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Astron. Astrophys. 364, 835-844 (2000)

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4. Density and temperature measurements

The observed plasma clearly has two components, one with complex structures in the active region (the "prominence" plasma), and the other outside these structures ("active region corona"), as shown in Fig. 2. Thus we present density and temperature measurements carried out in both plasmas.

4.1. Prominence plasma

The temperature and density maps of the cooler plasma inside the active region structures is displayed in Fig. 3, where also the [FORMULA] 1005 Å intensity map is shown. The intensity, density and temperature for the plasma observed at solar X=45" are shown in Fig. 4, where the uncertainties of each measurements are also reported. It is important to note that [FORMULA] and [FORMULA] emission outside the bright structures in Fig. 3 is due mainly to instrument-scattered light. Plasma diagnostics has not been carried out at these positions.

[FIGURE] Fig. 3. Diagnostic results in the active region structures. From the top: 1 - [FORMULA] 1005 Å intensity map of the structures (in he black portion line intensity is due mainly to instrument-scattered light); 2 - Temperature map from the [FORMULA]/[FORMULA] line ratio; 3 - Density map from the [FORMULA] line ratio; 4 - [FORMULA]/[FORMULA] line ratio, yielding an estimate of the Ne/Mg abundance.

[FIGURE] Fig. 4. Diagnostic results in the active region structures, at solar X=45". From the top: 1 - [FORMULA] 1005 Å intensity 2 - Temperature from the [FORMULA] line ratio; 3 - Density from the [FORMULA] line ratio; 4 - [FORMULA] line ratio, yielding an estimate of the relative Ne/Mg abundance.

Making use of density-sensitive 1190/1191 [FORMULA] line ratio, we find that the electron density (log value) varies from 9.0 to 9.7 (Ne in cm-3) inside the bright structures. Uncertainties at each pixel position are 0.15 dex. These are given by the combination of the uncertainties in the fitted line profile parameters and in the SUMER intensity calibration. The random fluctuation of the measured density between 10" and 40" in Fig. 3 are probably due to the strong contribution of the scattered light to line intensities and to the weakness of true line emission.

The temperature, determined via the [FORMULA] 1189/1191 line ratio, is also displayed in Fig. 3. It is possible to see that, where the [FORMULA] and [FORMULA] emission is strongest, the ratio is almost constant, with a value of about 5-6[FORMULA] K. Outside the brightest positions, the ratio increases and shows a greater noise, due to the weakness of the diagnostic lines and to the increasing importance of the scattered light component to line intensity. The theoretical [FORMULA] ratio has been calculated in each pixel adopting the measured density value, to take into account the density sensitivity of the [FORMULA] 1191.64 Å line. The prominence plasma is thus relatively cooler and denser compared to the surrounding unstructured plasma (see next section).

The pixel-to-pixel variation of the electron density suggests the presence of many unresolved plasma structures, with very similar electron temperatures and different density.

It is interesting to note that the use of different ion fraction datasets to calculate the [FORMULA] theoretical ratio does not affect the results significantly, although some difference can be seen between ion fractions coming from different sources (Fig. 5). This is consistent with the results reported by Allen et al. (2000).

[FIGURE] Fig. 5. Ion fraction of the ions used in the present work. Full line : Mazzotta et al. (1998). Dashed line : Arnaud & Rothenflug (1985) (Si, S, Ar), Landini & Monsignori Fossi (1991) (K).

4.2. Active region corona

Temperature and density for the off-limb plasma are displayed in Fig. 6. We have used density-sensitive 1196/1213 [FORMULA] line ratio to determine Ne outside the structure. When the plasma density is fairly low, we must take account of photoexcitation of ground levels by photospheric radiation in calculating level populations for many ions. There is a critical density below which ground levels' populations are affected by such a process. In our dataset Ne is sufficiently low to let photoexcitation play an important role for [FORMULA] density diagnostics. Using CHIANTI and taking account of this process, the inferred log Ne values as a function of height are shown in Fig. 6 (top). Uncertainties increase with height, and the last 5 density values are only an estimate of an upper limit, due also to the "flattening" of the theoretical ratio. Apart from the very last ratios, these values lie more or less on a straight line as a function of height.

[FIGURE] Fig. 6. Top : Density vs. height from the [FORMULA] line ratio; Bottom : temperature versus height from the [FORMULA] line ratio.

We also measured electron temperature T outside the structure from [FORMULA] 1018/[FORMULA] (1196+1213) line ratio as a function of height. It is to be noted here that such a measurement could be biased to any problem in the relative Ar/S abundance. Argon is a high-FIP element while sulfur is just at the border (its FIP is 10.4 eV), so this can provide additional uncertainty to our results. However, our study has shown (see Sect. 5.2) that S/Ar abundance remains photospheric all across the field of view, so that this uncertainty should be reduced.

We find from this ratio that the electron temperature remains more or less constant with height with log T values between 6.24 and 6.26, assuming photospheric abundances. These values are slightly higher than those provided by Feldman et al. (1998) and Allen et al. (2000) in the quiet off-limb solar corona.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001