# Glossary of Solar Terrestrial Terms

skip terms menu## 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.

## Limb

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.

## Loss

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.

## LUF

See Lowest Usable Frequency.