Spectral Lines in FUV and EUV for Diagnosing Coronal Magnetic Field

Abstract

The diagnostic capabilities of spectral lines in the far-ultraviolet (FUV) and extremeultraviolet (EUV) wavelength ranges are explored in terms of their Hanle and Zeeman sensitivity to probe the vector magnetic fields in the solar corona. The temperature range covered is log10(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 Å. The Stokes V/I signal for a FUV line is found to be of the order of 10−4 for a longitudinal field strength of 10 gauss, which further reduces to 10−5 for wavelengths below 1200 Å. Due to such low signals, the present study aims to find combinations of spectral lines having different Hanle sensitivity but with identical peak formation temperature to probe the 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 the 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 Hanlesensitive lines for a given temperature range suitable for probing the coronal vector magnetic field and they are located in close proximity to each other in terms of their wavelength.