# Measuring relative abundances in the solar corona with optimised linear combinations of spectral lines

**Authors:** Natalia Zambrana Prado, Éric Buchlin  
**arXiv:** [1910.02886](https://arxiv.org/abs/1910.02886)  
**Date:** 2019-10-07  
**Summary page:** https://www.summarizepaper.com/en/paper/1910.02886v2/

## Key points

- The study focuses on measuring FIP (first ionization potential) bias in the corona for understanding coronal and heliospheric physics.
- Elemental abundances in some coronal structures differ significantly from photospheric abundances, with a dependence on the FIP of the element.
- The authors aim to build a method for optimal determination of FIP biases in the corona from spectroscopic observations that is independent from Differential Emission Measure (DEM) inversions.
- They optimize linear combinations of spectroscopic lines of low-FIP and high-FIP elements so that the ratio of their corresponding radiances yields the relative FIP bias with good accuracy for any DEM in a small set of typical DEMs.
- These optimized linear combinations allow retrieval of a test FIP bias map with good accuracy for all DEMs in the map.
- The results compare well with a FIP bias map obtained from observations using a DEM-dependent method.
- The study has several practical applications, including obtaining FIP bias maps from existing observations not specifically designed for this purpose, designing observations to obtain optimal FIP bias maps with given instruments, releasing constraints on spectroscopic line lists required to build FIP bias maps, and ultimately helping design solar or stellar UV spectrometers with FIP bias measurement capability.
- This method provides a convenient, fast, and accurate way to compute relative FIP bias maps that can be used to optimize existing observations and design new ones.
- The study's findings have implications for advancing our understanding of coronal and heliospheric physics as well as improving observational techniques in astronomy.

## Summary

The study focuses on the measurement of FIP (first ionization potential) bias in the corona, which is important for understanding coronal and heliospheric physics. Elemental abundances in some coronal structures differ significantly from photospheric abundances, with a dependence on the FIP of the element. The authors aim to build a method for optimal determination of FIP biases in the corona from spectroscopic observations that is independent from Differential Emission Measure (DEM) inversions. They optimize linear combinations of spectroscopic lines of low-FIP and high-FIP elements so that the ratio of their corresponding radiances yields the relative FIP bias with good accuracy for any DEM in a small set of typical DEMs. These optimized linear combinations allow retrieval of a test FIP bias map with good accuracy for all DEMs in the map. The results compare well with a FIP bias map obtained from observations using a DEM-dependent method. The authors present theoretical background information on contribution functions and differential emission measure to evaluate quantities involved in radiance equations. They also discuss how two spectroscopic lines emitted by ions of two different elements can be used to determine first ionization potential bias. The study has several practical applications, including obtaining FIP bias maps from existing observations not specifically designed for this purpose, designing observations to obtain optimal FIP bias maps with given instruments, releasing constraints on spectroscopic line lists required to build FIP bias maps, and ultimately helping design solar or stellar UV spectrometers with FIP bias measurement capability. Overall, this method provides a convenient, fast, and accurate way to compute relative FIP bias maps that can be used to optimize existing observations and design new ones. The study's findings have implications for advancing our understanding of coronal and heliospheric physics as well as improving observational techniques in astronomy.