SOHO Joint Observing Plan 063: Absorption by H and He Continua in Prominences

T. Kucera, V. Andretta, A. Poland, J. Gurman

First Draft 4/17/97

Purpose:
To determine the relative abundances of H, He I, and He II in prominences by observing absorption by these species of coronal lines observed by CDS and EIT.

Method:
The upper limit to the resonance continuum occurs at 911 for H, 504 E for He I, and 228 E for He II. Lyman absorption by prominences and other structures is clearly visible in emission of hot coronal lines observed by CDS and EIT. By comparing the absorption in lines absorbed at different wavelengths from 171 - 625 E , we plan to determine the absorption due to each of the three species, calculate the column depth of each and, in this way, find their relative abundances.

Scientific Case:
Determining the relative abundances of H and He is important in evaluating models of the solar atmosphere, prominences, and formation of elements in stellar cores. In the past, attempts have been made to measure the H/He abundance ratio using IR lines (Hirayama et al. 1998), and in the quiet corona using ultraviolet emission lines (e.g. Gabriel et al. 1995), but in general, this value is not very well determined for prominences.

References:

Gabriel, A., H., Culhane, J.L., Patchett, B.E., Breeveld, E.R., Lang, J., Parkinson, J.H., Payne,: J., and Norman, K., 1995, Adv. Space Res. Vol 15, p.(7)63.
Hirayama, T. Nakagomi, Y., and Okamoto, T., 1979, Physics of Solar Prominences,: Proceedings of IAU Colloquium #44, Oslo, Aug 1978., eds: E. Jensen, P. Maltby, F.Q., Orrall, Inst of Theoretical Astrophyysics, Blinderen.

Observing Program:
The target is a prominence extending to the near side of the limb and showing some absorption in EIT Fe images. Targets which are too large should probably be avoided, as that will make it especially difficult to determine the background emission.

1. CDS:             NIS
                    Slit:  4x240" slit
                    Integration time : 120 s, Compression: None
                    16 lines: H absorbed: Si X/Mg X (624.7/624.9 E), 
                        Al XI (550.03 E), Ca X (557.77 E), 
                        Si (XII 520.66 E)  
                        H & He absorbed: Si VIII (319.83 E), 
                        Mg VIII/Fe XVI (335.3 E), Si IX (345.12,349.87 E), 
                        Si X (347.40 E), Mg IX (368.07 E)
                        Context lines: He I (584.33 E), He II (303.78 E),  
                        O III (599.59 E), O IV (554.51 E), O V (629.73 E), 
                        Ne VI  (562.8 E)
                   Line window width: 22 pix 
                   Raster: 30 locations, 4" apart for 120"x240" coverage 
                        of prominence
                   Duration: 1 hr/raster

2: EIT:  Observations of a subsection of the Sun every 5 
             or 10 minutes with Fe XV (284  E), Fe IX/X (171 E), 
             and Fe XII (195 E). He II (304  E) images might be 
             taken for context every half hour or so.

3: SUMER: Lyman continuum observations when possible