A C O M P A R I S O N OF T H E O R E T I C A L F e x l I E M I S S I O N LINE

S T R E N G T H S W I T H E U V O B S E R V A T I O N S OF A SOLAR ACTIVE

R E G I O N

F. P. K E E N A N

Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 lh~V, N. Ireland

J. G. D O Y L E

Armagh Observatory, Armagh BT61 9DG, N. Ireland

S. S. T A Y A L

Department of Physics and Astronomy, Louisiana State University. Baton Rouge, LA 70803-4001, U.S.A.

and

R. J. W. H E N R Y

Department of Physics, Auburn University, Auburn, AL 36849, U.S.A.

(Received 19 November, 1990; in revised form 5 February, 1991)

Abstract. New theoretical electron-density-sensitive FexII emission line ratios

Rl = 1(3s23p s 4S3 2 - 3s3p 4 4Ps.2)/l(3s23p3 2p3.2 - 3s3p 4 205.,2)

and

R2 = l(3s23p 3 2p32 - 3s3p 4 2D~,2)/I(3s23p3 4S3 2 - 3sZ3p 3 2P3,z)

are derived using R-matrix electron impact excitation rate calculations. We have identified the FexJl 3s23p 3 4S3 2 - 3s3p 4 4p5:2, 3s23p s 2p3.2 - 3s3p 4 ZDs,z, 3sZ3p 3 4S3.2 - 3s23p 3 2P3.2 and 3s23p 3 4S3. a - _ 3s23p3 2pj '2 transitions in an active region spectrum obtained with the Harvard S-055 spectrometer

on board Sk)'lab at wavelengths of 364.0, 382.8, 1241.7, and 1349.4 ~,, respectively. Electron densities determined from the observed values of R~ (logN e _~ 11.0) and R a (logN~-~ 11.4) are significantly larger than the typical active region measurements , but are similar to those derived from some active region spectra observed with the Slcylab 2082A instrument, which provides observational support for the atomic data adopted in the line ratio calculations, and also for the identification of the Fe xlI transitions in the S-055 spectrum. However the observed value of R 3 = I( 1349.4 .,~)/I(1241.7 .~) is approximately a factor of two larger than one would expect from theory which, considering that the 1349.4 A. line lies at the edge of the S-055 wavelength coverage, may reflect errors in the instrument efficiency curve. Another possibility is that the 1349.4 .~ transition is blended, probably with S in 1350.1 A..

I. Introduction

Emission lines arising from transitions in FexII have been observed in many solar features, including active regions (Dere, 1982), flares (Widing and Doyle, 1990) and sunspots (Sandlin et al., 1986). They may be used to infer the electron density of the emitting plasma through diagnostic line ratios (see Dere etal., 1979), although to

Solar Physics 135: 353-360, 1991. �9 1991 Kluwer Academic Publishers. Printed in Belgium.

354 F . P . K E E N A N ET AL.

calculate such ratios reliably accurate atomic data must be employed, especially for electron impact excitation rates (Dufton and Kingston, 1981).

Over the past few years we have calculated electron impact excitation rates for transitions in Fexn using the R-matrix code of Burke and Robb (1975). These atomic data (Tayal, Henry, and Pradhan, 1987; Tayal and Henry, 1988) have subsequently been used to produce Ne-diagnostics for solar spectra in the wavelength intervals 1240-3070 A (Tayal and Henry, 1988), 338-364 ]~ (Tayal et al., 1989) and 186-197 'A (Tayal et al., 1991). In this paper we extend this work to derive ratios for both E U V

(2 ~ 1300 A) and X U V (2 = 380 A) transitions that we identified in the spectrum of an active region obtained with the Harvard S-055 spectrometer on board Skylab (see Section 3), and show that these are potentially useful as electron density diagnostics.

2. Theoretical Ratios

Level populations for a wide range of electron temperatures and densities have been published for FexII by Tayal et al. (1989, 1991). Briefly, the model ion consisted of the 3s23p3 4S, 20, 2p; 3s3p4 4p, 2D, 2p, 2S; 3s23p2 (3P)3d 2p, (3p)3d 4p, (iS)3d 2D,

(1D)3d 2D, (~D)3d 2p, (3p)3d 2F, (1D)3d 2 S and (3p)3d 2D states, making a total of 29 fine structure levels. Collisional excitation and de-excitation by electrons and spon- taneous radiative de-excitation were the only atomic processes considered in the calcu- lation, and the plasma was assumed to be optically thin. Further details may be found in Tayal etal . (1989, 1991).

