Glossary of Solar Terrestrial Terms

Central Meridian

The north-south meridian of the Sun that passes through the centre of the disc as viewed from the Earth.

Chromosphere

"Colour sphere" or layer of the Sun's atmosphere between the photosphere and the corona. Appears as a red ring around the solar limb during a solar eclipse. Plage regions are visible in the chromosphere, usually overlying sunspot groups. EUV radiation is produced in plage.

Circuit

In an SWS HF prediction, this is the name used to designate communications between two locations, e.g., Sydney-London.

Core

The region of very high density and temperature located at the centre of the Sun.

Corona

The outer atmosphere of the Sun with low density and high temperature. Visible as an extended bright region about the Sun during solar eclipses.

Coronal Hole

A low density region of the corona with relatively low temperature. Coronal holes are the sources of high speed solar wind streams.

Coronal Mass Ejection

An ejection of material from the Sun into interplanetary space. If the material is directed towards the Earth then the event may result in a disturbance to the Earth's magnetic field and ionosphere.

Critical Frequency

The highest frequency that can be reflected vertically from an ionospheric layer.