Resonance Scattering of 30.4 nm Chromospheric Radiation by Coronal Singly Ionized Helium Observed with EIT

A diffuse emission is observed above the solar limb in the 304 Å channel of the Extreme-Ultraviolet Imaging Telescope (EIT) onboard the SOHO spacecraft. Part of this emission is attributed to the presence of residual singly-ionized helium in the solar corona, which resonantly scatters the intense helium Lyman alpha radiation of the chromosphere. This emission can be distinguished from other coronal emissions in the EIT bandpass. Maps of the helium ion density integrated along the line of sight are derived. These agree well with models in the low latitude, closed magnetic field regions of the solar corona. However, the helium ions' abundance seems to be enhanced in the polar, open field regions above coronal holes. This may be related to acceleration processes of the fast solar wind close to the Sun.