As noted by, for example, Keenan and Norrington (1987), level populations may be used to derive emission line ratios R through the expression

R = I(2~j)/I()o,,,,,) - bj Aj, 2ran , (1) N , , A ..... Z o

where 2ij, 2mn and I(2ij), I(2m,,) are the wavelengths and intensities (in energy units) of the lines, respectively', Nj and N n are the upper level populations of the relevant transitions and Aj; and An, . are the Einstein A-coefficients. In Figures 1 and 2 we use the FexTt level populations listed in Tayal et al. (1991) in conjunction with the A-values of Tayal and Henry (1986) to plot the emission line ratios

and

R , = I(3s23p 3 4S3,, 2 - 3s3p 4 4ps,2)/[(3s23p3 2 p 3 , 2 - 3s3p 4 2D5.,,2)

R 2 = l (3s23p 3 2P3. 2 - 3s3p 4 2Ds,,,2)/'I(3s23p3 4 S 3 , , 2 - 3s23p 3 2P3,,,2)

as a function of electron density at the temperature of maximum FexIl fractional abundance in ionisation equilibrium, T e = Tma x = 1.5 x 106 K (Arnaud and Rothen- fiug, 1985). We note that the results are relatively independent of the adopted electron temperature with, for example, a change of _+ 0.2 dex in log T e leading to only a ~ 103.;, change in R 1 o r R 2 at N e = 1 0 9 c m - 3 , and only ~ 6~ at N e = 1012 c m - 3 .

Fex[t EMISSION LINE S T R E N G T H S IN .AN ACTIVE REGION 355

1.5

(~

1.0

1.5

I i I I

i I I [

9 10 1] 12

[0g N e Fig. 1. The theoretical F e x n emission line ratio

R I = I ( 3 s 2 3 p 3 4S3. 2 - 3s3p 4 aPs.2),."I(3s23p3 2P3 2 - 3s3p 4 ZDs.2) = I(364.0 A)/I(382.8 ~ ) ,

where I is in energ3' units, plotted as a function of electron density at the electron temperature of maximum Fe Xlr fractional abundance in ionisation equilibrium, T~ = Tm~• = 1.5 x 106 K.

However , an inspection of the figures reveals that R 1 and R 2 a r e very sensitive to

variations in the electron density and, hence, may be useful as Ne-diagnostics. For example, R l and R2 vary by factors of ~ 8.7 and ~ 45, respectively, between N e = 109 and 10 ~2 cm -3

3. Results and Discuss ion

We have identified the F e x I I 3s23p 3 4 3 3 2 - 3s3p 4 4p5.2, 3s23p 3 2P3,. 2 - 3s3p 4 205..2, 3s23p 3 4P3. 2 - 3s23p 3 2P3. 2, and 3sZ3p 3 4S3. 2 - 3s23p 3 2Pl, 2 transitions at wave-

lengths of 364.0, 382.8, 1241.7, and 1349.4.A, respectively, in an active region spectrum obtained with the Harvard S-055 E U V spectrometer on board Skylab (Doyle, Mason, and Vernazza, 1985). This instrument, which covered the wavelength region 280-1350 A, observed a spatial area of 5 x 5 arc sec with a spectral resolution of approximately 1.5 A. ( F W H M ) using an integration time of 0.04 s and a step length of 0.2112 A. It is discussed in detail by Reeves, Huber , and Timothy (1977) and Reeves et al. (1977).

356 F . P . K E E N A N ET AL.

l_

r-F"

0

0.5

0.0

-0.5

I I I I

-1.0 l I I , g 10 11 12

tog Ne Fig. 2. The theoretical F e x n emission line ratio

R 2 = I ( 3 s 2 3 p 3 2 p y 2 - 3s3p 4 2Ds,2),, 'I(3s23p3 4 S 3 , 2 - 3sZ3p 3 2 p 3 : 2 ) = I(382.8 .~t)/I(1241.7 ~ ) ,

where I is in energy units, plotted as a function of electron density at the electron temperature of maximum Fe XlI fractional abundance in ionisation equilibrium, T e = Tma • = 1.5 x 106 K.

TABLE I

Observed Fe xll emission line intensities 1(2) (in erg cm 2 s - ' s r - ' ) and line ratios I(~. 1 )/1(22) in an active region, and the derived logarithmic electron densities.

2 t (#k) 22 (A) 1(2, ) 1(22) 1(21 )/I(22) l o g N

364.0 382.8 702 _+ 103 99 _+ 45 7.1 + 3.4(R,) 11.0_+~-.4 382.8 1241.7 99 _+ 45 62 _+ 17 1.6 _+ 0.8(R2) 1 1.4_+~

1349.4 1241.7 40 + 7 62 + 17 0.6 + 0.2(R3) -

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At~

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(y) x S9[: 09C

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358 F.P . KEENAN ET AL.

