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Astron. Astrophys. 327, 1230-1241 (1997)

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3. The Serts 89 spectrum

3.1. The data

The diagnostic method shown in Sect. 2 is applied to the solar spectrum observed with the Solar Euv Rocket Telescope and Spectrograph (S.E.R.T.S.) during the flight of May, 5 1989 (Thomas and Neupert 1994). During the flight an active region was observed by the telescope and a small rising subflare happened to be included in the field of view of the instrument.

The authors kindly supplied the averaged intensities for the lines, both for the Subflare and the Active Region component. The data are provided in Tables 2 and 3 where only the lines used in the following analysis are included. No lines identified in the 1994 paper as a blend were included in the line list. In the present paper only the Active Region intensities are considered.

The theoretical data adopted for this study come from the CHIANTI database (Dere et al. 1996), and have been used to calculate the [FORMULA] assuming Feldman chemical composition (Feldman et al. 1992). Since measured intensities are in ergs instead of photons, [FORMULA] defined in eq 5 have been multiplied for the photon energy.

The complete set of data for the active region has been analyzed and the results are discussed in detail in the following sections.

3.2. Differential emission measure analysis

Following the procedure described in Sect. 2.3 we have used all the density insensitive lines available to derive the d.e.m. of the emitting source. The selected lines are reported in Table 1.


[TABLE]

Table 1. The SERTS-89 intensities for Active Region and Subflare. Wavelengths are in Angstrom, intensities are in [FORMULA] and Tmax in K.


The resulting d.e.m. is provided in Fig. 4 as a function of electron temperature.

[FIGURE] Fig. 4. Differential emission measure for the SERTS-89 averaged Active Region fluxes. Details on the analyzed lines are reported in the text.

Ten lines (starred in the table) have been excluded for the following reasons that clearly become apparent during the d.e.m. procedure:

  • Mg V line at 353.08 is weaker than expected and does not agree with the other Mg V line.
  • Ne VI lines at 433.16 Å  and 435.63 Å  , Mg VIII lines at 430.44 Å  ,and also 436.73 Å  not included in the table, Mg VII line at 431.29 Å  and S XIV line at 445.67 Å  have always lower values than the other lines belonging to the same ions. As already noted by Young et al. 1997, this feature is most probably due to some underestimation of the intensity calibration curve around 430-450 Å  .
  • Cr XIII line at 328.26 disagrees with those of Fe XIII and Si XI, sharing common effective temperature, and are a factor 6 too high.
  • Fe XV line at 417.25 Å  is stronger than expected and does not agree with the rest of the Fe XV lines. This behavior has already been observed with the ratio method (Young et al. 1997, Feldman 1992, Brosius et al. 1996).
  • S XIII and S XIV lines are not consistent with the present ionization balance.

Several other lines deserve a few comments :

  • The observed intensities of both the two Cr XIV lines and Cr XIII ([FORMULA]) both disagree with those by Fe XIII, Si XI, Fe XV and Fe XVI, and are expected to have intensities around a factor 6 smaller. This feature is most probably due to an underestimation of the adopted abundance of Cr (Feldman et al. 1992). Cr XIII line at 328 shows the greatest discrepancy, but it is blended with a couple of Al VIII lines. Their presence is not able to justify the gap between Cr XIII and the other elements.
  • The observed intensities of the Ni XVIII lines are overestimated by a factor 1.5. Nevertheless the presence of only one ion makes it difficult to understand if the discrepancy should be attributed to the abundance of Nickel or to the ionization fraction of Ni XVIII.
  • Fe XVII 409.71 Å  seems to have a lower intensity than the other Fe XVII lines.
  • Fe X line at 365.57 Å  has a higher intensity than the other Fe X transition. This line is blended with a Ne V transition expected at 365.60 Å  which could provide 25 % of the total intensity.
  • Cr XIV line at 389.85 Å  is higher than expected by the theoretical ratio with the 412.04 Å. No line blending is expected.
  • Mg VII line at 278.41 Å  is blended with the Si VII transition at 278.44 Å , not reported in the SERTS89 catalogue.
  • There seems to be an inconsistency between the observed intensity of the Fe XI and Fe X lines, the former being unexpectedly weaker than the latter. We suggest that this behavior is determined by some problems of the relative ionization equilibrium abundances of the two ions. On the contrary there is fairly good agreement between Fe X and the Mg IX 368.07 Å  line which share common [FORMULA].
  • There is an huge inconsistency between Fe XIII lines ([FORMULA]) and the Be-like Si XI transition 303.23 Å  ([FORMULA]). The latter line seems to be stronger than expected. It is not clear if the problem is to be found in the atomic transition probabilities of the CHIANTI Si XI model, in those of Fe XIII (see Fe XIII analysis for further details) or in some problems in the abundance of Silicon. It is difficult to understand this behavior since Si XI is the only Silicon ion presenting density insensitive lines. Moreover Si XI line is on the wings of the very strong He II 304. Nevertheless several other Si lines are observed in the present spectrum and they can be used for checking the behavior of Si XI (see the Conclusions).
  • There is inconsistency between the observed intensities of Fe XVI ([FORMULA]) and S XIV ([FORMULA]). This problem may be caused both by relative element abundance problems or by the relative ionization equilibrium abundances of the two ions. Since S XIV is also in conflict with the Be-like S XIII it is not possible to determine which of the problems is the cause of this discrepancy.

