Glossary of Solar Terrestrial Terms

Daylight Fadeout

When flares occur, increased absorption of a radio wave in the D region may make part of the HF spectrum unusable. At times, the whole HF spectrum is affected, but normally the lower frequencies are affected most. Only those circuits with daylight sectors may be affected.

Decile

If a set of numerical values is ordered from lowest to highest and divided into ten equal parts, then the values separating the ten parts are called deciles. Hence, there are nine deciles which separate the ten sections. The upper and lower deciles have 90% and 10% of the values below them, respectively.

Deviative Absorption

Absorption of a radio wave near the point of refraction.

Differential Solar Rotation

Refers to the fact that the rotation of the Sun varies with latitude. Rotation of plasma near the equator of the Sun is generally faster.

Disc/Disk

When referring to the Sun, it is the visible hemisphere.

Diurnal

Through the day.

D region

The lowest region of the ionosphere where most HF absorption occurs. Present during daylight hours only. This region does not refract HF radio waves but does attenuate them.