XRISM observations of solar flare X-ray emission reflected in the Earth’s atmosphere

Abstract The X-ray Imaging and Spectroscopy Mission (XRISM), launched into low-Earth orbit in 2023, observes the reflection of solar flare X-rays in the Earth’s atmosphere as a by-product of celestial observations. Using a $\sim$1 yr data set covering 2023 October–2024 November, we report on our first results of the measurement of the metal abundance pattern and high-resolution Fe K spectroscopy. The abundances of Mg, Si, S, Ar, Ca, and Fe measured with the CCD detector Xtend during M- and X-class flares show the inverse-first-ionization-potential (inverse-FIP) effect, which is consistent with the results of Katsuda et al. (2020, ApJ, 891, 126) using the Suzaku satellite. The abundances of Si, S, and Ar are found to decrease with increasing flare magnitude, which is consistent with the theoretical model by Laming (2021, ApJ, 909, 17), whereas Ca exhibits an opposite trend. The large effective area and field of view of Xtend allow us to trace the evolution of the abundances in several X-class flare loops on a timescale of a few 100 s, finding an enrichment of low-FIP elements before flare peaks. The high-resolution Fe K spectrum obtained with the microcalorimeter Resolve successfully separates the Rayleigh- and Compton-scattered Fe xxiv$/$xxv lines and neutral or low-ionized Fe K$\alpha$ lines. The neutral/low-ionized Fe K$\alpha$ equivalent width shows an anti-correlation with hard X-ray flux with a best-fitting power-law slope of $-0.14 \pm 0.09$, suggesting that hard X-rays from flare loops are stimulating the Fe K$\alpha$ fluorescence. This work demonstrates that XRISM can be a powerful tool in the field of solar physics, offering valuable high-statistic CCD data and high-resolution microcalorimeter spectra in the energy range extending to the Fe K band.