On a possible cause of discrepancy between Yohkoh and SMM calcium abundance determinations in flares

Abstract

We have investigated problems concerning the discrepancy between estimates of absolute calcium abundances (A Ca) in solar flares as derived from soft X-ray high resolution spectra as obtained from Yohkoh and Solar Maximum Mission ( SMM) Bragg spectrometers. The analysis of these spectra allows for determination of absolute calcium abundance since both Ca xix resonance line and neighbouring continuum are reliably measured in these experiments. The abundance determinations are possible by means of fitting the synthetic spectra to the observed ones. In previous work, we attributed part (around 50%) of the observed disagreement to a possible instrumental problem in Yohkoh. In the present research, we consider in more details possible causes of the other part of the discrepancy. We first investigate whether the estimates of (A Ca) depend substantially on the extension of the observed short wavelength wing of the Ca xix resonance line. This problem is important in the case of the analysis of Yohkoh spectra, since the extension of the spectrum towards the short wavelengths depends on the position of the flare along North—South direction on the Sun. After verifying the robustness of results of the spectral fitting technique, we have considered possible physical scenario which may cause the observed discrepancy. The hypothesis tested was that the Ca abundance in flares is correlated with brightness in the source — namely that it is a few times higher in the centre of the source (flaring kernel) than outside. We have found that postulated model may account for a part of the noted discrepancy as the SMM spectrometer has been equipped with the 6 × 6 arcmin collimator and no collimator has been used on Yohkoh.