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This page contains brief definitions of some of the terms used when discussing space weather. For more background information, try the Science Briefs and Reports page.  
  A diagram of the magnetosphere.



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.

Astronomical unit

The average distance between the Earth and the Sun, about 150 million kilometers.

Aurora (or Northern and Southern Lights)

The bright emission of atoms and molecules in Earth's polar upper atmosphere that appears as permanent, ring shaped belts called the auroral oval around the north and south magnetic poles. Aurora are associated with a global electrical discharge process triggered by solar wind disturbances. The emissions are caused by energetic particles from Earth's magnetotail region impinging on the upper atmosphere. Aurora occur about 70 miles above Earth's surface. Contrary to popular belief, they do not produce any sounds.

Auroral electrojet

An electric current that flows horizontally through the ionosphere in the auroral region. See Ionospheric Electrojets.

Auroral Oval and Zone

The pattern or distribution of auroral light around Earth’s north and south magnetic poles. The auroral oval expands and contracts over a period of hours and days, depending on geomagnetic activity. The auroral zone is about six degrees wide, centered on a magnetic latitude of 67°.



Abbreviated C. A unit of temperature. Zero degrees Celsius is equal to 273.15 Kelvin. Also know as centigrade. Water freezes at zero degrees C and boils at one hundred degrees C.


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.


Short for Coronal Mass Ejection.


The transfer of energy via collisions of randomly moving atoms and electrons.


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 it's extra heat to it's surroundings, the bubble sinks again. Convection can occur when there is a substantial decrease in temperature with height, such as in the Sun's convection zone.

Convection Zone

A layer in a star in which convection currents are the main mechanism by which energy is transported outward. In the Sun, a convection zone extends from just below the photosphere to about seventy percent of the solar radius.


In solar astronomy, the innermost part of the Sun, where energy is generated by nuclear reactions.


The very hot outer layer of the Sun's atmosphere, composed of highly diffused, ionized gases with a temperature of 1,000,000 F° and above, and extending into interplanetary space. There is still considerable debate about how solar wind plasma escapes to make its way into space. We know that the Sun's corona contains "holes," regions of lower temperature and density and weak magnetic fields. These holes probably allow the high speed solar wind to spray out like water through a hose. Coronal holes show up in X-ray photographs as dark areas.

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 (CME)

High speed coronal mass ejections produce major disturbances in the solar wind. Often loop-like in appearance, coronal mass ejections rise as massive clouds of material from the solar atmosphere. Dangerous, high energy, charged particles are often produced in these disturbances and, when they are directed towards Earth, often produce large magnetic storms in the magnetosphere.


Telescope for observing the corona. Often contains an occulting disk which covers the disk of the Sun so that the corona may be more easily observed.

Cosmic Ray

High energy charged particles traveling through interstellar space at nearly the velocity of light.

Cross-tail Current

The sheet of electric current that flows across the center of the Earth's magnetotail. This current, like others in the magnetosphere, is driven by energy extracted from the solar wind.



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 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).


Electromagnetic Radiation

Radiation that travels through space at a 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.


A negatively charged elementary particle that normally resides outside (but is bound to) the nucleus of an atom.



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.


Geomagnetic Storm

see Magnetic Storm


Also called the solar-terrestrial environment. Geospace is the domain of solar-terrestrial 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).

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).



The study of the interior of the Sun by the analysis of its natural modes of oscillation.


A huge magnetic bubble containing the solar wind and the entire solar magnetic field. At the outermost boundary of the heliosphere, our solar wind meets the interstellar medium, a plasma that permeates our Milky Way galaxy. Scientists estimate that this boundary is between 9 and 15 billion kilometers away from the Sun, far beyond the orbits of all the planets. We should find out for certain sometime in the next century when one or more spacecraft - Voyagers 1 and 2 and Pioneers 10 and 11 - leave the heliosphere and enter interstellar space.

Hydrogen Alpha

Light emitted at a wavelength of 6563A from an atomic transition in hydrogen. This wavelength is in the red portion of the visible spectrum and is emitted in the solar chromosphere.


