Spectral lines in FUV and EUV for diagnosing coronal magnetic field

TL;DR

This paper investigates the use of FUV and EUV spectral lines, focusing on Hanle and Zeeman effects, to diagnose the vector magnetic field in the solar corona, highlighting the advantages of Hanle-sensitive line combinations.

Contribution

It identifies optimal pairs of Hanle-sensitive spectral lines in the FUV and EUV range for effective coronal magnetic field diagnostics, emphasizing their proximity in wavelength and sensitivity.

Findings

Hanle signals are stronger than Zeeman signals by at least an order of magnitude.

A pair of Hanle-sensitive lines always exists for a given temperature range.

Close wavelength proximity of Hanle-sensitive line pairs facilitates magnetic field measurements.

Abstract

The diagnostic capabilities of spectral lines in far ultraviolet (FUV) and extreme ultraviolet (EUV) wavelength range are explored in terms of their Hanle and Zeeman sensitivity to probe vector magnetic field in the solar corona. The temperature range covered is log$_{10}(T)=5.5-6.3$. The circular polarization signal due to longitudinal Zeeman effect is estimated for spectral lines in the wavelength range of 500 to 1600 \r{A}. The Stokes $V/I$ signal for a FUV line is found to be in the order of 10$^{-4}$ for a longitudinal field strength of 10 Gauss, which further reduces to 10$^{-5}$ for wavelengths below 1200 \r{A}. Due to such low signals, the present study aims to find combination of spectral lines having different Hanle sensitivity but with identical peak formation temperature to probe coronal magnetic field vector. The combination of Hanle sensitive lines is better suited because the Hanle signals are stronger by at least an order of magnitude compared to Zeeman signals. The linear polarization signals due to Hanle effect from at least two spectral lines are required to derive information on the full vector. It is found from this study that there is always a pair of Hanle sensitive lines for a given temperature range suitable for probing coronal vector magnetic field and they are located in close proximity with each other in terms of their wavelength.