1203.4091 (V. Andretta et al.)

Coronal Diagnostics from Narrow-Band Images around 30.4 nm    [PDF]

V. Andretta, D. Telloni, G. Del Zanna

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 in fact 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. When observing the off-limb corona, however, 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 semi-empirical 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 narrow-band 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.

View original: http://arxiv.org/abs/1203.4091