Interplanetary Magnetic Field, or IMF

The magnetic field of the Sun drawn out by the solar wind that fills interplanetary space. The direction of the IMF controls much of the activity in the magnetosphere. When the IMF points southward, the IMF can interconnect with Earth’s own magnetic field. This allows solar wind energy and particles to enter the magnetosphere leading to magnetically disturbed conditions like geomagnetic storms.


A atom that has lost or gained one or more electrons and has become electrically charged as a result.


The process by which ions are produced, typically occurring by collisions with atoms or electrons ("collisional ionization"), or by interaction with the electromagnetic radiation ("photoionization").


The area 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. Here, ultraviolet rays from the Sun and high energy particles from Earth’s magnetosphere bombard the gases of our upper atmosphere. They collide with the atoms, knowing off electrons and leaving the positively charged nuclei – called ions – floating free. Because of these ions and electronics, the ionosphere can conduct electricity.

Ionospheric Electrojets

Currents that flow horizontally in the ionosphere driven by energy from the solar wind and magnetosphere. During magnetic storms and substorms these currents intensify, and new electrojets may be formed. Also see Auroral Electrojet.

International Solar-Terrestrial Physics Science Initiative (ISTP)

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. The Solar and Heliospheric Observatory (SOHO) is part of this program.


Light Year

The distance light travels in 1 year.


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.


A map showing the strength of the magnetic field in different locations.

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 electric magnetic fields.


The region of space above the atmosphere and bounded by the magnetopause that is under the direct influence of Earth's magnetic field. The solar wind blows Earth’s magnetic field into a shape of a comet. As the solar wind flows past the magnetosphere, it acts like a cosmic generator producing millions of amps of electric power resulting in auroras and magnetic storms.

Magnetic Storms and Substorms

A series of geospace disturbances -- namely, auroral activity, increase in the radiation levels in the inner magnetosphere, and rapid changes in Earth's magnetic field -- caused by increased energy input from the solar wind. Magnetic storms can have measurable effects worldwide, such as radio communication blackouts and power grid failures. Storms are far less frequent than magnetic substorms which are initiated by processes in Earth's magnetotail and are restricted to the auroral ovals. Both storms and substorms are triggered by conditions in the solar wind such as a southward directed IMF.


The location in space where Earth's magnetic field balances the pressure of the solar wind. The magnetopause forms the outer magnetic surface of Earth's magnetosphere: it is the boundary between Earth's magnetic field and the magnetic field generated by the Sun. It's closest approach on the side facing the Sun (called the dayside) is 63,000 km from Earth, about 1/6th the distance to the moon.


The long magnetic tail of a magnetosphere drawn out by the flow of the solar wind. Earth’s magnetotail is located on its night side (in the direction away from the Sun). It extends 100 of thousands of kilometers and is a major energy source for the magnetosphere. Auroral substorms are caused by powerful charged particles in the magnetotail.

Maunder Minimum

During one seventy-five year period, now called the Maunder Minimum (1645-1715), sunspot activity virtually ceased, and temperatures fell enough to cause a "Little Ice Age" of severely cold weather across the northern hemisphere of Earth. During the 11th and 12th centuries, there was also a large warming coincident with enhanced solar activity.



An electrically neutral elementary particle. A neutron is 1839 times heavier than an electron.


The positively charged core of an atom, consisting of protons and neutrons (except for hydrogen), around which electrons orbit.



The pat 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 it's orbit.



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.


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. Plasmas respond to electric and magnetic forces.

Polar Cusps (or Cusps)

The funnel shaped magnetic field regions located near Earth's magnetic poles on the day side (i.e. the side which is facing the Sun). Solar wind plasma has near direct access to these regions.


One of the most spectacular features of the Sun are solar prominences. They appear to stream, loop and arch away from the Sun. The most recognizable prominences appear as huge arching columns of gas above the limb (edge) of the Sun. However, when prominences are photographed on the surface of the Sun, they appear as long, dark, threadlike objects and are called filaments. Like sunspots, prominences are cooler (about 10,000 degrees C) in relation to the much hotter background of the Sun’s outer atmosphere (about 1,500,000 degrees C). Prominences can also erupt from the Sun with a tremendous burst of energy.


