4 Practical use of the diagnostics

- Firstly, we can determine, which ionization processes occur in the medium, i.e.
whether photoionization dominates or if an additional process competes (such as a
collisional one). Indeed, in the case of a pure photoionized plasma, the intensity of the
resonance line *w*, is weak compared to those of the intercombination *x*+*y* and
forbidden *z* lines. On the contrary, if there is a strong *w* line, this means
that collisional processes are not negligible and may even dominate. This combined with the
relative intensity of the K
line (H-like) can give an estimate of the ratio of
the ionic abundance of H-like/He-like and according to Fig. 7, this can also
give an indication of the electronic temperature
in the case of a hybrid
plasma, since *G* is sensitive to .
Figure 10 gives the
temperature range where *G* is insensitive to
and
for pure photoionized plasmas.

- Next, density diagnostics can be used. The ratio *R*=*z*/(*x*+*y*) changes
rapidly over approximatively two decades of density, around the critical value, which is
different for each He-like ion (see Fig. 10). In this narrow density range,
when the density increases the 1s2s ^{3}S_{1} level (metastable) is depopulated
by electron impact excitation to the 1s2p ^{3}P_{0,1,2} levels which imply that
the intensity of the forbidden *z* line decreases while the intensity of the
intercombination *x*+*y* lines increases (see Fig. 11). Outside this range, at
the low density limit (intense *z* and a constant *R* value), *R* gives an
upper limit for the value of the gas density producing the He-like ion. At higher densities
(the forbidden *z* line disappears since the density value is greater than the
critical density and hence *R* tends to zero), *R* gives a lower density limit.
Thus if the physical parameters deduced from each He-like ion do not correspond, this could
be the signature of stratification of the WA.
- Once the density is determined from the ratio *R*, an estimate of the size
of the medium ()
becomes possible, since
,
where
is the column density of the WA.

- In addition, the distance *r* of the medium from the central ionizing source
could be deduced, since the density and the distance are related by the "ionization
parameter''
.
Note that the determination of
is
dependant of the shape of the incident continuum.

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