Varying Calcium Abundances in Solar Flares seen by Solar Maximum Mission

We report on calcium abundance $A({\rm Ca})$ estimates during the decay phases of 194 solar X-ray flares using archived data from the Bent Crystal Spectrometer (BCS) on Solar Maximum Mission (operational 1980~--~1989). The abundances are derived from the ratio of the total calcium X-ray line emission in BCS channel~1 to that in neighboring continuum, with temperature from a satellite-to-resonance line ratio. Generally the calcium abundance is found to be about three times the photospheric abundance, as previously found, indicating a ``FIP'' (first ionization potential) effect for calcium which has a relatively low FIP value. The precision of the abundance estimates (referred to hydrogen on a logarithmic scale with $A({\rm H}) = 12$), is typically $\sim \pm 0.01$, enabling any time variations of $A({\rm Ca})$ during the flare decay to be examined. For a total of 270 short time segments with $A({\rm Ca})$ determined to better than 2.3\% accuracy, many (106; 39\%) showed variations in $A({\rm Ca})$ at the $3\sigma$ level. For the majority, 74 (70\%) of these 106 segments $A({\rm Ca})$ decreased with time, and for 32 (30\%) $A({\rm Ca})$ increased with time. For 79 out of 270 (29\%) we observed constant or nearly constant $A({\rm Ca})$, and the remaining 85 (31\%) with irregular time behavior. A common feature was the presence of discontinuities in the time behavior of $A({\rm Ca})$. Relating these results to the ponderomotive force theory of Laming, we attribute the nature of varying $A({\rm Ca})$ to the emergence of loop structures in addition to the initial main loop, each with its characteristic calcium abundance.