Kavli Affiliate: Eric Miller
| First 5 Authors: Hiromasa Suzuki, Hiromasa Suzuki, , ,
| Summary:
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 $sim$one-year
data set covering from October 2023 to November 2024, 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., ApJ, 2020 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 (Laming,
ApJ, 2021), 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 the best-fit 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.
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