Variations in iron and calcium abundances during solar flares

Adv. Space Res. Vol. IS, No. 7, pp. (7)3747)40. 1995 chpynga 0 1995 CmPAR Riotedis Chat&Uain.Aufi bb m-served. 0273-I 1’1719 f $9.50 + 0.00

0273-l 177(94)ooo17-4

VARIATIONS IN IRON AND CALCIUM ABUNDANCES DURING SOLAR FLARES E. Antonucci md R. Martin lstituto di Fisicu, University of Torino, Via P. Giuria 1, 10125 Torino, Italy

ABSTRACT Evidence for variations in iron and calcium abundances during the impulsive phase of solar flares has been obtained by analyzing the Ca XIX and Fe XXV spectra, detected with the Bent Crystal Spectrometer of the Solar Maximum Mission. The plasma thermal conditions have been investigated by considering different temperature indicators: namely, the temperatures To., and TF~, derived from the intensity ratios of the dielectronic recombination satellites to the resonance line, and the temperature Toa,ze, calculated from the ratio of the resonance lines of Ca XIX and Fe XXV, which is also depending on the Fe/Ca abundance ratio. The observed values of To* and Tl+ can be ascribed to the specific characteristics of the plasma thermal distribution, the corresponding values of Toa,pe can be explained by allowing also for variations in the Fe/Ca abundance ratio relative to the photospheric ratio by a factor within 0.2 and 2.4. According to the observed abundance variations, the events analyzed can be divided in Ca-rich and Fe-rich flares. ANALYSIS OF THE INDICATORS OF PLASMA TEMPERATURE The relative abundance of iron to calcium in the solar flare plasmas is studied by analyzing the intensities of the soft X-ray lines detected with the Bent Crystal Spectrometer, flown on the Solar M&mum Mission, in the Ca XIX (3.165-3.226 A) and Fe XXV (1.843-1.896 A) spectral regions. The sample of flares included in our analysis consists of 26 events, detected in 1980 with one event detected in April 1984 (Table 1). The lines emitted from the flare plasma in the Ca XIX and Fe XXV spectral regions allow an accurate measurement of the temperature of this plasma either by means of a diagnosis based on the intensity of the dielectronic recombination satellites relative to the resonance line, or from the ratio of the Ca XIX and Fe XXV resonance lines. The ratio of the dielectronic satellites to the resonance line intensity depends exclusively on the plasma temperature, thus allowing us to define both a calcium and an iron temperature, To,, and TF~, for the Ca XIX and Fe XXV spectrum respectively, completely independent of the ionization conditions of the emitting plasma. The calcium and iron temperature, however, assume the same value only in the ideal case of an isothermal plasma. For non-isothermal plasmas, these two quantities are different, being derived from a ratio of line intensities, each one, Ij, given by the integral of the differential emission measure of the plasma, 4(T) = nf dV/dT, where n, is the plasma density and dV is the volume element, weighted with the emission function Gj(T) = (Ni/Nz)j(T) Cj(T) of the line:

Ij

0~ Aj

J

(Ni/NE)j(T) Cj(T) 4(T) dT.

(1)

The quantity Aj is the abundance of the element relative to hydrogen, (Ni/Nz)j(T) is the ionization balance fractional term, Cj(T) is the collisional excitation coefficient. The temperature Toa,ze, obtained from the intensity ratio of the resonance lines of Ca XIX and (7)37

(7)X8

E. Antooucci and R. Martin T&U&J

Solar flares included in the analysis

Flare Date 08 Apr 1980 10 Apr 1980 09 May 1980 21 May 1980 13 Jun 1980 25 Jun 1980 29 Jun 1980 29 Jua 1980 29 Jun 1980 01 Jul 1980 05 Jul 1980 12 Jul 1980 14 JuI 1980 21 Jul 1980 23 Aug 1980 24 Aug 1980 31 Aug 1980 24 Sep 1980 20 Ott 1980 05 Nov 1980 05 Nov 1980 06 Nov 1980 12 Nov 1980 18 Nov 1980 19 Nov 1980 24 Apr 1984

Class M4 M4 M7 Xl M4 M3 M4 M4 X2 M9 M4 Xl M8 M2 Ml M2 Ml Ml Ml M4 M4 Ml M3 M9 Xl3

