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TIME VARIATIONS OF THE CALCIUM
ELEMENTAL ABUNDANCE IN FLARES FROM NOAA ACTIVE REGIONS 2562 AND 2779
J. Sylwester, M.-C. Zolcinski-Couet, R. Bentley, J. Lemen
To cite this version:
J. Sylwester, M.-C. Zolcinski-Couet, R. Bentley, J. Lemen. TIME VARIATIONS OF THE CALCIUM ELEMENTAL ABUNDANCE IN FLARES FROM NOAA ACTIVE REGIONS 2562 AND 2779.
Journal de Physique Colloques, 1988, 49 (C1), pp.C1-189-C1-193. �10.1051/jphyscol:1988136�. �jpa-
00227456�
JOURNAL DE PHYSIQUE
Colloque C1, Supplement au n 0 3 , Tome 49, Mars 1988
TIME VARIATIONS OF THE CALCIUM ELEMENTAL ABUNDANCE IN FLARES FROM NOAA ACTIVE REGIONS 2562 AND 2779
J. SYLWESTER, M.-C. Z O L C I N S K I - C O U E T * , R.D. BENTLEY* and J.R. L E M E N * *
Space Research Centre, Polish Academy of Sciences, Kopernika 11, PL-51-622 Wroclaw, Poland
" ~ u l l a r d Space Science Laboratory, Holmbury St Mary, GB-Dorking.
RH5 6NT, Surrey, Great-Britain
* * Lockheed Palo Alto Research Laboratory, Dept. 91-20,
Building 255, 3251 Hanover Street, Palo Alto, CA 94304, U.S.A.
ABSTRACT
Analysis of flare spectra obtained with the Bent Crystal Spectrometer aboard the Solar M a x i m u m Mission (SMM) satellite enables the determination of the fluxes in the resonance (w) line of Ca XIX and the near-by continuum. The line-to-continuuA in- tensity ratio is a sensitive measure of the calcium elemental abundance (Aca) relative to hydrogen in the emitting plasma. In previous investigations we found the calcium abundance varies from flare to flare. In the present investigation we analyse the varia- tions of Aca for flares which occurred in two active regions well observed by SMM. We conclude that it is not possible to correlate the abundance variations with the time of the flare occurrence as suggested in an earlier paper for flares produced from a single active region. Further, we find no convincing correlation of abundance variation with any other flare characteristics. This negative result will stimulate future work on the physical interpretation of the calcium abundance variation.
INTRODUCTION
Reliable measurements of the X-ray continuum flux constitute one of the most complicated tasks for astrophysical Bragg spectroscopy. Spectrometers devoted to such purposes must possess high resolution and high sensitivity. The Bent Crystal Spec- trometer aboard SMM has the unique capability to reliably observe the continuum flux at wavelengths short of the helium-like resonance (w) line of Ca XIX ( ~ S O - ~ P I , X = 3.1781A). Both the w line and the continuum photon fluxes are effectively formed only in plasmas which have temperatures above w 8 x
lo6
K. Such high temperatures are normally observed only during solar flares. The computed emission functions for the Ca XIX w line and the near-by continuum have similar temperature dependences (Bely-Dubau et al. 1982; Mewe e t al. 1985), and therefore, the ratio of their fluxes is not sensitive to the temperature structure of the source. In our previous investigations (Syl- wester e t al. 1984; Lemen e t al. 1986) we found that the value of the line-to-continuum ratio varies between flares but is constant for a given flare during its decay. It was concluded that only a variation in the coronal calcium abundance could explain this be- haviour since all other causes, such as instrumental effects, could be ruled out. Among the many active regions which produced the flares analysed by Lemen e t al. (1986), the largest number of abundance estimates (seven) were made for flares observed from NOAA active region 2779 (6-19 November 1980). The time history of the calcium abundance for these seven flares led us to postulate that there may exist a systematic trend for the value of Ac, to increase with time for a given active region. The present paper discusses a thorough analysis of the Aca estimates for all flares which have been observed by the BCS from AR 2779. For comparison, we also present results for AR 2562 (11-21 July 1980).Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988136
C1-190 JOURNAL DE PHYSIQUE
DATA SELECTION
The following procedure was used to select the flares from the two active regions of interest:
1. The period when the active region was potentially visible by SMM was defined.
2. All statistically significant flare-like events were identified from the BCS light curve.
Only those flares which produced a t least 80 counts s-' in BCS channel 1 a t the flare peak were considered.
3. The flare spectra were processed. Typically, the data were summed into 20s time intervals to improve the statistical quality.
