On the Detectability of Oxygen X-ray Fluorescence and its Use as a Solar Photospheric Abundance Diagnostic

TL;DR

This study explores the potential of using oxygen K-alpha X-ray fluorescence as a diagnostic tool for determining solar photospheric oxygen abundance, through Monte Carlo simulations of line emission and sensitivity analysis.

Contribution

It demonstrates that oxygen K-alpha fluorescence can serve as a viable and sensitive diagnostic for solar oxygen abundance, with potential to resolve the solar 'oxygen crisis.'

Findings

Line equivalent widths range from 0.02 to 0.2 Å depending on plasma temperature.

Line remains uncontaminated by blends at coronal temperatures ≤ 3 million K.

Changes in oxygen abundance by a factor of 2 cause 35-60% variation in line strength.

Abstract

Monte Carlo calculations of the O Kalpha line fluoresced by coronal X-rays and emitted just above the temperature minimum region of the solar atmosphere have been employed to investigate the use of this feature as an abundance diagnostic. While quite weak, we estimate line equivalent widths in the range 0.02-0.2 AA, depending on the X-ray plasma temperature. The line remains essentially uncontaminated by blends for coronal temperatures T =< 3e6 K and should be quite observable, with a flux >~ 2 ph/s/arcmin^2. Model calculations for solar chemical mixtures with an O abundance adjusted up and down by a factor of 2 indicate 35-60% changes in O Kalpha line equivalent width, providing a potentially useful O abundance diagnostic. Sensitivity of equivalent width to differences between recently recommended chemical compositions with ``high'' and ``low'' complements of the CNO trio important for interpreting helioseismological observations is less accute, amounting to 20-26% at coronal temperatures T ~< 2e6 K. While still feasible for discriminating between these two mixtures, uncertainties in measured line equivalent widths and in the models used for interpretation would need to be significantly less than 20%. Provided a sensitive X-ray spectrometer with resolving power >= 1000 and suitably well-behaved instrumental profile can be built, X-ray fluorescence presents a viable means for resolving the solar ``oxygen crisis''.