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A

angstrom: A unit of length equal to one hundred-millionth of a centimeter.

active region: An area of the solar atmosphere where the Sun’s magnetic field is concentrated. The concentration and bipolar nature of the magnetic field results in the formation of dark areas such as sunspots and bright areas known as faculae. These regions also produce flares and plages.

aurora: Light radiated by ions and atoms in the Earth’s upper atmosphere, mostly in polar regions, the result of bombardment by energetic electrically charged particles from the ionosphere and magnetosphere.
 

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B

bow shock: The shock wave that flanks the magnetosphere on the day side. It causes the solar wind flow to slow down and flow around the magnetosphere.
 

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C

chromosphere: The part of the Sun's (or another star's) atmosphere between the photosphere and the corona.

convection: Circulation of a fluid or gas.

corona: The Sun’s outer atmosphere, with a temperature of greater than a million degrees, that gives rise to the solar wind.

coronal mass ejection: A vast magnetic blob of plasma that erupts from the Sun's corona and travels through space at high speed. Coronal mass ejections may cause intense geomagnetic storms when they strike the Earth and accelerate vast quantities of energetic particles in both interplanetary space and the magnetosphere.
 

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D

disk: The visible surface of the Sun projected against the sky.
 

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E

extreme ultraviolet: Electromagnetic radiation, invisible to the naked eye, with wavelengths shorter than ultraviolet radiation and longer than x-rays. These wavelengths are mainly responsible for the existence of the ionosphere.
 

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F

facula: Brighter-than-average regions on the Sun’s surface that typically appear near a group of sunspots just before the sunspots themselves appear.

flare: A sudden outburst of energy from the Sun that occurs near concentrated magnetic fields (known as active regions) on the Sun’s visible surface. Flares emit high-energy charged particles (ions and electrons) and all forms of electromagnetic radiation into space.
 

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G

gamma rays: High-energy electromagnetic radiation, invisible to the naked eye, with wavelengths shorter than x-rays. Gamma rays are emitted from the Sun during the most energetic flares on the solar surface.

gauss: A unit of magnetic field strength.

geomagnetic field: The Earth’s magnetic field.

geomagnetic storm: A worldwide large disturbance in the Earth’s magnetic field, accompanied by intense auroras in the northern and southern polar regions and intensifications of the particle radiation trapped in Earth's magnetosphere (the Van Allen belts).

granulation: The cellular structure of the photosphere. "Granules" are formed by convection, each one is quite large, about 700 to 1000 km (400 - 600 miles) in diameter.

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H

heliopause: The outer edge of the heliosphere, where the solar wind runs into the interstellar medium. At the heliopause, the pressure of the solar wind balances that of the interstellar medium.
 

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I

infrared radiation: Electromagnetic radiation, invisible to the naked eye, with wavelengths longer than visible light and shorter than microwaves.

interstellar Medium: Electrified gas and dust between the stars.

ionosphere: The highest region of the Earth’s atmosphere containing free electrons and ions.
 

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K

Kelvin (K): A unit of temperature with a magnitude equal to that of the degree Celsius, and based on a scale in which 0 K is absolute zero (-273.15 °C). A temperature expressed in Kelvins is equal to the Celsius temperature minus 273.15.

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L

limb: The edge of the Sun or planet visible to an observer or instrument.

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M

magnetic field: A map of the direction and strength of magnetic forces around any object (such as the Sun or planet) that is magnetic. Magnetic fields are caused by electric currents in the Sun.

magnetometer: A device used to measure the magnetic fields in space and on the ground, and its changes. For example, a magnetometer can measure changes caused by solar storms.

magnetopause: The boundary of the magnetosphere, lying inside the bow shock, usually about 10 Earth radii toward the Sun. The solar wind is deflected inside the bow shock to flow around the magnetopause in the magnetosheath.

magnetosheath: The region between the bow shock and the magnetopause, characterized by very turbulent plasma. For Earth, along the Sun-Earth axis, the magnetosheath is about two Earth radii thick.

magnetosphere: The region surrounding a planet within which the planetary magnetic field is the dominant force on electrically charged particles that can be trapped within the magnetosphere, making radiation belts like Earth's Van Allen belts.

magnetotail: A comet-like extension of a planet’s magnetosphere formed on the planet’s dark night side by the interaction of the solar wind and the magnetosphere. It can extend hundreds of planetary radii away from the Sun.

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N

noctilucent clouds: Clouds formed at extremely high-altitude that shine at night. A bit of a mystery, scientists aren't sure why or how they are formed.
 

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P

penumbra: A dark region that surrounds an even darker central area of a sunspot.

photosphere: The visible portion of the Sun.

plage: Bright regions of gases with concentrated magnetic fields in the solar chromosphere. They appear near groups of sunspots and in the regions surrounding sunspots that are about to emerge.

plasma: A low-density gas in which the individual particles are charged and which contains an equal number of positively charged ions and negatively charged electrons.

plasmasphere: A region of the inner magnetosphere that contains relatively cool (low energy) and dense plasma. This area can be considered an outer extension of the ionosphere.

prominence: Complex structures of relatively cool, dense solar material that extend into the outer chromosphere and inner corona. They are generally static and believed to be supported by magnetic forces. They can appear as loops on the edge of the solar disk or limb. Their shape is probably controlled by the Sun's magnetic field. Sometimes they erupt, often in association with coronal mass ejections.
 

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R

radiation: Radiation has two different meanings. One is the stream of particles emitted by decaying nuclei such as uranium. This energy often takes the form of alpha or beta particles, or neutrons. A second use is as part of the term “electromagnetic radiation," which refers to energy traveling in the form of electromagnetic waves or photons. For example, yellow light is a form of electromagnetic radiation, as are radio waves and x-rays.

radiation belts: Regions of of high-energy particles traped by the magnetic influence of the Earth. These belts are sometimes called "Van Allen" belts because of their discovery in 1958 by Professor J.A. Van Allen. Radiation belts are composed of electrons, protons, and smaller numbers of other ions.
 

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S

solar activity: Activity of sunspots, flares and CME's which follow the solar cycle.

solar cycle: A predictable 11-year cycle when defined by solar activity, including the number of sunspots, flares, and CMEs, which follow this cycle. When defined by the solar magnetic field directions, the cycle is 22 years long.

solar flare: An explosive release of energy of the Sun.

solar maximum: A period of increased solar activity when the number of sunspots reaches a maximum in the 11-year solar cycle.

solar minimum: A period of decreased solar activity when the number of sunspots reaches a minimum in the 11-year solar cycle.

solar wind: The charged particles (plasma), primarily protons and electrons, that are continuously emitted from the Sun and stream outward throughout the solar system at speeds of hundreds of kilometers per second.

sunspot: A region of the solar surface that is dark and relatively cool; it has an extremely high magnetic field.
 

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U

ultraviolet radiation: Electromagnetic radiation, invisible to the naked eye, with wavelengths shorter than violet light and longer than x-rays.

umbra: The dark central area of a sunspot.
 

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W

white light: Electromagnetic radiation composed of all wavelengths of light that is visible to the naked eye (red through violet).

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X

x-rays: High-energy electromagnetic radiation, invisible to the naked eye, with wavelengths shorter than ultraviolet radiation and longer than gamma rays.