Solar Week Home page
	Glossary

Active Region
A region on the sun which is bright in X-rays and which is very dynamic in nature. Usually an active region incorporates sunspots, plage (bright areas in the chromosphere overlying sunspots), filaments (cool, relatively dense regions of the corona that appear dark against the background of the chromosphere) and hot, x-ray emitting loops. Active regions contain strong magnetic fields. Flares occur within active regions.

Chromosphere
The layer of the sun's atmosphere betweenthe photosphere and the corona. Appears as a red ring during a solar eclipse.

Corona
The outer atmosphere of the sun. Low in density, high in temperature. Appears as a bright region extending in all directions.

Coronal Mass Ejection
A coronal mass ejection, or CME, is an ejection from the sun of material into interplanetary space. CMEs can disturb the earth's magnetic field or ionosphere if directed toward the earth.

Electromagnetic Radiation
Electromagnetic radiation, or light, can be considered to be composed of particles (photons) or waves. Its properties depend on its wavelength: longer waves are less energetic than shorter waves - photons with long wavelength have less energy than short-wavelength photons. Electromagnetic radiation is usually described as bands of radiation of similar wavelength, e.g., infrared, radio waves, microwaves, gamma rays, X-rays... (These bands of radiation roughly correspond to the range of wavelengths which can be detected by different instruments.) Only a small fraction of the entire range of electromagnetic radiation can be detected by the human eye: visible light, or what in everyday-life is referred to simply as light. The human eye cannot detect wavelengths longer than those of the visible light, such as those of infrared light, microwaves (wavelengths of centimetres), or radio waves (wavelengths of metres). Wavelengths shorter than visible light cannot be seen either: ultraviolet light, X-rays, gamma rays (the most energetic).

Electromagnetic Spectrum
The electromagnetic spectrum is the complete range of wavelengths of the electromagnetic radiation.

Flare
An explosion on the sun that releases large amounts of energy and particles. Typically a flare occurs within an active region. Flares are more likely to occur at a solar maximum.

Ionization
The process of removal of an electron from an atom forms a positive ion. The process of attachment of an electron to an atom creates a negative ion.

Ionosphere
The part of the earth's atmosphere that is ionized by the sun's radiation. it extends above the earth from about 60km (about 36 miles) on out. Radio waves move easily through the ionosphere.

Luminosity
The amount of radiation emitted by a star or celestial object in a given time.

Magnetosphere
The outer magnetic field of the earth. The magnetosphere surrounds the earth, with its shape affected by the solar wind.

Pixel
Single picture element of a detection device.

Resolution
Ability to discriminate fine detail in an image, a spectrum or data. The angular resolution of a telescope is the smallest angle between two point objects that produces distinct images. In a spectrum, the resolution determines how well closely spaced features in the wavelength spectrum can be detected.

Solar Cycle
The sun's activity changes over a period of about eleven years. At the solar maximum, the sun is very active and produces a lot of Ultraviolet (UV) radiation.

Solar Flare
Every so often a small region of the Sun erupts in a flash of radiation and energetic particles. We call these eruptions solar flares. The main driving force behind solar flares is the energy contained in the Sun's magnetic field. Like an elastic band, magnetic field stores energy when it is twisted: on the Sun the motions of the solar surface from which the field emanates does the twisting. This stores energy in the field (or elastic band) in the form of additional tension and pressure. A solar flare occurs because the magnetic field can only store a limited amount of energy. When the energy in the field gets too large it is suddenly released into the solar atmosphere.

Solar Maximum
The time at which the sun is most active. Activity is determined by the number of and area covered by sunspots.

Solar Minimum
The time at which the sun is least active.

Solar Wind
The outflow of material from the sun's hot corona. The wind blows into space at a speed of about 800km per second (680 miles per hour), carrying with it the magnetic fields that originate in the sun.

Spectrometer
Instrument used to determine the distribution of energy within a spectrum of wavelengths.

Spectrum
Electromagnetic radiation whose beam is dispersed like a natural rainbow so that components with different wavelengths are separated in space in order of increasing or decreasing wavelength.

Sunspots
When the Sun is viewed through a solar telescope dark spots can be observed on the surface. These are called sunspots. They appear dark to the eye because they are cooler than the surrounding gas although they are still quite hot!

Ultraviolet Radiation
Light waves of shorter wavelength than violet light. The UV light cannot be seen with the human eye. UV light can be harmful to living things.

Solar Week Home page