New Solar Flare Calcium Abundances with no Surprises: Results from the SMM Bent Crystal Spectrometer

Abstract

The calcium abundance in flare plasmas is estimated using X-ray spectra from the Solar Maximum Mission Bent Crystal Spectrometer (BCS) during the decays of 194 flares (GOES classifications from B6.4 to X13) occurring between 1980 and 1989. Previous work by Sylwester et al. found that the abundance varied from flare to flare. That analysis is improved on here using updated instrument parameters and by including all calcium lines viewed by the BCS instead of only the resonance line, so greatly enhancing the photon count statistics. The abundance variations are confirmed with the average abundance, $A(\mathrm{Ca})$ (expressed logarithmically with $A(\mathrm{H})=12$), equal to $6.77\pm 0.20$ for 194 flares (141 of which are new in this study). This range corresponds to factors of between 1.7 and 7.2 larger than the photospheric abundance and so our results are in line with a ``FIP" (first ionization potential) effect whereby low-FIP elements like Ca (FIP = 6.11~eV) have enhanced coronal abundances. The Ca flare abundance is uncorrelated with solar activity indices, but weak correlations are suggested with GOES flare class and duration (larger $A(\mathrm{Ca})$ for smaller and shorter flares). The ponderomotive force theory of Laming explaining the FIP effect gives a range of parameters within which our estimates of $A(\mathrm{Ca})$ agree with the theory. However, this then gives rise to disagreements with previous estimates of the flare silicon and sulfur abundances, although those of argon and iron are in good agreement. Small adjustments of the theory may thus be necessary.

arXiv commentAccepted for publication in Astrophysical Journal. Paper has 17 pages with 11 figures. Table 1 in its entirety is available from the Authors (email addresses on p. 1)

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