On the formation of Lyman $\beta$ and the O I 1027 and 1028 \AA\ spectral lines

TL;DR

This study analyzes the formation and diagnostic capabilities of Lyman β and O I spectral lines for understanding the solar chromosphere and transition region, highlighting their sensitivity to temperature and velocity at different heights.

Contribution

It provides a detailed synthesis and response function analysis of Lyman β and O I lines, demonstrating their complementary roles in probing the solar atmosphere's thermal and dynamic properties.

Findings

Lyman β sensitive to temperature from middle chromosphere to transition region

O I lines form lower in the middle chromosphere and are sensitive to lower height velocities

Spectral lines at 1025 Å can jointly trace the thermal and gas dynamic structure of the solar atmosphere

Abstract

We study the capabilities of Lyman $\beta$ and the O I 1027 and 1028 \AA\ spectral lines for understanding the properties of the chromosphere and transition region. The oxygen transitions are located in the wing of Lyman $\beta$ that is a candidate spectral line for the solar missions Solar Orbiter/SPICE and Solar-C (EUVST). We examine general spectroscopic properties of the three transitions in the quiet Sun by synthesizing them assuming non-local thermal equilibrium taking into account partial redistribution effects. We estimate the heights where the spectral lines are sensitive to the physical parameters computing the response functions to temperature and velocity using a 1D semi-empirical atmospheric model. We also synthesize the intensity spectrum using the 3D enhanced network simulation computed with the BIFROST code. The results indicate that Lyman $\beta$ is sensitive to the temperature from the middle chromosphere to the transition region while it is mainly sensitive to the line-of-sight velocity at the latter atmospheric layers, around 2000 km above the optical surface. The O I lines form lower in the middle chromosphere, being sensitive to the LOS velocities at lower heights than those covered by Lyman $\beta$. The spatial distribution of intensity signals computed with the BIFROST atmosphere, as well as the inferred velocities from the line core Doppler shift confirm the previous results. Therefore, these results indicate that the spectral window at 1025 \AA\ contains several spectral lines that complement each other to seamlessly trace the thermal structure and gas dynamics from the middle chromosphere to the lower transition region.