![]() The IRIS Si IV 1403 Å spectral window also includes the O IV line at 1404.81 Å, which is blended with the S IV transition at 1404.85 Å, and the S IV line at 1406.06 Å ( Figure 5.2), which is included in selected linelists (large linelists since October 2013, flare linelists since May 2015).The S IV line ratio is sensitive to higher electron densities (up to ~10 13 cm -3) compared to the O IV line ratios (up to ~10 12 cm -3) and therefore is particularly useful to diagnose very high densities, which might occur during flares and energetic events. In particular, if the plasma is outside ionization equilibrium, the estimated density may vary significantly (see e.g. Most of the density diagnostic techniques assume that the emitting plasma is thermal and in ionization equilibrium, which might not always be the case in the solar atmosphere. The other advantage is that the lines are close in wavelength, minimizing any calibration effects. These lines are particularly suitable for density measurements as their ratio is largely independent of the electron temperature, and only weakly dependent on the electron distribution Dudík et al. The best density-sensitive line ratios observed by IRIS is given by the intersystem (spin-forbidden) O IV transitions at 1399.77 Å and 1401.16 Å included in the Si IV 1403 Å spectral window. Gabriel & Mason, 1982 and Phillips et al. For more details on density sensitive line ratios see e.g. ![]() The ratio of two allowed lines could also be density sensitive if one of the lines is excited from a metastable state. The density sensitivity of two forbidden lines arises from the relative importance of collisional and radiative de-excitation. Two allowed transitions or an allowed and a forbidden transition can be found. Density diagnostics based on either the ratio of two forbidden transitions, For an allowed line excited from the ground state, the intensity is proportional to \(N_\textrm\). Transitions with small A-values, see Sect. The density sensitivity of line ratios from a single ion requires the existence of metastable levels (which can decay radiatively only via forbidden In addition, the lines should have a similar temperature dependence, so that their ratio will be mostly independent on the plasma temperature. Ideally, lines from the same ion should be used, in order to remove the uncertainties associated with the ion fractional abundance and chemical abundance. The intensity ratio of spectral lines with a different dependence on the electron number density represents a reliable density diagnostic.
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