A positively charged elementary particle. A proton is 1836 times heavier than an electron.


Radiative Zone

An interior layer of the Sun, lying between the core and the convection zone, where energy travels outward by radiation.

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. Earth’s radiation belts were the first major discovery of the space age. In 1958 a Geiger counter on board the Explorer 1 spacecraft, detected so much radiation scientists first thought the detector had malfunctioned.


Solar Cycle

The approximately 11-year quasi-periodic variation in frequency or number of sunspots, coronal mass ejections, solar flares, and other solar activity.

Solar Eclipse

The passing of the moon between the Sun and Earth. In a total solar Eclipse the moon blocks out the light from the solar disk, allowing us to see the solar corona more clearly. Coronagraphs essentially create artificial eclipses so that the corona can be studied continually.

Solar Flare

An explosive release of electromagnetic radiation and huge quantities of charged particles from a small area of the solar surface. Solar flares are marked by a sudden brightening near a sunspot or prominence. The radiation released includes x-rays and radio waves. Solar flares and CMEs often occur together, but what connections may exist between them is a matter of debate.

Solar Maximum

The month(s) during the solar cycle when the number of sunspots reaches a maximum. The most recent solar maximum occurred in July 1989. The next solar maximum is expected in 2001.

Solar Minimum

The month(s) during the solar cycle when the number sunspots is lowest. The most recent minimum occurred in 1996.

Solar Wind

The hot, fast, and tenuous plasma convecting from the solar corona. Typically, the solar wind is "blowing" at 400 km/s (about a million miles per hour), has temperatures in excess of 104oK and has a density of about 10 ions and electrons per cubic centimeter. (Typical molecular densities for the atmosphere at the surface of the earth are about 1022 times larger than the solar wind density.)


The Solar and Heliospheric Observatory


The expanse in which the solar system, stars and galaxies exist. The space between the Sun and Earth is not empty -- it is filled with high energy particles hot enough to conduct electricity (a plasma) and electric and magnetic fields.

Spectral Line

A line in a spectrum due to the emission or absorption of electromagnetic radiation at a discrete wavelength. Spectral lines result from discrete changes in the energy of an atom or molecule. Different atoms or molecules can be identified by the unique sequence of spectral lines associated with them.


An instrument that spreads light or other electromagnetic radiation into it's component wavelengths (spectrum), recording the results photographically or electronically.


An instrument for measuring the intensity of radiation as a function of wavelength. See Spectrograph.


Electromagnetic radiation arranged in order of wavelength. A rainbow is a natural spectrum of visible light from the Sun. Spectra are often punctuated with emission or absorption lines, which can be examined to reveal the composition and motion of the radiating source.

Substorm current wedge

A Birkeland current system that is formed at the onset of a substorm. A portion of the cross-tail current is diverted into the ionosphere, producing a new westward electrojet and a brightening of the aurora.


A dark, fringed blemish on the solar surface caused by a concentration of the Sun's magnetic field. They look dark because they are cooler than the plasma surrounding them. Sunspots appear in groups and last from several hours to several months. The number of sunspots increases and decreases over an eleven-year cycle. The next peak is in 2001. Scientists think that the sunspot cycle has been going on for millions of years and that other stars also have sunspots.


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 successful implemented, could become an attractive source of power on the Earth.



Ultraviolet (UV)

Ultraviolet is a range of radiation wavelengths of the electromagnetic spectrum between 5 and 400 nm, just beyond the violet in the visible light spectrum.

Upper Atmosphere

see Ionosphere.



White Light (WL)

Visible light that includes all colored and therefore all visible wavelengths.




The part of the electromagnetic spectrum whose radiation has much greater frequencies and smaller wavelengths than those of ultraviolet radiation. Because rays are absorbed by the Earth's atmosphere, X-ray astronomy is performed in space.



A Japanese and US satellite which observes X-rays from the Sun. Launched in 1991.


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