An average electron density Ne can be deduced from the total emission measure
< EM > and an estimate of the dimension Dh along the line of sight
of the
emitting plasma (or the volume V = As Dh,
with As being the observed projected area), assuming that the elemental abundances are known:
< Ne2 > ~ < EM > / Dh .
In most cases, when the < Ne2 > are compared to the
< Ne > 2 values that are calculated from direct
measurements of the average (along the line of sight) density < Ne >
in the transition region,
large discrepancies are found. This has been interpreted
as due to the transition region having a filamentary structure,
so that most of the plasma
occupies only a small fraction (the filling factor) of
the observed volume.
Typical values are of the order of 0.01 (cf. Dere et al. 1987).
