8.3  Examples

Within CHIANTI, DENS_PLOTTER is a high-level widget for the analysis of density-sensitive ratios of lines from the same ion. It allows inclusion of proton rates and photoexcitation. The calling sequence is simple:

 IDL > dens_plotter,'si_9'

Figure 19: The density-sensitive Si IX ratio and its variation with temperature.

Figure 20: Some useful density-sensitive line ratios observed by CDS.

Table 2: Density sensitive line ratios observed by CDS, ordered by the temperature of maximum of the contribution functions shown in column 4, calculated with the ionization equilibrium calculations of Arnaud and Rothenflug (1985). The ranges for which the ratios can be used are shown in column 5. The numbers in parentheses indicate the number of transitions that compose the self-blends. The lines indicated with a (!) have problems in either their observation or in the atomic physics. The lines indicated with a (*) are the best selection within a wider choice. The detectors in parentheses indicate that the lines are also visible in second order. N means NIS, and G means GIS;

Ion	Ratio (Å)	Detector	log Tmax	log Ne
Si IX (*)	349.9 (3) / 341.949	N 1 (G 4)	6.02	7.5 - 9.5
Si IX	345.100 / 341.949	N 1 (G 4)	6.02	7.5 - 9.5
Si IX	349.9 (3) / 345.100	N 1 (G 4)	6.02	7.5 - 9.5
Si X	356.0 (2) / 347.402	N 1 (G 4)	6.12	8.0 - 10.0
Fe XII	338.278 (!) / 364.467	N 1 (G 4)	6.16	7.0 - 12.0
Fe XIII	321.4 / 320.80	N 1	6.21	8 - 10
Fe XIII	318.12 / 320.80	N 1	6.21	8 - 10
Fe XIII	359.6 (2) (!) / 348.18	N 1 (G 4)	6.21	8 - 10
Fe XIII (*)	320.8 (bl) / 348.18	N 1	6.21	8 - 10
Fe XIII (*)	203.8 (2) / 202.044	G 1	6.21	8.5 - 10.5
Fe XIV	353.83 / 334.17	N 1 (G 4)	6.25	9.0 - 11.0

Table 2 presents a list of useful density-sensitive line ratios available within the CDS channels.

1. Temperature effects should be taken into account.

   Figure 19 shows the density-sensitive Si IX ratio calculated at three temperatures (T=10⁶ K is the temperature of maximum ionization fraction for Si IX), showing that these temperature variations do not appreciably affect the ratio.

2. Photoexcitation can be important for some ions.

   The radiation that comes from the solar photosphere can excite transitions between levels in the ground configuration of ions in the corona, mainly affecting the level balance calculations for low densities (N_e £ 10⁸ cm^-3).

   The effect is stronger close to the photosphere, and tends to die out with height because of the decrease of the photospheric radiation field with distance. This decrease can be accounted for by estimating a geometric dilution factor that is a function of height above the photosphere. Si IX should be affected more than other ions.

3. Most of the observed diagnostic lines are very weak, which makes the data analysis difficult.
4. For some ions, the atomic data are yet not reliable.

5. Many diagnostic ratios are affected by blending which varies depending on the type of source observed, and which is particularly strong at moderate resolutions.
6. Only ions that have peak emissivity at the temperatures where the source emits can be expected to give meaningful results.