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Glossary of Solar Terrestrial Terms
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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.)
Recombination
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.
Reflection
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.
Refraction
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.
Retardation
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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Educational 
MAG: Normal
ION: Moderate