized gases, and extending into interplanetary space. The hot gasses in the solar corona form the solar wind.
See also: The Dynamic Sun.
Coronal Mass Ejection: A vast region of hot, dense, and high speed solar wind propagating away from the Sun.
Cosmic Rays: Nuclear and subatomic particles moving through space at speeds close to the speed of light. They are thought to come from stars in the Milky Way galaxy.
Dynamics Explorer (or DE): The name of a two-spacecraft mission launched in 1981 and operated until 1989 to study the auroral regions and their interaction with the Earth's magnetosphere.
Electromagnetic: Relating to the interplay between electric and magnetic fields. In particular, light (as well as x-rays, microwaves, gamma rays, etc.) is electromagnetic radiation, consisting of alternating electric and magnetic fields which move through space.
Electromagnetic Spectrum: The array of electromagnetic radiation, arranged in order of wavelength, from long-wavelength radio emissions to short-wavelength gamma rays. Also refers to a narrower band of wavelengths, called the visible spectrum, as when light dispersed by a prism shows its component colors. Spectra are often striped with emission or absorption lines, which can be examined to reveal the composition and motion of the light source.
Fission: The splitting of heavier atomic nuclei into lighter ones. In the case of heavy atoms (e.g., uranium, plutonium), this will release energy. Fission is how nuclear power plants produce energy.
See also: Fusion.
Fusion: The combining of lighter elements into heavier ones. For lighter elements (e.g., hydrogen, helium) this processes releases energy. Fusion is how stars produce energy, and is being researched as a way to produce power on Earth.
See also: Fission See also: The Dynamic Sun.
Gamma Rays: High energy, very short wavelength electromagnetic radiation which can be generated by nuclear reactions (i.e., fission and fusion).
Geospace: Also called the solar-terrestrial environment, geospace is the domain of sun-earth interactions. It consists of the particles, fields, and radiation environment from the Sun to Earth's space plasma environment and upper atmosphere. Geospace is considered to be the fourth physical geosphere (after solid earth, oceans, and atmosphere).
See also: Geostationary; See also: Geosynchronous.
Geosynchronous: Refers to an orbit with a period equal to one day. A satellite in geosynchronous orbit above Earth's equator will stay over the same point on Earth at all times. Communications satellites are often put in geosynchronous orbits so that satellite dishes on earth can remain pointed at the same point in the sky at all times.
Gyroradius: A charged particle moving in a magnetic field will orbit around the magnetic field lines. The radius of this orbit is called the gyroradius (also known as the Larmor radius). The gyroradius is larger for faster or more massive particles and smaller for stronger magnetic fields.
Heliosphere: The vast region starting at the Sun's surface and extending to the limits of the solar system, well beyond the orbits of the most distant planets.
See also: The Dynamic Sun.
Hydrosphere: The water on or around the surface of a planet.
Ion, Ionize: An ion is an atom which has lost or gained one or more electrons so that it has a net electrical charge. Normally atoms have equal numbers of negatively charged electrons and positively charged protons so that the total charge of the atom is zero.
Ionosphere: The region of Earth's atmosphere that extends from about 50 to 300 miles above the surface of the planet and is made up of multiple layers dominated by electrically charged, or ionized, atoms.
Lithosphere: The crust of a planet.
Magnetic Field: A field of force around the Sun and the planets, generated by electrical currents, in which a magnetic influence is felt by other currents. The Sun's magnetic field, like that of Earth, exhibits a north and south pole linked by lines of magnetic force.
See also: Plasma, the Fourth State of Matter
Magnetic Field Lines: A magnetic field has both a strength and a direction at each point in space. For example, at each point on the earth, the magnetic field -- and thus a compass -- points a particular direction, roughly toward the North. Magnetic fields are therefore generally represented as lines: the direction of the line gives the direction of the field, and the number of lines indicates the strength.
Magnetic Storms and Substorms: A series of terrestrial disturbances -- namely, the precipitation of auroras and rapid changes in Earth's magnetic field -- caused by high-speed blasts of the solar wind. Magnetic storms have measurable effects worldwide, such as radio communication blackouts and power grid failures. Magnetic storms are far less frequent than magnetic substorms which are initiated by processes in Earth's magnetotail and are restricted to the auroral ovals.
See also: Space Weather.
MagnetoHydroDynamics (or MHD): Just as HydroDynamics is the study of the motion and dynamics of fluids such as water, MHD is the study of plasma motion and dynamics in the presence of a magnetic field.
Magnetopause: The location in space where Earth's magnetic field balances the pressure of the solar wind. It is located about 63,000 km from Earth in the direction of the Sun, or about 1/6th the distance to the moon's orbit.
Magnetosphere: The region of space above the atmosphere, and bounded by the magnetopause, that is under the direct influence of Earth's magnetic field.
See also: The Magnetosphere
Magnetotail: The region on the night side of the Earth where the magnetic field is stretched backwards by the force of the solar wind.
Perihelion: The point in the orbit of a planet, comet, etc. which is closest to the sun.
See also: Aphelion.
Photosphere: The visible region of the sun: above the photosphere are the chromosphere, then the corona.
Plasma: One of the four states of matter. (The other three are solid, liquid and gas.) Consists of a gas of positively charged and negatively charged particles with approximately equal concentrations of both so that the total gas is approximately charge neutral. A plasma can be produced from a gas if enough energy is added to cause the electrically neutral atoms of the gas to split into positively and negatively charged atoms and electrons.
See also: The Plasma State of Matter.
Polar Cusps (or Cusps): The funnel shaped magnetic field regions located above Earth's magnetic poles. Solar wind plasma has near direct access to these regions.
Radiation Belt: Magnetized planets, like Earth, are encircled by zones of particle radiation known as the "Van Allen belts," in which charged particles spiral to and fro, trapped by the planet's magnetic field.
Radiation: Energy transmitted through space as waves or particles.
Science Discovery: A hands-on science program at the University of Colorado, Boulder. Each year, it offers over one hundred after-school and summer classes and camps, visits classrooms across the state through a program called Science From CU, develops hands-on curricula, and conducts the Science Explorers teacher training workshops in ten locations.
Solar Flare: An explosive release, marked by a sudden brightening near a sunspot or prominence, of electromagnetic radiation and huge quantities of charged particles from a small area of the solar surface.
See also: Ever-Changing Sun Causes Space Weather
See also: The Dynamic Sun.
Solar-terrestrial environment: See also: Geospace.
Solar Wind: The hot, fast, and tenuous plasma convecting from the solar corona. Typically, the solar wind is "blowing" at 400 km/s, has temperatures in excess of 100000 K, and has a density of about 10 ions and electrons per cubic centimeter. (For comparison, water boils at about 373 K. Typical molecular densities for the atmosphere at the surface of Earth are about 1022 times larger than the solar wind density.)
See also: Ever-Changing Sun Causes Space Weather
Space Weather: The conditions and processes occuring in space which have the potential to affect the near Earth environment. Space Weather processes can include changes in the interplanetary magnetic field, coronal mass ejections from the sun, and disturbances in the Earth's magnetic field. The effects can range from damage to satellites to disruption of power grids on Earth.
See also: Space Weather.
Sunspot Cycle: The recurring, eleven-year rise and fall in the number of sunspots.
See also: The Dynamic Sun.
Sunspot: A dark, fringed blemish on the solar surface that is caused by a concentration of the Sun's magnetic field lines.
See also: Ever-Changing Sun Causes Space Weather