Time (UT) 03:05:33 09:19:37 07:12:26 21:00:34 22134~11 15:51:18 02:34:48 10:42:22 18~23~43 16:27:18 22:42:59 11:16:38 08:25:40 02:59:19 21:27:39 16:11:29 12:51:33 07:35:00 18:33:11 22:27:00 22:35:12 17:28:31 17:01:21 14:54:20 05:35:02 23:58:26

Log 7.3 7.3 7.5 7.3 -

Tr 7.8 7.8 7.8 7.8

7.3 - 7.8

7.4 7.5 7.5 7.3 7.5 7.4 7.3 7.3 7.3 7.3 7.4 7.3 -

7.8 7.8 7.8 7.8 7.8 7.8 7.8 7.8 7.8 7.8 7.8 7.8

Fe/Ca Fe Fe Ca Fe

rich rich rich rich

Fe rich Fe rich Fe rich Fe rich Fe rich Ca rich Ca rich Fe rich Fe rich Fe rich Ca rich

Fe rich Ca rich Ca rich Ca rich

7.4 - 7.8 7.4 - 7.8

Ca rich Ca rich

7.3 - 7.8 7.4 - 7.8

Ca rich

Fe rich

2.21 p 2.30 2.30 - 2.40 0.45 - 0.47 2.16 - 2.21 1.74 1.88 1.13 1.27 1.33 - 1.36 0.42 - 0.44 0.66 - 0.68 1.52 - 1.62 1.21 - 1.25 1.90 - 1.94 0.43 - 0.47 0.89 - 0.92 1.79 - 1.86 0.92 - 0.99 0.52 - 0.56 0.58 - 0.59 0.19 0.56 - 0.57 0.47 0.85 1.46 - 1.81 0.32 - 0.33

Fe XXV, on the other hand, is not only dependent on the thermal properties but it is also affected by the iron to cakium abundance ratio Ar./Ao, and by the ionization conditions of the plasma. Thus for plasmas in ionization equilibrium, a difference of Tos,r+ from To. and TF= can be only in part due to the thermal distribution; in general it is also due to the iron to calcium abundance ratio. In the majority of cases (- 80%), flare plasmas are non-isothermal, and this condition impIies a difference in the observed calcium and iron temperatures, as shown in Fig. 1 a, for the very impulsive flare observed on July 1, 1980. Considering this example, values coincident with the observed intensities of the lines emitted in the Ca XIX spectral region and with the observed line ratios used to derive Tre can be computed by assuming an emitting plasma with a differential emission measure function $(T), which consists of a ‘hot’ component centered at log Ti = 7.2, with Ti close to To,, and a ‘superhot’ component centered at log Tr = 7.6, both with a gaussian profile characterized by a full-width-halfmaximum equal to A log(T) = 0.35. This function can be found by adopting a simple fitting method based on the difference between Tc. and Trc, which is a measure of nonisothermal conditions /l/. The assumed plasma thermal distribution is only one of the possible differential emission measures compatible with the intensities and line ratios of the limited set of spectral lines available. However, although the function 4(T) yields exactly the observed values of T oa and Tre, it is impossible to compute from 4(T) a temperature Tcs,re equal to the observed one (continuous line in Fig. 1 b). The quantity Tos,re derived from the function b(T), plotted as a dashed line in Fig. 1 b, results to be much larger than the observed one (both the observed and simulated values of Toe,re

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1 July

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observed simulated simulated

(pW = 6.9) (pW = 0.9) (,o = 0.4)

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400 Time (s) Figure 1: July 1, 1980 flare: a) evolution of the observed Ca and Fe temperatures To,, and TF~, derived from the intensity ratios of the dielectronic recombination satellites to the resonance line in the Ca XIX and Fe XXV spectrum, respectively; b) evolution of the observed temperature Tos,~= (continuous curve) derived from the ratio of the resonance lines of Ca XIX and Fe XXV. The dashed curve and the diamonds give the simulated temperature Tc~,F~ inferred for a plasma with the same difkrential emission measure, which is used to derive Tca and TF~ fitting the observed values, but different Fe/Ca abundances. are derived for the abundances given by Veck and Parkinson /2/). This discrepancy can be removed if we allow for a variation in the relative iron to calcium abundance. This is equivalent to imposing the condition to fit not only the ratios but also the intensities of lines in the Fe XXV spectral region, when modeling the thermal distribution of the flare plasma. In the case of the July 1, 1980 flare, if the relative abundance is decreased from p = (Avc/Aos)vp / (Ave/ ACT)+ = 0.9 to a value p = 0.4, where (Aw/Ao,)vp is the relative abundance given by Veck and Parkinson /2/ and (Ave/Aoa)*h is the photospheric value /3/, the simulated values of Tos,ve (diamonds, Fig. 1 b) coincide with the observed ones. IRON TO CALCIUM

