Glossary of Solar Terrestrial Terms

Filament

A cool, relatively dense region in the corona that appears dark against the background chromosphere. On the Sun's limb, filaments are called prominences.

Flare

An explosion on the Sun usually releasing large amounts of energy and particles, and usually occurring within an active region. Flares are more likely at solar maximum.

fmin

The lowest frequency displayed on an ionogram. It can be an indication of the amount of absorption occurring in the ionosphere.

foE

The critical frequency of the E region. The maximum frequency which can be refracted vertically from this region.

foF2

The critical frequency of the F2 region. It is the maximum frequency which can be refracted vertically by the F2 region. Frequencies higher than this normally penetrate the ionosphere.

F Region

A region in the ionosphere located above about 140 km and capable of refracting HF radio waves. During the day, the region often divides into a lower F1 region and an upper F2 region. At night there is only an F region. The F region may be used for HF sky wave communications at any time.