4. Synthetic spectra were fitted to the flare decay spectra using a least-squares pro- cedure (Lemen et al., 1984).
5. Based on values of the fitting parameters, the average electron temperature and the line-to-continuum intensity ratio were estimated.
6. Values of Aca were determined from the line-to-continuum results for each flare.
A more detailed description of the method for determining Aca will be given in a forthcoming paper. In all, 35 flares were processed for AR 2779, and 17 flares were processed for AR 2562.
RESULTS
Derived values of the calcium abundance for the flares from AR 2562 and 2779 are presented in Tables I and I1 and plotted as a function of time in Figures 1 and 2, respectively. The tables also contain additional characteristics of the investigated flares. The points which were used in the original analysis are indicated in the figures.
On average, the calcium abundance was systematically higher for flares from AR 2562 relative to flares from AR 2779.
I
Active Region 2562 results1
Day of July, 1980
Fig. 1. The calcium abundance, Aca, as a function of the day in July 1980 for flares from AR 2562. The error bars represent la uncertainties for the abundance measurements due to counting rate statistics. The possible uncertainties in the atomic data used in the analysis would cause the values of Aca to be systematically biased for all flares by no more than about 15 per cent.
Active Region 2779 results
I
Day of November, 1980
Fig. 2. The same as in Figure 1, except for AR 2779, which was observed in November 1980.
TABLE
1: Mean calcium abundances for flares in AR 2562July 1980 Classification HXRBS' BCS
Day. 'Time Ha GOES Length max total max Aca f
(UT) (8) (cnts) (cnts) (cnts)
Notes: The numbers in parenthesis refer to the power of 10.
'Based on Dennis et al., 1985.
+maximum in BCS channel 1 unobserved, lower limit given.
*No data available.
CI-192 JOURNAL DE PHYSIQUE
TABLE 2: :Mean calcium a b u n d a n c e s f o r flares i n AR 2779
Nov. 1980 Classification HXRBS' BCS
Day Time H a GOES Length max total max Aca f
(UT)
(4
(cnts) (cnts) (cnts)Notes: The numbers in parenthesis refer t o the power of 10.
'Based on Dennis e t al., 1985.
f maximum in BCS channel 1 unobserved, lower limit given.
*No data avaiIab1e.
DISCUSSION
The results of the present analysis do not support our earlier hypothesis that there is a trend for the value of Aca, observed from flares produced by a single active region, to increase with time. We have postulated that this trend - if present - would be consistent with the assumption that the increase of the abundance is due to long- acting pre-flare processes operating within the transition region. The occurrence of
a flare makes it possible to observe the transition region plasma after it is heated and evaporated into the coronal portion of the loops. The calcium abundance in the evaporated plasma (heated to temperatures above 10' K) would thus reflect the level of the heavy elemental abundance enhancement within the transition region. Assuming that the thermal diffusion and/or gravitational settlement processes are responsible for the increase of the abundance in the pre-flare transition region (Nakada, 1969;
Tworkowski, 1975), the characteristic time over which the abundance is observed to increase would lead to quantitative estimates of the rates for the processes involved.
However, our results indicate that this time dependence, if present at all, is not the only factor contributing to the observed abundance variations.
With the aim to understand better the flare-to-flare variations of the calcium abun- dance, we attempted to correlate this behaviour with the following flare parameters:
- Ha: classification, - GOES classification,
- Maximum and total hard X-ray flux,
- Duration of the hard X-ray component, - Rate of increase of the emission measure, - Rate of decrease of the electron temperature.
No significant correlation was found between the above parameters and values of Ac, Further studies are planned to search for correlations between the measured Ac, vari- ations and other specific flare characteristics, namely:
- Location within the active region, - Flare area as observed in X-rays, - Flare loop aspect ratio,
- Total energy deposited in the flare.
At present, however, we conclude that no correlation between the observed abun- dance variation and several flare characteristics can be identified.
ACKNOWLEDGEMENTS
J.S. acknowledges the support of the UK Science and Engineering Research Council and the Polish Academy of Sciences Programme CPBP 01.20.
REFERENCES
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Dennis B.R., Orwig L.E., Kiplinger A.L., Gibson B.R., Kennard G.S., and Tolbert A.K.:
1985, NASA, Technical Memorandum No. 86236.
Lemen J.R., Phillips K.J.H., Cowan R.D., Hata J., and Grant I.P.: 1984, Astron. As- trophys., 135,313.
Lemen J.R., Sylwester J., and Bentley R.D.: 1986, Adv. Space Res., Vol. 6 , No. 6, 245.
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