FORECAST SOL: Moderate yellow MAG: Normal green ION: Normal green
Space Weather

Glossary of Solar Terrestrial Terms

Radio Emission

The Sun emits radio waves. The intensity of the radio emission from the Sun often increases during solar flares (radio bursts) and above large sunspot groups (radio noise storms). In addition, the Sun produces a background radio flux which depends on frequency and varies in the course of the solar cycle. Units for solar radio flux are 10-22 watts per square metre (also known as a solar flux unit or s.f.u.)


The bonding of ions and free electrons to form neutral atoms and molecules.

Recurrent Disturbance

A disturbance (usually geomagnetic) which repeats at an interval of approximately 27 days, the approximate rotation rate of some features on the Sun.


Although a radio wave is actually refracted in the ionosphere, it is often permissible to substitute a simple triangular ray path for the real ray path, as if the ray were reflected from a mirror. Thus radio waves are often referred to as being reflected from the ionosphere.


The bending of a wave when it crosses a boundary between media due to a change in velocity of the wave. Until it reaches the ionosphere, a radio wave propagates in a straight line. Once in the ionosphere, it may be refracted back towards the ground. The amount of refraction depends on the electron density of the ionosphere and the operating frequency.

Refractive Index

An index to define the amount of refraction a wave will undergo when it enters a medium.


The delay in propagation of a radio wave near the critical frequency caused by the slowing down of the wave by the ionosphere.

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