3.3. Analysis of the observed intensities

3.3.1. Density insensitive ions

The He II, C IV, Mg V, Ne VI lines show no density dependence and are used just to check the agreement among lines of the same ions. Only lines of NeVI deserve a few comments: the observed spectrum of Ne VI includes 5 fully resolved lines. Lines 399 and 401.9 originate from the same upper level 2s2p2 -2 P [FORMULA], while lines 433 and 435 originate from level 2s2p2 -2 S [FORMULA]. All the observed lines do not depend on electron density, so they are expected to have very similar L functions. Nevertheless, as noted in the d.e.m. analysis, while lines 399, 401.1 and 401.9 show an excellent agreement the L functions of lines 433 and 435 are lower than the other by a factor [FORMULA]. It is worth noting that the 433 and 435 lines have identical (within the uncertainties) L functions, satisfying in this way the requirement for density independent lines.

Since the behavior of 433 and 435 lines is common for the SERTS-89 spectral lines in the range 430-450 [FORMULA] (see Sect. 3.2), we think that there is a problem in the intensity calibration for this spectral range. It is worth noting that this method allows to estimate the correction factor for the intensities much more easily than the ratio method. The other lines pertaining to the other ions do not present any problem.

3.3.2. Density sensitive ions

  • Ne V: The L-functions for Ne V are plotted in Fig 5; for each line the observed flux plus and minus error identifies on the plot a strip of possible L([FORMULA]) solutions for that line, and the area common to all the lines, marked by crosses, identifies the L([FORMULA]) solutions. The observed Ne V spectrum contains 3 lines, two of which (358.455 and 359.378) are originated from the same upper level 2s2p3 -3 S1. The L functions of these two lines are expected to be nearly identical. Line 416.208 is density dependent for N [FORMULA] cm-3. None of these lines is blended with unresolved transitions. We note that also the line 365.603 should have been observed, but it is blended with the observed Fe X line at 365.558, as noted during the d.e.m. analysis. The electron density measurement provides unexpectedly a very low value. Unfortunately no other density sensitive lines formed at such low temperatures are available in SERTS 89 spectrum so we are not able to check the consistency of this measure of [FORMULA]. The three lines allow to identify a common solution with [FORMULA] and [FORMULA] ; this ion is the coldest one (log [FORMULA] = 5.50) among those giving a density evaluation.
  • Mg VI: Mg VI shows five lines, sufficiently density sensitive for [FORMULA] to provide a density diagnostic. The L functions of lines 399.275 and 400.668 perfectly agree but do not supply any density measurements since they originate from two levels of the same multiplet whose population depends on [FORMULA] in the same way. The other lines don't agree among each other. The behavior of line 270.4 is uncertain since no unresolved contributions are expected and probably the problems are to be found in the atomic calculations.
  • Mg VII: Six lines of Mg VII have been observed; a very narrow interval of solutions is common to all the lines except 278.407. However, a small correction of line 434.917 (about 1.5 larger) allows any log density value larger than 7.6. Line 278.407 has higher L values than the rest of the lines by a factor 3. This difference is far greater than the experimental uncertainties and is most probably due to the unresolved contribution of Si VII 278.443 line. From the comparison of the L functions we are able to evaluate the expected contribution of the Mg VII line to be around 43.2 [FORMULA] while the Si VII transition intensity should be 86.4 [FORMULA]. Unfortunately no averaged Active Region intensity for any Si VII line has been reported (see Tables 2 and 3) and it is not possible to check directly this conclusion.
  • Mg VIII: For Mg VIII (Fig. 6) lines 436.726 and 430.445 do not agree with the others that identify a common region [FORMULA] and [FORMULA]. The L functions of the two lines are in good agreement with each other but have values lower by a factor 2 than those of the L functions of the other lines. This feature, already outlined by the Ne VI and Mg VII transitions in the same wavelength region, is probably due to intensity calibration problems.
  • Si VIII: Si VIII lines 314.345, 316.220 and 319.839 overlap and the intersection with 276.85 allows a common solution for [FORMULA] and [FORMULA].
  • Si IX: Si IX allows the evaluation of only a lower limit [FORMULA]. Line 296.137 only very marginally agrees with the others, showing a too strong observed intensity. The reason for this behavior is probably due to an unresolved blending contribution.
  • Si X: Si X (Fig. 1) shows one of the best results; there are 5 density dependent lines which allow density diagnostic. Line 356 is a blend of two Si X transitions. The lines 271 and 277 originate from the same upper level and their L functions are nearly identical, as expected from theory. All the L functions meet for densities in the range 108.6 -109.8 cm-3.
  • Si XI Also the three Si XI lines agree over a rather narrow range of solutions: 108.8 -109.4 cm-3. The L functions for these transitions have values [FORMULA]. The values of the L function are higher than those found with Fe XIII, which shares the same log [FORMULA], while the value of the electron density is consistent with that provided by this ion.
[FIGURE] Fig. 5. The ratio of the observed intensity to the [FORMULA] is plotted versus the logarithmic density for all the lines of Ne V. For each line the observed intensity plus and minus the error is considered.


