# ARTS built-in documentation server

## Workspace Method refr_index_airFreeElectrons

### Description

Microwave refractive index due to free electrons.
The refractive index of free electrons is added to refr_index_air.
To obtain the complete value, refr_index_air should be set to 1
before calling this WSM. This applies also to refr_index_air_group.
The expression applied is n=sqrt(1-wp^2/w^2) where wp is the plasma
frequency, and w is the angular frequency (the function returns
n-1, that here is slightly negative). This expressions is found in
many textbooks, e.g. Rybicki and Lightman (1979). The above refers
to refr_index_air. refr_index_air_group is sqrt(1+wp^2/w^2).
The expression is dispersive. The frequency applied is the mean of
first and last element of f_grid is selected. This frequency must
be at least twice the plasma frequency.
An error is issued if free electrons not are part of abs_species
(and there exist a corresponding "vmr"-value). This demand is
removed if *demand_vmr_value* is set to 0, but use this option
with care.

**Authors: **Patrick Eriksson

### Synopsis

### Variables