Speaker
Description
Recent studies of coronal active regions and flare composition—particularly the
interpretation of STIX spectra using TEMIRA—indicate substantial variations in
iron abundance during flares. Consequently, these variations influence plasma
emissivities as observed in the two standard channels of GOES X-ray sensors
(XRS): the 0.5–4 Å (high-energy, H) and 1–8 Å (low-energy, L) bands. The
standard interpretation of XRS H and L bands relies on an isothermal assumption,
providing the average temperature (Tg) and emission measure (EMg) of the plasma
contributing to the bands' emission. The SolarSoft "goes" package allows the
determination of (Tg) and (EMg) values from momentarily observed H and L fluxes
using two plasma abundance models: coronal-extended (Feldman et al., 1992) and
photospheric (Grevesse N., Sauval A.J., 1998). We first demonstrate how the
assumed abundance model influences the temperature dependence of the
"contribution functions" calculated for the H and L channels, and we discuss the
respective differences in the derived Tg and EMg values. Next, we recalculate
the contribution functions using modern CHIANTI spectra (version 11.0.3 with
Giulio Del Zanna's modifications) for these two abundance models and discuss the
consistency of the results. Since TEMIRA results indicate a large variation in
Fe abundance (ranging from 7.4 to 9.0) between flares, as well as substantial
changes during individual flares, we investigated the respective variation in
GOES XRS emissivities across this wide range of Fe abundances. We show that to
obtain more physically meaningful average values for Tg and EMg, one must use
appropriate, momentary Fe abundances, A(Fe) (such as those obtained from
TEMIRA); otherwise, the results are substantially biased.