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 (Ne) and temperature (Te). These results are presented in the form of ratio–ratio diagrams, which should in principle allow both Ne and Te 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 (Ne), temperature (Te) and their inhomogeneities through spectroscopic diagnostic techniques for solar ions, is a problem of

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Figure 1.  Plot of the theoretical Ne V log of emission line ratios R1 = 416.20/359.39 against R2 = 365.61/416.20 and R3 = 416.20/358.48 against R2 = 365.61/416.20 for a range of logarithmic electron temperature (log Te = 5.2 – 5.7; Te in K) and logarithmic electron densities (log Ne = 8–11; Ne in cm–3). Points of constant Te are connected by solid lines while those of constant Ne are joined by dashed lines.

Figure 2.  Plot similar to Figure 1 but with R1 = 416.20/359.39 against R4 = 358.48/572.34 and R3 = 416.20/358.48 against R4 = 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 Ne, Te, 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 general1–3.

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 Ne and Te. We, therefore, investigate this problem from another

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Figure 3.  Plot similar to Figure 1 but with R5 = 569.83/416.20 against R6 = 569.83/359.39 and R5 = 569.83/416.20 against R7 = 569.83/358.48.

viewpoint. We plot ratio–ratio diagrams for a grid
of (log Ne, log Te) values appropriate to the line formation. Using this approach, it is possible to simul-
taneously determine Ne and Te 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 space4,5.

The Ne V solar ion has its maximum ionic concentration at 2.8   105 K as per the ionization equilibrium calculations6. 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 Ne or Te when the other plasma parameter has been independently estimated. In Figures 1–3 we plot several ratio–ratio diagrams, such as log R1 vs log R2, log R3 vs log R2 and so on, for a suitably chosen grid of (log Ne, log Te) 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 paper8.

In view of the non-availability of measured line intensities, we have computed theoretical line intensities, making use of a schematic spherically symmetric model atmosphere9. This exercise has been done only to ascertain the applicability of this technique to deduce Ne and Te. The theoretical line intensity ratios thus obtained are: log R1 = 0.16, log R2 = – 0.32, log R3 = 0.39, log R4 = –0.32, log R5 = – 0.31, log R6 = – 0.15 and log R7 = 0.07. The derived Ne and Te from the diff-
erent sets of ratio–ratio diagrams are presented in
Table 1.

1373b.jpg (14584 bytes)

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.

  1. Dwivedi, B. N., Space Sci. Rev., 1994, 65, 289–316.
  2. Mason, H. E. and Monsignori-Fossi, B. C., Astron. Astrophys. Rev., 1994, 6, 123–179.
  3. Dwivedi, B. N. and Mohan, A., Curr. Sci., 1996, 70, 709–718.
  4. Keenan, F. P., Foster, V. J., Reid, R. H. G., Doyle, J. G., Zhang, H. L. and Pradhan, A. K., Astron. Astrophys., 1995, 300, 534–538.
  5. Mohan, A., Dwivedi, B. N. and Raju, P. K., J. Astrophys. Astron., 1998, submitted.
  6. Arnaud, M. and Rothenflug, R., Astron. Astrophys. Suppl., 1985, 60, 425–457.
  7. Meyer, J. P., Astrophys. J. Suppl., 1985, 57, 151–171.
  8. Dwivedi, B. N. and Mohan, A., Solar Phys., 1995, 158, 237–
  9. Elzner, L. R., Astron. Astrophys., 1976, 47, 9–18.

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