Elemental abundances in the solar corona are known to be different from those observed in the solar photosphere. The ratio of coronal to photospheric abundance shows a dependence on the first ionisation potential (FIP) of the element. We estimate FIP bias from direct measurements of elemental abundances from soft X-ray spectra using data from multiple space missions covering a range of solar activity levels. This comprehensive analysis shows clear evidence for a decrease in FIP bias around maximum intensity of the X-ray flare with coronal abundances briefly tending to photospheric values and a slow recovery as the flare decays. The departure from coronal abundances are larger for the low FIP elements Ca, Fe and Si than for S which have a mid FIP value. These changes in the degree of fractionation might provide inputs to model wave propagation through the chromosphere during flares.

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Advancements in computer-controlled polishing, metrology, and replication have led to an x-ray mirror fabrication process that is capable of producing high-resolution Wolter microscopes. We present the fabrication and test of a nickel–cobalt replicated full-shell x-ray mirror that was electroformed from a finely figured and polished mandrel. This mandrel was designed for an 8-m source-to-detector-distance microscope, with 10× magnification, and was optimized to reduce shell distortions that occur within 20 mm of the shell ends. This, in combination with an improved replication tooling design and refined bath parameters informed by a detailed COMSOL Multiphysics® model, has led to reductions in replication errors in the mirrors. Mandrel surface fabrication was improved by implementing a computer-controlled polishing process that corrected the low-frequency mandrel figure error and achieved <2.0 nm RMS convergence error. X-ray tests performed on a pair of mirror shells replicated from the mandrel have demonstrated <10 μm full-width at half-maximum (FWHM) spatial resolution. Here, we discuss the development process, highlight results from metrology and x-ray testing, and define a path for achieving a program goal of 5 μm FWHM resolution.

Soft X-ray spectroscopy of the Sun is an important tool to understand the coronal dynamics and composition. The solar coronal X-ray spectrum below 1 keV is the least explored with high-resolution spectroscopy. Recent observations with Hinode XRT using coarse spectroscopy along with high-resolution imaging have shown that abundances in the coronae have variability associated with structures on the Sun. Disk averaged abundances with better spectral resolution spectrometers show time variability associated with flares. Both spatial and temporal variabilities seem to be related to changes in the magnetic field topology. Understanding such short term variabilities is necessary to model the underlying dynamics and mixing of material between different layers of the Sun. A Sensitive high-resolution spectrometer that covers the range in plasma temperatures and emission line complexes would uniquely reveal the entire evolution of flares. We are investigating a design of a multi-layer mirror-based X-ray spectrograph in the spectral range from 0.5 to 7 keV. The instrument operates in four asynchronous spectral channels operating one at a time. The multi-layer mirror placed at the focus of a Wolter type I telescope reflects a narrow band X-rays to the CCD which is placed at Nasmyth defocus. Converging X-rays from the front end optics helps to increase the spectral range of each channel while preserving the spectral resolution. This design is estimated to achieve a spectral resolution of 20 eV in the spectral range of 0.5–7 keV. With such high spectral resolution, we can resolve individual spectral features e.g., 6.7 keV Fe complex which can be used to diagnose high-temperature transient plasma during flares. The instrument design estimated performance and the science capabilities of this instrument will be discussed in detail in the paper.

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