High-Resolution Solar X-ray Spectroscopy from Archived Solar Maximum Mission Data

TL;DR

This study analyzes archived high-resolution X-ray spectra from the Solar Maximum Mission to determine plasma temperatures, examine Fe XVII line emission, and address discrepancies in atomic data, providing insights into solar active regions and flare plasma.

Contribution

It offers the first detailed analysis of FCS spectra with temperature diagnostics and investigates Fe XVII line ratios, highlighting the need for revised atomic data and proposing future instrument designs.

Findings

Fe XVII lines are evident at temperatures below 3 MK.

The underintensity of Fe XVII 3C and 3D lines is not due to resonant scattering.

Spectra during flare impulsive stages could reveal plasma densities.

Abstract

Archived high-resolution X-ray spectra in the 13~\AA\ to 22~\AA\ range from the Flat Crystal Spectrometer (FCS), an instrument on the Solar Maximum Mission operating in the 1980s, are analyzed with reference to nonflaring active regions, and to the \ion{Fe}{17} line emission in light of laboratory and atomic data for nearby \ion{Fe}{16} satellites. The satellites allow temperature to be found for these relatively low-temperature spectra, at which more conventional temperature-dependent line ratios are unavailable. By this means, the spectra can be arranged by temperature, showing that the \ion{Fe}{17} lines are evident at temperatures of $<3$~MK. We confirm that the problem of the underintense Fe XVII 3C and 3D lines is not due to resonant scattering, and instead suggest that, for comparison with CHIANTI spectra, the problem may lie with a needed revision of collisional excitation rates. The line ratio 3G/3H is in theory density-dependent but for \ion{Fe}{17} the ratio is in the low-density limit. However, we suggest that spectra taken during the impulsive stage of flares might reveal a departure from this limit and so allow densities to be derived and hence properties of the flaring plasma. Suggestions for the design of future crystal spectrometers are made in the light of the fluorescence background in FCS spectra.