Glossary of Space Weather-Related Terms

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Aurora: Glowing, dancing curtains of light in the upper atmosphere of a planet. Auroras are caused by the interaction between the planet’s magnetic field and
charged particles from Earth’s magnetosphere. Aurora Borealis are the Northern Lights and Aurora Australis are the Southern Lights.

Auroral Oval: The ring-shaped belts of auroral activity around the north and south magnetic poles. The auroral oval expands and contracts over a period of hours and days, depending on geomagnetic activity.

Auroral Substorms: They are caused by streams of charged particles in the magnetotail that are accelerated to high energies and collide with atoms in the upper atmosphere surrounding Earth's poles. This interaction causes the beautiful and eerie looking aurora.


Chromosphere: The irregular layer of the solar atmosphere that is located above the photosphere and beneath the corona. The temperature in the chromosphere rises from 6000° C to about 20,000° C. It is hotter than the photosphere but not as hot as the corona. At these higher temperatures hydrogen emits light that gives off a reddish color (called H-alpha emission). This colorful emission can be seen in prominences that project above the surface of the Sun during total solar eclipses. This is what gives the chromosphere its name (color-sphere). The chromosphere also produces most of the UV light from the Sun.

Convective Zone: In the convective zone, rising and falling currents carry heat from the radiative zone to the surface. This nonstop churning is similar to what happens when you boil water on a stove.

Core: The innermost part of the Sun. The core produces colossal amounts of energy, including all of the Sun's light and heat. The temperature and pressure are so great in the Sun’s core that hydrogen atoms are squeezed together to form helium. This reaction is called nuclear fusion. The temperature at the center of the Sun is about 15,000,000 °C (27,000,000 °F) and the density is about 10 times that of gold or lead.

Corona: The very hot outer layer of the Sun's atmosphere, composed of highly diffused, superheated, ionized gases, and extending into interplanetary space. Some of the hot gasses in the solar corona escape the Sun to form the solar wind.

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 in 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.

Cosmic Ray: High energy charged particles traveling through interstellar space at nearly the velocity of light. Most are produced in supernova explosions.


Electromagnetic Spectrum: The entire range of all the various kinds or wavelengths of electromagnetic radiation or light, including (from long to short wavelengths): radio, infrared, visible, ultraviolet, X-rays and gamma rays.

Electron: A sub-atomic particle that has a negative electric charge.


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.

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.

Fusion: The combining of lighter elements into heavier ones. For lighter elements (e.g., hydrogen, helium) this process releases energy. Fusion is how stars produce energy, and is being researched as a way to produce power on Earth.


Geosynchronous: Refers to an orbit (about 22,300 miles above Earth’s surface) 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.


Heliosphere: 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 this century when one or more spacecraft - Voyagers 1 and 2 and Pioneers 10 and 11 - leave the heliosphere and enter interstellar space.


Infrared Radiation: Infrared (IR) radiation is electromagnetic radiation whose wavelength is longer than that of visible light, but shorter than microwaves. The name means "below red".

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.


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.

Magnetic Field Lines: A magnetic field has both a strength and a direction at each point in space. For example, at each point Earth, the magnetic field -- and thus a compass -- points in 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 (also called auroral substorms) which are initiated by processes in Earth's magnetotail and are restricted to the auroral ovals.

Magnetosphere: The solar wind pushes and stretches Earth's magnetic field into a vast, comet-shaped region called the magnetosphere. The magnetosphere and Earth's atmosphere protect us from the solar wind and other kinds of solar and cosmic radiation.

Magnetotail: 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.

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.


Photosphere: The photosphere is what our eyes perceive as the visible surface of the Sun. Here, energy escapes from the interior and streams into the Sun's atmosphere and beyond. The photosphere is home to dark Sunspots.

Plasma: One of the four states of matter. (The other three are solid, liquid and gas.) Plasma is very hot gas that conducts electricity and responds to electric and magnetic forces. The Sun is made almost entirely of plasma. This form of matter is rare on Earth (it's in candle flames, neon signs, fluorescent lights) but incredibly common in outer space — 99% of what we can see in the universe is plasma. Plasma 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.

Prominences: A structure in the Sun’s corona consisting of cool plasma supported by magnetic fields. Prominences are dark structures when seen against the bright solar disk, but appear bright when seen over the solar limb. Prominences seen against the disk of the Sun are also known as filaments. The most recognizable prominences appear as huge arching columns of gas above the limb of the Sun. Like Sunspots, prominences are cooler (about 10,000 °C) in relation to the much hotter background of the Sun’s outer atmosphere (about 1,500,000 °C). Prominences can also erupt from the Sun with tremendous energy.

Proton: A sub-atomic particle that has a positive electric charge. It is 1836 times more massive than an electron.


Radiation: Energy transmitted through space as waves or particles.

Radiation Belt: Magnetized planets, like Earth, are encircled by zones of particle radiation known as the "Van Allen belts," in which high-energy charged particles spiral to and fro, trapped by the planet's magnetic field.

Radiative Zone: In the radiative zone, energy from the core slowly travels outward. This region is so dense that the Sun's energy takes about 150,000 years to work its way through.


Solar Cycle: The approximately 11-year period during which the frequency and number of Sunspots, coronal mass ejections, solar flares, and other solar activity rises and falls. Also called the Sunspot cycle.

Solar Eclipse: The passing of the Moon between the Sun and Earth. In a total solar eclipse, the Moon blocks all light from the solar disk, allowing us to see the solar corona more clearly.

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 11-year solar cycle when the number of Sunspots reaches a maximum.

Solar Minimum: The month(s) during the 11-year solar cycle when the number of Sunspots is lowest.

Solar System: The Sun and the family of objects that orbit it. The Solar System includes things like planets, moons, comets and asteroids, and a mysterious form of matter called a plasma.

Solar Wind: A continuous stream of tiny charged particles coming from the Sun. The solar wind interacts with the magnetic field and atmosphere of Earth causing auroras. The solar wind pours out of the Sun at 200 tons per second and a million miles per hour.

Space Weather: The conditions and processes occurring 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 Earth's magnetic field. The effects of space weather can range from damage to satellites to disruption of power grids on Earth.

Spectrum: Electromagnetic radiation arranged in order of wavelength from Ion wavelength radio waves to short wavelength gamma rays. A rainbow is a natural spectrum of visible light from the Sun. Spectra are often punctuated by emission or absorption lines, which can be examined to reveal the composition and motion of the radiating source.

Sunspot: A dark, fringed blemish on the solar surface caused by a concentration of the Sun's magnetic field. Sunspots 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 called the solar cycle. Some individual spots cover areas 20 times the diameter of Earth.

Sunspot Cycle: The recurring, eleven-year rise and fall in the number of Sunspots.


X-rays: The part of the electromagnetic spectrum whose radiation has much 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.