Oxygen Abundance in Coronal Streamers

Abstract

In a previous work we developed a stationary proton-electron MHD model of the solar corona, suitable for the minimum-activity epochs. In the present work, we use empirically derived information from the past solar minimum for the oxygen parameters (temperatures and outflow velocity) to develop an oxygen model in the frame of our previous model. Solar and Heliospheric Observatory UVCS observations of past minimum coronal streamers in the O vi k1032 and O vi k1037 lines have revealed that oxygen abundances are very much depleted within the stable closed-field regions of the quiescent streamers. This characteristic appears as a relatively dark region, often seen in the core of the quiescent equatorial streamers when imaged in oxygen lines. Using the oxygen abundance as a free parameter, we compute the expected O vi line emissivities from our models and synthesize images considering the projection effects in detail. We find here that, in order to obtain oxygen images with dark streamer cores similar to those observed, very low element abundances have to be assumed. The required depletion factors are lower than previous estimates that do not account for projection effects, which is especially important at 1.75 R and higher.