ABUNDANCE

VARIATIONS

IN FLARES

The same analysis has been performed for each flare of the selected sample, at the time given in Table 1, which for non-isothermal flares is characterized by the largest discrepancy of the quantities To,, TF~ and Tos,ve. For non-isothermal flares, it is always possible to find a set of two-component differential emission measures, whose principal characteristic is to include both a ‘hot’ component at a temperature close to To. and a ‘superhot’ component, all of them consistent with the observed iron and calcium temperatures and the intensity of the calcium lines. The differential emission measures constituting a given set differ mainly

E. Autonucci and R. Martin

c1w

for the parameters, emission measure and temperature, of the ‘superhot’ component. For the each set of b(T) functions, the peak temperature of the ‘superhot’ component, Ts, ranges within the limits given in Table 1. For a given 4(T) function, the intensities of the emission lines in the Fe XXV region can then be fitted to the observed ones only by adjusting the relative iron to calcium abundance of the flare plasma. This is equivalent to deriving a simulated To.,v=, coinciding with the observed one. The method of analysis adopted here is explained in detail in paper 11,‘. The values found in this way for the relative iron to calcium abundance p, normalized to the photospheric value ( Ave) / Aoa)vh = 14.73 /3/, depend only weakly on the specific differential emission measure profile chosen within the possible solutions considered here. As an example, for the April 8, 1980 flare, a ‘superhot’ component temperature varying between 7.3 and 7.8 in log Tz causes a variation of the normalized iron to calcium abundance p within 2.21-2.30 (Table 1). The excursion of the relative abundance associated with the variation of the parameters of the ‘superhot’ component is assumed as the uncertainty in the measurement of p. A subset consisting of 7 of the events of Table 1 have been also studied by Fludra et al. /4/ to derive the abundances of calcium and iron relative to hydrogen, by means of a different method, that is by deriving the differential emission measure of the flare plasma with an inversion technique. Notwithstanding the fact that they limit the analysis to the decay of flares and use a different ionization balance calculation /5/ from the one adopted in our analysis /6,7/, in both analyses the flares which have p > 1 are the same. Although the set considered by Fludra et al. has very few examples of flares with ps l,andifthi s is the case p is close to unity, again the events with p 2 1 are the same in the two samples. Considering the Ca and Fe abundances derived by Fludra et al. /4/, we can suggest that the relative enhancement of iron with respect to calcium, found in our analysis, corresponds to an actual iron enrichment; conversely, an enhancement of calcium relative to iron corresponds to an enrichment of calcium in the flare plasma. In Table 1 we have therefore identified flares as Fe-rich and Ca-rich, pointing out those flares with the largest enrichments of either iron or calcium. The principal result of this study is that in flare plasmas the relative iron to calcium abundances can vary remarkably with respect to the photospheric value, being the normalized abundance ratio included within 0.2 and 2.4 (Table 1). Namely, in flare plasmas we find either enhancements of iron relative to calcium by factors 22, or enhancements of calcium with respect to iron by factors 5 5. Considering that calcium is ionized at much lower temperature than iron, and therefore at cooler layers of the solar atmosphere with respect to iron, the present results might be indicative of differences in the height of the site where the flare process occurs. REFERENCES 1. E. Antonucci and R. Martin, Astrophys. J., submitted (1994). 2. N. Veclc and J. Parkinson, h4NRaS , a,

41 (1981).

3. D.V. Reames, Pmceedingr of the First Soho Workshop, Annapolis, =A (1992).

SP-348, 315

4. A. Fludra, R.D. Bentley, J.L. Culhane, J.R. Lemen and J. Sylwester, Ado. Space Res., u, 155 (1991). 5. M. Arnaud and R. Rothenflug, A&on.

Adtrophyd. Suppl.

,

6Q, 425 (1985).

6. V.L. Jacobs, J. Davis, P.C. Kepple and M. Blaha, Astrophys. J., 2Lb, 605 (1977). 7. V.L. Jacobs, J. Davis, J.E. Rogerson, M. Blaha, J. Cain and M. Davis Astrophys. J., a, 1119 (1980).