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 both concentrated and contorted. The contortion 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: Luminous and colorful "curtains of light" typically seen in the night skies of the high northern and southern latitudes. During times of increased solar activity, they can be seen at much lower latitudes. The aurora is produced when electrons from the sun’s solar wind disturb the earth’s magnetic field and interact with molecules in the earth’s upper atmosphere.

C

Chromosphere: The layer of the solar atmosphere immediately above the visible surface of the sun with flame-like structures called "spicules." The chromosphere extends a few thousand kilometers above the sun’s surface. 

Convection: The transfer of heat from one part of a fluid or gas to another, carried out by the movement of molecules in the substance.

Corona: The outermost layer of the sun’s atmosphere, which extends more than two million kilometers above the sun’s visible surface. The appearance and shape of the corona varies with the sun’s activity cycle. The corona is most prominent when the number of sunspots reaches a maximum in the solar cycle.

Coronal mass ejection: A vast magnetic bubble of plasma that erupts from the Sun's corona and travels through space at high speed. Coronal mass ejections may cause intense geomagnetic storms and accelerate vast quantities of energetic particles.

D
 
Disk: The visible surface of the sun projected against the sky.  

E

Extreme ultraviolet: Electromagmetic radiation, invisible to the naked eye, with wavelengths shorter than ultraviolet radiation and longer than X rays. 

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 atomic particles and all forms of electromagnetic radiation into space. 

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 large eruptions on the solar surface. 

Gauss: A unit of magnetic field strength. 

Geomagnetic field: The earth’s magnetic field.

Geomagnetic storm: A worldwide disturbance in the earth’s magnetic field. 

I

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

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. 

L

Limb: The edge of the sun or planet visible to an observer or instrument.  

M

Magnetic field: A map of the magnetic forces around any object (such as the sun or planet) that is magnetic. The map is created by measuring the influence of the field on a small magnetic compass.  

Magnetosphere: The region around an astronomical object (like a sun or planet) where the motion of charged atomic particles is influenced by the shape, strength, and direction of the object’s magnetic field . 

P

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

Photosphere: The 100-kilometer-thick gaseous outermost layer of the sun that emits virtually all of the sun’s visible light. Although the photosphere is sometimes called the "surface" of the sun, it is not solid.  

Plage: Bright regions of gases heated by concentrated magnetic fields in the solar chromosphere during the sun’s active periods. They appear near groups of sunspots just before the sunspots emerge.  

Plasma: Sometimes called "the fourth state of matter," plasma is a gas composed of charged atoms (ions) and electrons. Plasma is created when a gas absorbs enough heat or other energy to release electrons from their orbits around atomic nuclei. 

Prominence: Eruptions of clouds of solar material that extend into the outer chromosphere and inner corona . They can appear as loops (when they follow the sun’s magnetic field ) or as sprays (when ejected by the sun’s magnetic field ). 

S

Solar cycle: An 11-year cycle during which the number of sunspots varies predictably.  

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 outward flow of charged particles from the sun into space. 

Sunspot: Cooler, darker area on the sun’s surface thought to be caused by concentrated, contorted magnetic fields that suppress convection of hot matter from the sun’s core. 

U

Ultraviolet radiation: Electromagmetic 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 .  

X

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

W

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