Predicting the COSIE-C Signal from the Outer Corona up to 3 Solar Radii

Abstract

We present estimates of the signal to be expected in quiescent solar conditions, as would be obtained with the COronal Spectrographic Imager in the EUV in its coronagraphic mode (COSIE-C). COSIE-C has been proposed to routinely observe the relatively unexplored outer corona, where we know that many fundamental processes affecting both the lower corona and the solar wind are taking place. The COSIE-C spectral band, 186–205 Å, is well-known as it has been observed with Hinode EIS. We present Hinode EIS observations that we obtained in 2007 out to 1.5  R ⊙ , to show that this spectral band in quiescent streamers is dominated by Fe xii and Fe xi and that the ionization temperature is nearly constant. To estimate the COSIE-C signal in the 1.5–3.1  R ⊙ region we use a model based on CHIANTI atomic data and SoHO UVCS observations in the Si xii and Mg x coronal lines of two quiescent 1996 streamers. We reproduce the observed EUV radiances with a simple density model, photospheric abundances, and a constant temperature of 1.4 MK. We show that other theoretical or semi-empirical models fail to reproduce the observations. We find that the coronal COSIE-C signal at 3  R ⊙ should be about 5 counts/s per 3.″1 pixel in quiescent streamers. This is unprecedented and opens up a significant discovery space. We also briefly discuss stray light and the visibility of other solar features. In particular, we present UVCS observations of an active region streamer, indicating increased signal compared to the quiet Sun cases.