8.4  A better (global) approach

It is common in the literature to use only a few well-behaved line ratios, without considering all the observed lines.

Figure 21: Top: Emissivity ratios calculated with CHIANTI; Below: [from Keenan et al. (2002)] In most literature a large number of line ratios are often used.

Or consider a large number of ratios. The fact that with many lines a large number of line ratio combinations is possible, and the large number of ratios sometimes complicates the interpretation.

A different approach, preferable when more than two lines from an ion are observed, is to plot the values of

Fji =   I(lij) Ne h nij  Nj(Ne)   Aji

(26)

as a function of electron density, calculated at a fixed temperature. All the curves should cross at one point, if the plasma is isodensity. The F_ji curves should be calculated at the effective temperature, i.e. at the temperature where the bulk of the ion emission is (see Del Zanna et al. 2002 and Del Zanna et al 2004).

Figure 22: [From Del Zanna et al. 2004] The F_ji curves from a whole-Sun grazing incidence spectrum. Note that the intensities of the 257.26 and 182.3 have been rescaled taking into account the known blends.

In some cases, as for ions of the Be-like sequence, the method is very sensitive to the temperature, as Fig. 23 shows.

8.4.1  Temperature effect

Figure 23: [from Del Zanna et al. 2002] The f_ji curves of the O V lines observed in the STIS spectrum of AU Mic, calculated at two temperatures. Note the large temperature effect.