Varying Calcium Abundances in Solar Flares Seen by the Solar Maximum Mission

Abstract

We report on calcium abundance A (Ca) estimates during the decay phases of 194 solar X-ray flares using archived data from the Bent Crystal Spectrometer (BCS) on the 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 3 times the photospheric abundance, as previously found, indicating a “first ionization potential” (FIP) 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 (H) = 12), is typically ∼± 0.01, enabling any time variations of A (Ca) during the flare decay to be examined. For a total of 270 short time segments with A (Ca) determined to better than 2.3% accuracy, many (106; 39%) showed variations in A (Ca) at the 3 σ level. For the majority, in 74 (70%) of these 106 segments A (Ca) decreased with time, and for 32 (30%) A (Ca) increased with time. For 79 out of 270 (29%) we observed constant or nearly constant A (Ca), and the remaining 85 (31%) with irregular time behavior. A common feature was the presence of discontinuities in the time behavior of A (Ca). Relating these results to the ponderomotive force theory of Laming, we attribute the nature of varying A (Ca) to the emergence of loop structures in addition to the initial main loop, each with its characteristic calcium abundance.

Keywords

• Solar flares