On the Origin of the Balmer and Lyman Emission Lines

TL;DR

This paper investigates the origins of hydrogen Balmer and Lyman emission lines in solar prominences, comparing observations and analyzing their physical properties to understand their spatial and excitation differences.

Contribution

It provides a detailed comparison of Lyman and Balmer line emissions, revealing differences in excitation and spatial origin, and estimates brightness temperatures using line optical thickness.

Findings

Lyman and Balmer emissions originate from different spatial regions.

Effective hydrogen atom densities differ significantly between the two series.

Brightness temperature from Lyman lines is largely independent of upper level number.

Abstract

We show how the observed hydrogen Balmer and Lyman emission lines constrain the modeling of quiescent solar prominences. We compare space observations of Lyman lines with ground-based observations of Balmer lines for quiescent solar prominences of comparable brightness defined by their H-beta emission. The effective number densities of hydrogen atoms emitting from the same upper level u deduced from the corresponding emerging Lyman and Balmer line emissions show large differences that diminish with increasing level number and converge at the highest level numbers. Hydrogen atoms excited in u=5 contribute 250 times less, and those in u=8 still contribute 65 times less to the Lyman than to the corresponding Balmer emission, supporting the idea of distinct spatial origin of the emissions of both series. This is also indicated by the line widths. The high optical thickness of all Lyman members allows the brightness temperature T_b to be estimated from the spectral radiance at line center, where T_b is found to be largely independent of the upper level number, in contrast to the (known) behavior of the Balmer lines.