Probing the solar plasma through ratio–ratio
diagnostic technique – Ne V emission lines |

**A. Mohan and B. N. Dwivedi**

Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India

**We present here Ne V line intensity ratios as a function of electron
density ( N_{e}) and temperature (T_{e}). These results are
presented in the form of ratio–ratio diagrams, which should in principle allow both N_{e}
and T_{e} to be deduced for the Ne V line emitting region, a non-isothermal
and inhomogeneous solar plasma. We also discuss the importance of this investigation to
analyse and interpret the spectral data from the coronal diagnostic spectrometer (CDS) on
the spacecraft SOHO (solar and heliospheric observatory).**

THE
inference of plasma density (*N*_{e}), temperature (*T*_{e}) and
their inhomogeneities through spectroscopic diagnostic techniques for solar ions, is a
problem of

**Figure 1.** Plot of the theoretical Ne V log of emission
line ratios *R*_{1} = 416.20/359.39 against *R*_{2} = 365.61/416.20
and *R*_{3} = 416.20/358.48 against *R*_{2} = 365.61/416.20
for a range of logarithmic electron temperature (log *T*_{e} = 5.2 – 5.7;
*T*_{e} in K) and logarithmic electron densities (log *N*_{e} = 8–11;
*N*_{e} in cm^{–3}). Points of constant *T*_{e} are
connected by solid lines while those of constant *N*_{e} are joined by dashed
lines.

**Figure 2.** Plot similar to
Figure 1 but with *R*_{1} = 416.20/359.39 against *R*_{4} = 358.48/572.34
and *R*_{3} = 416.20/358.48 against *R*_{4} = 358.48/572.34.

universal importance for both laboratory and cosmic plasmas. Without
the knowledge of these physical parameters, almost nothing can be said regarding the
generation and transport of mass, momentum and energy. Various diagnostic techniques have
been developed to deduce physical parameters such as *N*_{e}, *T*_{e},
differential emission measure, flows, and elemental abundances, making use of these
spectra. This topic has been reviewed recently for solar plasmas in particular and
astrophysical plasmas in general^{1–3}.

A fundamental property of hot solar plasmas is their inhomogeneity. The
emergent intensities of spectral lines from optically thin plasmas are determined by
integral along the line of sight through the plasma. The line-ratio diagnostics uses an
observed line intensity ratio to determine density or temperature from theoretical
density – or temperature-sensitive line-ratio curves, based on atomic model
and taking account of physical processes for the line formation. As the solar atmosphere
is highly structured and inhomogeneous, the line intensity ratios in general, depend both
on *N*_{e} and *T*_{e}. We, therefore, investigate this problem
from another

** **

**Figure 3.** Plot similar to Figure 1 but with *R*_{5} = 569.83/416.20
against *R*_{6} = 569.83/359.39 and *R*_{5} = 569.83/416.20
against *R*_{7} = 569.83/358.48.

viewpoint. We plot ratio–ratio diagrams for a grid

of (log *N*_{e}, log *T*_{e}) values appropriate to
the line formation. Using this approach, it is possible to simul-

taneously determine *N*_{e} and *T*_{e} from the
measured/computed values of line intensity ratios. This

diagnostic technique seems to be more realistic, given the adequate pairs of line
intensity ratios observed from space^{4,5}.

The Ne V solar ion has its maximum ionic concentration at 2.8 ´ 10^{5} K as per the ionization equilibrium
calculations^{6}. The photospheric abundance of neon relative to hydrogen is
3.5 ´ 10^{–5} (ref. 7). We have
carried out an extensive computation of Ne V line emissivities over a relevant density and
temperature range. Under solar conditions, these ratios are usually sensitive to
variations in both the electron density and temperature. Hence in principle they should
only be used to determine *N*_{e} or *T*_{e} when the other
plasma parameter has been independently estimated. In Figures 1–3 we plot several
ratio–ratio diagrams, such as log *R*_{1} vs log *R*_{2},
log *R*_{3} vs log *R*_{2} and so on, for a suitably
chosen grid of (log *N*_{e}, log *T*_{e}) values
appropriate to the solar transition region. Using these figures it is possible to
simultaneously determine both the electron density and temperature from the
measured/computed values of the line intensity

ratios. The physical processes involved and atomic data used in the
present investigation are the same as described in a previous paper^{8}.

In view of the non-availability of measured line intensities, we have
computed theoretical line intensities, making use of a schematic spherically symmetric
model atmosphere^{9}. This exercise has been done only to ascertain the
applicability of this technique to deduce *N*_{e} and *T*_{e}.
The theoretical line intensity ratios thus obtained are: log *R*_{1} = 0.16,
log *R*_{2} = – 0.32, log *R*_{3} = 0.39,
log *R*_{4} = –0.32, log *R*_{5} = – 0.31,
log *R*_{6} = – 0.15 and log *R*_{7} = 0.07.
The derived *N*_{e} and *T*_{e} from the diff-

erent sets of ratio–ratio diagrams are presented in

Table 1.

It should, however, be worthwhile to point out here that this diagnostic technique can be successfully applied to realistic cases, given the adequate observation. Such observation is expected shortly from the CDS instrument on SOHO. This technique may also be of great advantage to laboratory plasma diagnostics.

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ACKNOWLEDGEMENT. A. Mohan is supported by the Department of Science and Technology, New Delhi under the SERC Young Scientist Programme.

Received 20 July 1998; accepted 10 November 1998