The SUMER Lyman-alpha line profile in quiescent prominences

TL;DR

This paper presents the first high-quality, geocoronal-absorption-free observations of the hydrogen Lyman-alpha line profile in quiescent prominences, revealing profile asymmetries and supporting multi-thread prominence models.

Contribution

It provides novel SOHO-SUMER Lyman-alpha line profiles of prominences, comparing them with models and other lines, and analyzes the influence of magnetic field orientation on line profiles.

Findings

Lyman-alpha profiles show blue-peak dominance and increased reversal when line-of-sight is perpendicular to magnetic field.

Prominence emission contrast is very low and differs from disk log-normal distribution.

Features in Si III are not always co-spatial with Lyman-alpha emission.

Abstract

Aims: Out of a novel observing technique, we publish for the first time, SoHO-SUMER observations of the true spectral line profile of hydrogen Lyman-alpha in quiescent prominences. With SoHO not being in Earth orbit, our high-quality data set is free from geocoronal absorption. We study the line profile and compare it with earlier observations of the higher Lyman lines and recent model predictions. Methods: We applied the reduced-aperture observing mode to two prominence targets and started a statistical analysis of the line profiles in both data sets. In particular, we investigated the shape of the profile, the radiance distribution and the line shape-to-radiance interrelation. We also compare Ly-a data to co-temporal 1206 Si III data. Results: We find that the average profile of Ly-a has a blue-peak dominance and is more reversed, if the line-of-sight is perpendicular to the field lines. The contrast of Ly-a prominence emission rasters is very low and the radiance distribution differs from the log-normal distribution of the disk. Features seen in the Si III line are not always co-spatial with Ly-a emission. Conclusions: Our empirical results support recent multi-thread models, which predict that asymmetries and depths of the self-reversal depend on the orientation of the prominence axis relative to the line-of-sight.