[TABLE]

Table 2. The SERTS-89 intensities for Active Region and Subflare. Wavelengths are in Angstrom, intensities are in [FORMULA] and Tmax in K.



[TABLE]

Table 3. Lines used for the d.e.m. analysis, listed in order of increasing temperature.


[FIGURE] Fig. 6. The ratio of the observed intensity to the [FORMULA] is plotted versus the logarithmic density for all the lines of Mg VIII. For each line the observed intensity plus and minus the error is considered.

A large number of highly ionized iron lines have been detected in the SERTS 89 observation and they provide a wealth of strong emission lines extremely useful for density diagnostic.

  • Fe X: The two Fe X lines are almost density independent; they agree reasonably within the experimental uncertainties but do not give any density diagnostic.
  • Fe XI Two pairs of Fe XI lines originate from the same upper level: 352.672 and 369.163 decay from the 3s3p5 -3 P2 level and their L functions completely overlap, while 358.667 is totally incompatible with 341.114 which decays from the same upper level 3s3p5 -3 P1 and shares common solutions with the other pair. The disagreement is probably due to unresolved contributions from lines belonging to Si XI, Ne IV and Ne VI to the Fe XI line 358.667. Line 308.575 allows to find solutions for any [FORMULA].
  • Fe XII: The Fe XII Active Region spectrum shows 7 lines density sensitive in the range 108 -1012 cm-3 (Fig. 7). The L functions of 6 lines meet for densities in the range 109.9 -1010.6 cm-3 within the limits of their error bars. Line 291 seems to have a slightly high observed intensity but no contributions from other Fe XII line or other ions' lines are expected. Line 195 has a too low L function. A problem arises with line 200, not shown in Fig. 7, because its L function is a factor 10 greater than all the others and no contributions from other lines are expected. This line is a second order lines but since no other second order lines (like Fe XII 195) suffer from a too high L function we do not expect to have any intensity calibration uncertainties. The behavior of this line is not understood.
  • Fe XIII: Several observed lines of Fe XIII are strongly density dependent in the density range 108 -1011 cm-3, and most of them meet for densities between 109.3 and 109.6 cm-3 (Fig. 8). Nevertheless the Active Region spectrum of Fe XIII presents a very strange feature: there is not good agreement between the lines with wavelength shorter than 300 Å  and those with wavelength longer than 300 Å. For instance lines 202 and 348 never meet and lines 203, 221 cross lines 318, 320 and 359 for densities much lower than those found with other lines. If we plot all the L functions of lines longward 300 Å  in a separate figure we see that the agreement between these lines is very good and the determination of the electron density of the emitting source much less controversial. The same behavior is found for lines shortward than 300 Å  and it is important to notice that the values of electron density measured with this two sets of lines are consistent: 109.35 -109.75 for lines with [FORMULA] Å , 109.1 -109.6 for [FORMULA] Å. The values of the L functions at the crossing point instead are different: lines longward 300 Å  have L functions higher than the others by a factor [FORMULA]. One could think to a systematic effect on the experimental intensity calibration: this feature should be plainly visible also for all the observed lines but all the other ions do not show this behavior. We think that the explanation of this behavior is to be found in the atomic physics calculations that provide the adopted collisional and radiative transition rates. The only thing to be noted about single lines is that the observed intensity of line 318 appears to be stronger than the other 300 Å  lines by a factor 1.4; it is worth noting that a similar behavior of line 318 is seen in nearly all CDS NIS-1 spectra (P.R. Young, private communications).
  • Fe XIV: There are 9 observed Fe XIV lines which are density sensitive in the range 108 -1011 cm-3 and can potentially give density diagnostic. Nevertheless the behavior of the Fe XIV is quite complex. Lines with wavelength shorter than 260 Å  are consistent with each other and their L functions cross in the density range 109.3 -109.6 cm-3. All the other lines do not cross this meeting point and have a greater L function than expected. The two lines 252 and 264 originate from the same upper level but their L functions agree only at the limit of their error bars, lines 334.2 and 353.8 meet for densities in the range 109.5 -109.9 cm-3 but the values of the L functions in that point is greater than the one provided by the lines with [FORMULA] 260 Å  by a factor 2.5. The intercombination line at 447 has L values even higher. It is not easy understand the reason for this behavior, since there are not unresolved contributions for all these lines and the problem is most likely to be found in the theoretical data used for calculating the [FORMULA] of these lines.
  • Fe XV: The electron densities greater than 10 [FORMULA] satisfy four Fe XV lines out of five. Line 417.245 is only marginally too high.
  • Fe XVI: Fe XVI presents perfectly overlapping four L-functions with no density diagnostic, as expected.
[FIGURE] Fig. 7. Results of the method for Fe XII. See comments in the text
[FIGURE] Fig. 8. Results of the method for Fe XIII. See comments in the text

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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