Coronal Diagnostics from Narrowband Images around 30.4 nm

TL;DR

This study assesses the relative contributions of He II 30.4 nm and Si XI 30.3 nm lines in solar corona imaging, combining observations and models to improve coronal diagnostics around solar minimum.

Contribution

It provides a detailed estimation of line contributions in the 30.4 nm band, enhancing the interpretation of EUV images of the solar corona.

Findings

Si XI dominates the He II line from just above the limb to ~2 R_Sun in streamers.

Contribution of Si XI decreases beyond ~2-3 R_Sun, becoming negligible.

The results depend strongly on the coronal temperature profile.

Abstract

Images taken in the band centered at 30.4 nm are routinely used to map the radiance of the He II Ly alpha line on the solar disk. That line is one of the strongest, if not the strongest, line in the EUV observed in the solar spectrum, and one of the few lines in that wavelength range providing information on the upper chromosphere or lower transition region. However, when observing the off-limb corona the contribution from the nearby Si XI 30.3 nm line can become significant. In this work we aim at estimating the relative contribution of those two lines in the solar corona around the minimum of solar activity. We combine measurements from CDS taken in August 2008 with temperature and density profiles from semiempirical models of the corona to compute the radiances of the two lines, and of other representative coronal lines (e.g., Mg X 62.5 nm, Si XII 52.1 nm). Considering both diagnosed quantities from line ratios (temperatures and densities) and line radiances in absolute units, we obtain a good overall match between observations and models. We find that the Si XI line dominates the He II line from just above the limb up to ~2 R_Sun in streamers, while its contribution to narrowband imaging in the 30.4 nm band is expected to become smaller, even negligible in the corona beyond ~2 - 3 R_Sun, the precise value being strongly dependent on the coronal temperature profile.