Variation in the intensity ratio at each wavelength point of the Si iv 1394/1403 \AA\ lines. Spectral diagnostics of a bifurcated eruption

TL;DR

This study investigates the variation of the Si IV 1394/1403 Å line intensity ratio in a bifurcated solar eruption, revealing that resonance scattering and opacity influence the ratio, which can serve as a diagnostic tool.

Contribution

It introduces the analysis of wavelength-dependent intensity ratio variations as a diagnostic for resonance scattering and opacity effects in solar spectral lines.

Findings

Most Si IV line ratios remained above 2 in the observed event.

The wavelength-dependent ratio varied from 2.0 to 3.3, indicating complex radiative transfer effects.

Different Doppler velocities were observed in the two lines at certain positions.

Abstract

Aims. This study aims to investigate the deviation of the intensity ratio of the \ion{Si}{IV} 1394 \AA\ and 1403 \AA\ emission lines from the expected value of 2 in the optically thin regime, as observed in many recent studies. Methods. We analyzed the integrated intensity ratio ($R$) and the wavelength-dependent ratio ($r(\Delta\lambda)$) in a small bifurcated eruption event observed by the Interface Region Imaging Spectrograph (IRIS). Results. Despite the relatively complex line profiles, most of the intensity ratio $R$ of \ion{Si}{IV} lines remained greater than 2 in the loops. The ratio $r(\Delta\lambda)$ varied in the line core and wings, changing distinctly from 2.0 to 3.3 along the wavelength. At certain positions, the \ion{Si}{IV} 1394 \AA\ and 1403 \AA\ lines exhibited different Doppler velocities. Conclusions. When diagnosing the spectra of small active region events, not only the impact of opacity but also the influence of resonance scattering should be considered. We propose that the ratio $r(\Delta\lambda)$ can serve as an indicator of the resonance scattering and opacity effect of the \ion{Si}{IV} line.