We have determined Fexn line strengths, and hence ratios, in the active region by using the STARLINK reduction package DIPSO (Howarth and Maslen, 1984) to fit Gaussian profiles to the spectrum. In the case of the 364.0 and 1241.7 A transitions, which lie in the wings of Mg vu 364.7 A and N v 1242.8 A, respectively, line strengths were estimated by employing the procedures developed by us for application to blended absorption lines in stellar spectra (see Brown et al., 1986, for more details). The FexH line intensities and ratios R 1 = 1(364.0 ,A)/I(382.8 A_) and R 2 = 1(382.8 A)/I(1241.7 A) are summarised in Table I, along with the observed value of the ratio R 3 = = 1(1349.4 A)/I(1241.7 A), while in Figure 3 we show the active region spectrum from 355-370 ,~ and 1235-1250 A, to illustrate the quality of the observational data.

Also listed in Table I are the electron densities determined from the observed values of RI and R 2 and the calculations in Figures 1 and 2. An inspection of the table reveals that the electron densities estimated from R~ and R 2 are in good agreement, with a discrepancy of only 0.4 dex. Unfortunately, there are no high temperature diagnostic line ratios in the active region spectrum with which to compare these results (see Doyle, Mason, and Vernazza, 1985). In addition, the derived values of N e are approximately an order of magnitude larger than the typical active region results of Dere (1982). However, our electron densities are similar to those deduced from some active region spectra observed with the S082A spectrograph on board Skylab. For example, Dere (1982) found log N e = 10.6 for McMath 12686 from the 1(233.9 A)/I(243.8 A) ratio in Fexv, while Tayal etal . (1991) deduced logN e = 10.8 for McMath 12375 from I(186.9 A)/I(193.5 A) and I(196.6 A)/I(193.5 A) in FexlI.

In the case of R 3, the theoretical results are insensitive to changes in the adopted electron temperature, and depend only weakly on the electron density, varying from ( R 3 = 0 . 6 ) i s , therefore, approximately a factor of two larger than one would expect from theory.This may be due to errors in the S-055 sensitivity curve near the edge of the instrument wavelength coverage (where the 1349.4 A transition lies), or blending of the 1349.4 A feature, possibly with Sin 1350.1 A (Sandlin et al., 1986).

4. Conclusions

There are two principal conclusions. (1) Theoretical FeII emission line ratios

R 1 = 1(3s23p 3 483. , 2 -- 3s3p 4 4ps,2)/I(3sZ3p3 2P3.. 2 - 3s3p 4 205,.2)

and

R 2 = 1 (3823p 3 2 P . ~ , 2 - 3s3p 4 2D5 , .2 ) / l (3 s23p3 483.,. 2 - 3 8 2 3 p 3 2 P 3 / 2 ) ,

determined using electron impact excitation rates calculated with the R-matrix code, are found to be relatively insensitive to changes in T e, but to vary by factors of ~ 8.7 (R1) and ~ 45 (R2) between Are = 10 9 and 1012 cm- 3, respectively. Hence, they are potenti- ally useful as Ne-diagnostics for the upper solar transition region/corona.

Fextl EMISSION LINE STRENGTHS IN AN ACTIVE REGION 359

(2) T h e F e x I I 3 s 2 3 p 3 483/2 - 3 s 3 p 4 4/95.,2 , 3 s 2 3 p 3 2p3.. 2 - 3 s 3 p 4 2 0 5 . 2 , 3 s 2 3 p 3 4S3.2 -

3 s 2 3 p 3 2/93/2 and 3s23p34S3 , .2 - 3s23p32Pl... 2 transitions have been identified in an active region spectrum obtained with the Harvard S-055 spectrometer on board Skylab at wavelengths of 364.0, 382.8, 1241.7, and 1349.4.~, respectively. Electron densities derived from the observed values of

and

R 1 = I(364.0 A)/I(382.8 .A)

R 2 = I(382.8 A)..'I(1241.7 A)

are in good agreement, and are similar to those determined from some active region spectra observed with the S082A instrument on board Skylab (Tayal et aL, 1991). This provides observational support for the atomic data adopted in the line ratio calculations, and also for the identification of the Fexn transitions in the S-055 spectrum. A comparison of the observed value of R 3 = I(1349.4 .A)/I(1242.0 A) with theory implies that the measured 1349.4 .~ line intensity has been overestimated by approximately a factor of two, due to either errors in the S-055 instrument sensitivity curve, or blending with possibly SilI 1350.1 .~.

Acknowledgements

We would like to thank Prof. A. E. Kingston for his continued interest in this work, and an anonymous referee for several suggestions which improved the clarity of the paper. This research was supported in part by the U.S. Department of Energy, Division of Chemical Sciences and by NATO travel grant 0469/87. Research at Armagh Observa- tory is grant-aided by the Department of Education for Northern Ireland.

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