Coronal Elemental Abundance: New Results from Soft X-Ray Spectroscopy of the Sun
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Coronal Elemental Abundance: New Results from Soft X-Ray Spectroscopy of the Sun Shyama Narendranath1 · P. Sreekumar2 · Netra S. Pillai1 · Singam Panini3 · K Sankarasubramanian1 · Juhani Huovelin4
Received: 28 May 2020 / Accepted: 17 November 2020 / Published online: 8 December 2020 © Springer Nature B.V. 2020
Abstract 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 ionization 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 the 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. Keywords Flares · Spectrum · X-ray · FIP · Cronal abundance
1. Introduction The abundance of elements in the solar photosphere is well established from many decades of systematic observations of the Sun using a variety of techniques; this has resulted in high confidence photospheric abundance values. However, abundance studies in the solar corona and coronae of other stars, have not kept pace compared to photospheric studies.
B S. Narendranath
[email protected] P. Sreekumar [email protected]
1
Space Astronomy Group, U R Rao Satellite Centre, ISRO, Bengaluru, India
2
Indian Space Research Organisation HQ, Bengaluru, India
3
Norwegian University of Science and Technology, Trondheim, Norway
4
Division of Geophysics and Astronomy, Department of Physics, University of Helsinki, Helsinki, Finland
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Though it is widely believed that there exists a steady flow of photospheric material into the corona, possibly channeled by structures associated with magnetic field configurations, the coronal abundance shows distinct deviations from photospheric values. For the solar case, studies show enhanced abundance of elements with low first ionization potential (FIP) over photospheric values, often categorized as a FIP bias (ratio of low FIP element coronal abundances to corresponding photospheric values). Stellar coronae also exhibit this effect (Drake, Laming, and Widing, 1997; Laming and Drake, 1999; Garcia-Alvarez et al., 2004) and the inverse, termed inverse-FIP (IFIP) effect (see Laming, 2015, for a review). Studies in the past such as those by Feldman (1992), Fludra and Schmelz (1999), Phillips et al. (2003), Feldman and Laming (2006), Sylwester et al. (2010), Sylwester et al. (2012), Schmelz et al. (2012), Narendranath et al. (2014), Dennis et al. (2015) and Moore et
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