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

Glossary of Solar Terrestrial Terms

Least Squares Approximation

A method where a polynomial approximation is chosen to relate to a given function in way that minimises the squares of the errors. Used for data smoothing and approximating in differentiation.


The edge of the solar disc.

Lower Decile

The smallest decile, one of nine, that has 90% of the ordered values above it.

Lower Decile MUF

See Optimum Working Frequency.


In the ionosphere it refers to the removal of free electrons from the ionosphere.

Lowest Usable Frequency

The lowest frequency which allows an acceptable grade of HF service. Unlike the ALF, the LUF is dependent on the system parameters.

Low Latitude

That geographic region of the Earth that lies between the equator and about 20 degrees.


See Lowest Usable Frequency.

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